zf

migration.ex (50239B)

---

 defmodule Ecto.Migration do
   @moduledoc """
   Migrations are used to modify your database schema over time.
 
   This module provides many helpers for migrating the database,
   allowing developers to use Elixir to alter their storage in
   a way that is database independent.
 
   Migrations typically provide two operations: `up` and `down`,
   allowing us to migrate the database forward or roll it back
   in case of errors.
 
   In order to manage migrations, Ecto creates a table called
   `schema_migrations` in the database, which stores all migrations
   that have already been executed. You can configure the name of
   this table with the `:migration_source` configuration option.
 
   You can configure a different database for the table that
   manages your migrations by setting the `:migration_repo`
   configuration option to a different repository.
 
   Ecto also locks the `schema_migrations` table when running
   migrations, guaranteeing two different servers cannot run the same
   migration at the same time.
 
   ## Creating your first migration
 
   Migrations are defined inside the "priv/REPO/migrations" where REPO
   is the last part of the repository name in underscore. For example,
   migrations for `MyApp.Repo` would be found in "priv/repo/migrations".
   For `MyApp.CustomRepo`, it would be found in "priv/custom_repo/migrations".
 
   Each file in the migrations directory has the following structure:
 
   ```text
   NUMBER_NAME.exs
   ```
 
   The NUMBER is a unique number that identifies the migration. It is
   usually the timestamp of when the migration was created. The NAME
   must also be unique and it quickly identifies what the migration
   does. For example, if you need to track the "weather" in your system,
   you can start a new file at "priv/repo/migrations/20190417140000_add_weather_table.exs"
   that will have the following contents:
 
       defmodule MyRepo.Migrations.AddWeatherTable do
         use Ecto.Migration
 
         def up do
           create table("weather") do
             add :city,    :string, size: 40
             add :temp_lo, :integer
             add :temp_hi, :integer
             add :prcp,    :float
 
             timestamps()
           end
         end
 
         def down do
           drop table("weather")
         end
       end
 
   The `up/0` function is responsible to migrate your database forward.
   the `down/0` function is executed whenever you want to rollback.
   The `down/0` function must always do the opposite of `up/0`.
   Inside those functions, we invoke the API defined in this module,
   you will find conveniences for managing tables, indexes, columns,
   references, as well as running custom SQL commands.
 
   To run a migration, we generally use Mix tasks. For example, you can
   run the migration above by going to the root of your project and
   typing:
 
       $ mix ecto.migrate
 
   You can also roll it back by calling:
 
       $ mix ecto.rollback --step 1
 
   Note rollback requires us to say how much we want to rollback.
   On the other hand, `mix ecto.migrate` will always run all pending
   migrations.
 
   In practice, we don't create migration files by hand either, we
   typically use `mix ecto.gen.migration` to generate the file with
   the proper timestamp and then we just fill in its contents:
 
       $ mix ecto.gen.migration add_weather_table
 
   ## Mix tasks
 
   As seen above, Ecto provides many Mix tasks to help developers work
   with migrations. We summarize them below:
 
     * `mix ecto.gen.migration` - generates a
       migration that the user can fill in with particular commands
     * `mix ecto.migrate` - migrates a repository
     * `mix ecto.migrations` - shows all migrations and their status
     * `mix ecto.rollback` - rolls back a particular migration
 
   Run `mix help COMMAND` for more information on a particular command.
   For a lower level API for running migrations, see `Ecto.Migrator`.
 
   ## Change
 
   Having to write both `up/0` and `down/0` functions for every
   migration is tedious and error prone. For this reason, Ecto allows
   you to defined a `change/0` callback with all of the code you want
   to execute when migrating and Ecto will automatically figure out
   the `down/0` for you. For example, the migration above can be
   written as:
 
       defmodule MyRepo.Migrations.AddWeatherTable do
         use Ecto.Migration
 
         def change do
           create table("weather") do
             add :city,    :string, size: 40
             add :temp_lo, :integer
             add :temp_hi, :integer
             add :prcp,    :float
 
             timestamps()
           end
         end
       end
 
   However, note that not all commands are reversible. Trying to rollback
   a non-reversible command will raise an `Ecto.MigrationError`.
 
   A notable command in this regard is `execute/2`, which is reversible in
   `change/0` by accepting a pair of plain SQL strings. The first is run on
   forward migrations (`up/0`) and the second when rolling back (`down/0`).
 
   If `up/0` and `down/0` are implemented in a migration, they take precedence,
   and `change/0` isn't invoked.
 
   ## Field Types
 
   The Ecto primitive types are mapped to the appropriate database
   type by the various database adapters. For example, `:string` is
   converted to `:varchar`, `:binary` to `:bytea` or `:blob`, and so on.
 
   In particular, note that:
 
     * the `:string` type in migrations by default has a limit of 255 characters.
       If you need more or less characters, pass the `:size` option, such
       as `add :field, :string, size: 10`. If you don't want to impose a limit,
       most databases support a `:text` type or similar
 
     * the `:binary` type in migrations by default has no size limit. If you want
       to impose a limit, pass the `:size` option accordingly. In MySQL, passing
       the size option changes the underlying field from "blob" to "varbinary"
 
   Any other type will be given as is to the database. For example, you
   can use `:text`, `:char`, or `:varchar` as types. Types that have spaces
   in their names can be wrapped in double quotes, such as `:"int unsigned"`,
   `:"time without time zone"`, etc.
 
   ## Executing and flushing
 
   Instructions inside of migrations are not executed immediately. Instead
   they are performed after the relevant `up`, `change`, or `down` callback
   terminates.
 
   However, in some situations you may want to guarantee that all of the
   previous steps have been executed before continuing. This is useful when
   you need to apply a set of changes to the table before continuing with the
   migration. This can be done with `flush/0`:
 
       def up do
         ...
         flush()
         ...
       end
 
   However `flush/0` will raise if it would be called from `change` function when doing a rollback.
   To avoid that we recommend to use `execute/2` with anonymous functions instead.
   For more information and example usage please take a look at `execute/2` function.
 
   ## Repo configuration
 
   ### Migrator configuration
 
   These options configure how the underlying migration engine works:
 
     * `:migration_source` - Version numbers of migrations will be saved in a
       table named `schema_migrations` by default. You can configure the name of
       the table via:
 
           config :app, App.Repo, migration_source: "my_migrations"
 
     * `:migration_lock` - By default, Ecto will lock the migration source to throttle
       multiple nodes to run migrations one at a time. You can disable the `migration_lock`
       by setting it to `false`. You may also select a different locking strategy if
       supported by the adapter. See the adapter docs for more information.
 
           config :app, App.Repo, migration_lock: false
 
           # Or use a different locking strategy. For example, Postgres can use advisory
           # locks but be aware that your database configuration might not make this a good
           # fit. See the Ecto.Adapters.Postgres for more information:
           config :app, App.Repo, migration_lock: :pg_advisory_lock
 
     * `:migration_repo` - The migration repository is where the table managing the
       migrations will be stored (`migration_source` defines the table name). It defaults
       to the given repository itself but you can configure it via:
 
           config :app, App.Repo, migration_repo: App.MigrationRepo
 
     * `:priv` - the priv directory for the repo with the location of important assets,
       such as migrations. For a repository named `MyApp.FooRepo`, `:priv` defaults to
       "priv/foo_repo" and migrations should be placed at "priv/foo_repo/migrations"
 
     * `:start_apps_before_migration` - A list of applications to be started before
       running migrations. Used by `Ecto.Migrator.with_repo/3` and the migration tasks:
 
           config :app, App.Repo, start_apps_before_migration: [:ssl, :some_custom_logger]
 
   ### Migrations configuration
 
   These options configure how each migration works. **It is generally discouraged
   to change any of those configurations after your database is deployed to production,
   as changing these options will retroactively change how all migrations work**.
 
     * `:migration_primary_key` - By default, Ecto uses the `:id` column with type
       `:bigserial`, but you can configure it via:
 
           config :app, App.Repo, migration_primary_key: [name: :uuid, type: :binary_id]
 
           config :app, App.Repo, migration_primary_key: false
 
     * `:migration_foreign_key` - By default, Ecto uses the `primary_key` type
       for foreign keys when `references/2` is used, but you can configure it via:
 
           config :app, App.Repo, migration_foreign_key: [column: :uuid, type: :binary_id]
 
     * `:migration_timestamps` - By default, Ecto uses the `:naive_datetime` as the type,
       `:inserted_at` as the name of the column for storing insertion times, `:updated_at` as
       the name of the column for storing last-updated-at times, but you can configure it
       via:
 
           config :app, App.Repo, migration_timestamps: [
             type: :utc_datetime,
             inserted_at: :created_at,
             updated_at: :changed_at
           ]
 
     * `:migration_default_prefix` - Ecto defaults to `nil` for the database prefix for
       migrations, but you can configure it via:
 
           config :app, App.Repo, migration_default_prefix: "my_prefix"
 
   ## Comments
 
   Migrations where you create or alter a table support specifying table
   and column comments. The same can be done when creating constraints
   and indexes. Not all databases support this feature.
 
       def up do
         create index("posts", [:name], comment: "Index Comment")
         create constraint("products", "price_must_be_positive", check: "price > 0", comment: "Constraint Comment")
         create table("weather", prefix: "north_america", comment: "Table Comment") do
           add :city, :string, size: 40, comment: "Column Comment"
           timestamps()
         end
       end
 
   ## Prefixes
 
   Migrations support specifying a table prefix or index prefix which will
   target either a schema (if using PostgreSQL) or a different database (if using
   MySQL). If no prefix is provided, the default schema or database is used.
 
   Any reference declared in the table migration refers by default to the table
   with the same declared prefix. The prefix is specified in the table options:
 
       def up do
         create table("weather", prefix: "north_america") do
           add :city,    :string, size: 40
           add :temp_lo, :integer
           add :temp_hi, :integer
           add :prcp,    :float
           add :group_id, references(:groups)
 
           timestamps()
         end
 
         create index("weather", [:city], prefix: "north_america")
       end
 
   Note: if using MySQL with a prefixed table, you must use the same prefix
   for the references since cross-database references are not supported.
 
   When using a prefixed table with either MySQL or PostgreSQL, you must use the
   same prefix for the index field to ensure that you index the prefix-qualified
   table.
 
   ## Transaction Callbacks
 
   If possible, each migration runs inside a transaction. This is true for Postgres,
   but not true for MySQL, as the latter does not support DDL transactions.
 
   In some rare cases, you may need to execute some common behavior after beginning
   a migration transaction, or before committing that transaction. For instance, one
   might desire to set a `lock_timeout` for each lock in the migration transaction.
 
   You can do so by defining `c:after_begin/0` and `c:before_commit/0` callbacks to
   your migration.
 
   However, if you need do so for every migration module, implement this callback
   for every migration can be quite repetitive. Luckily, you can handle this by
   providing your migration module:
 
       defmodule MyApp.Migration do
         defmacro __using__(_) do
           quote do
             use Ecto.Migration
 
             def after_begin() do
               repo().query! "SET lock_timeout TO '5s'"
             end
           end
         end
       end
 
   Then in your migrations you can `use MyApp.Migration` to share this behavior
   among all your migrations.
 
   ## Additional resources
 
     * The [Safe Ecto Migrations guide](https://fly.io/phoenix-files/safe-ecto-migrations/)
 
   """
 
   @doc """
   Migration code to run immediately after the transaction is opened.
 
   Keep in mind that it is treated like any normal migration code, and should
   consider both the up *and* down cases of the migration.
   """
   @callback after_begin() :: term
 
   @doc """
   Migration code to run immediately before the transaction is closed.
 
   Keep in mind that it is treated like any normal migration code, and should
   consider both the up *and* down cases of the migration.
   """
   @callback before_commit() :: term
   @optional_callbacks after_begin: 0, before_commit: 0
 
   defmodule Index do
     @moduledoc """
     Used internally by adapters.
 
     To define an index in a migration, see `Ecto.Migration.index/3`.
     """
     defstruct table: nil,
               prefix: nil,
               name: nil,
               columns: [],
               unique: false,
               concurrently: false,
               using: nil,
               include: [],
               nulls_distinct: nil,
               where: nil,
               comment: nil,
               options: nil
 
     @type t :: %__MODULE__{
       table: String.t,
       prefix: atom,
       name: atom,
       columns: [atom | String.t],
       unique: boolean,
       concurrently: boolean,
       using: atom | String.t,
       include: [atom | String.t],
       nulls_distinct: boolean | nil,
       where: atom | String.t,
       comment: String.t | nil,
       options: String.t
     }
   end
 
   defmodule Table do
     @moduledoc """
     Used internally by adapters.
 
     To define a table in a migration, see `Ecto.Migration.table/2`.
     """
     defstruct name: nil, prefix: nil, comment: nil, primary_key: true, engine: nil, options: nil
     @type t :: %__MODULE__{name: String.t, prefix: atom | nil, comment: String.t | nil, primary_key: boolean | keyword(),
                            engine: atom, options: String.t}
   end
 
   defmodule Reference do
     @moduledoc """
     Used internally by adapters.
 
     To define a reference in a migration, see `Ecto.Migration.references/2`.
     """
     defstruct name: nil, prefix: nil, table: nil, column: :id, type: :bigserial,
               on_delete: :nothing, on_update: :nothing, validate: true,
               with: [], match: nil
     @type t :: %__MODULE__{table: String.t, prefix: atom | nil, column: atom, type: atom,
                            on_delete: atom, on_update: atom, validate: boolean,
                            with: list, match: atom | nil}
   end
 
   defmodule Constraint do
     @moduledoc """
     Used internally by adapters.
 
     To define a constraint in a migration, see `Ecto.Migration.constraint/3`.
     """
     defstruct name: nil, table: nil, check: nil, exclude: nil, prefix: nil, comment: nil, validate: true
     @type t :: %__MODULE__{name: atom, table: String.t, prefix: atom | nil,
                            check: String.t | nil, exclude: String.t | nil, comment: String.t | nil, validate: boolean}
   end
 
   defmodule Command do
     @moduledoc """
     Used internally by adapters.
 
     This represents the up and down legs of a reversible raw command
     that is usually defined with `Ecto.Migration.execute/1`.
 
     To define a reversible command in a migration, see `Ecto.Migration.execute/2`.
     """
     defstruct up: nil, down: nil
     @type t :: %__MODULE__{up: String.t, down: String.t}
   end
 
   alias Ecto.Migration.Runner
 
   @doc false
   defmacro __using__(_) do
     quote location: :keep do
       import Ecto.Migration
       @disable_ddl_transaction false
       @disable_migration_lock false
       @before_compile Ecto.Migration
     end
   end
 
   @doc false
   defmacro __before_compile__(_env) do
     quote do
       def __migration__ do
         [
           disable_ddl_transaction: @disable_ddl_transaction,
           disable_migration_lock: @disable_migration_lock
         ]
       end
     end
   end
 
   @doc """
   Creates a table.
 
   By default, the table will also include an `:id` primary key field that
   has a type of `:bigserial`. Check the `table/2` docs for more information.
 
   ## Examples
 
       create table(:posts) do
         add :title, :string, default: "Untitled"
         add :body,  :text
 
         timestamps()
       end
 
   """
   defmacro create(object, do: block) do
     expand_create(object, :create, block)
   end
 
   @doc """
   Creates a table if it does not exist.
 
   Works just like `create/2` but does not raise an error when the table
   already exists.
   """
   defmacro create_if_not_exists(object, do: block) do
     expand_create(object, :create_if_not_exists, block)
   end
 
   defp expand_create(object, command, block) do
     quote do
       table = %Table{} = unquote(object)
       Runner.start_command({unquote(command), Ecto.Migration.__prefix__(table)})
 
       if primary_key = Ecto.Migration.__primary_key__(table) do
         {name, type, opts} = primary_key
         add(name, type, opts)
       end
 
       unquote(block)
       Runner.end_command()
       table
     end
   end
 
   @doc """
   Alters a table.
 
   ## Examples
 
       alter table("posts") do
         add :summary, :text
         modify :title, :text
         remove :views
       end
 
   """
   defmacro alter(object, do: block) do
     quote do
       table = %Table{} = unquote(object)
       Runner.start_command({:alter, Ecto.Migration.__prefix__(table)})
       unquote(block)
       Runner.end_command()
     end
   end
 
   @doc """
   Creates one of the following:
 
     * an index
     * a table with only the :id primary key
     * a constraint
 
   When reversing (in a `change/0` running backwards), indexes are only dropped
   if they exist, and no errors are raised. To enforce dropping an index, use
   `drop/1`.
 
   ## Examples
 
       create index("posts", [:name])
       create table("version")
       create constraint("products", "price_must_be_positive", check: "price > 0")
 
   """
   def create(%Index{} = index) do
     Runner.execute {:create, __prefix__(index)}
     index
   end
 
   def create(%Constraint{} = constraint) do
     Runner.execute {:create, __prefix__(constraint)}
     constraint
   end
 
   def create(%Table{} = table) do
     do_create table, :create
     table
   end
 
   @doc """
   Creates an index or a table with only `:id` field if one does not yet exist.
 
   ## Examples
 
       create_if_not_exists index("posts", [:name])
 
       create_if_not_exists table("version")
 
   """
   def create_if_not_exists(%Index{} = index) do
     Runner.execute {:create_if_not_exists, __prefix__(index)}
   end
 
   def create_if_not_exists(%Table{} = table) do
     do_create table, :create_if_not_exists
   end
 
   defp do_create(table, command) do
     columns =
       if primary_key = Ecto.Migration.__primary_key__(table) do
         {name, type, opts} = primary_key
         [{:add, name, type, opts}]
       else
         []
       end
 
     Runner.execute {command, __prefix__(table), columns}
   end
 
   @doc """
   Drops one of the following:
 
     * an index
     * a table
     * a constraint
 
   ## Examples
 
       drop index("posts", [:name])
       drop table("posts")
       drop constraint("products", "price_must_be_positive")
       drop index("posts", [:name]), mode: :cascade
       drop table("posts"), mode: :cascade
 
   ## Options
 
     * `:mode` - when set to `:cascade`, automatically drop objects that depend
       on the index, and in turn all objects that depend on those objects
       on the table. Default is `:restrict`
 
   """
   def drop(%{} = index_or_table_or_constraint, opts \\ []) when is_list(opts) do
     Runner.execute {:drop, __prefix__(index_or_table_or_constraint), Keyword.get(opts, :mode, :restrict)}
     index_or_table_or_constraint
   end
 
   @doc """
   Drops a table or index if it exists.
 
   Does not raise an error if the specified table or index does not exist.
 
   ## Examples
 
       drop_if_exists index("posts", [:name])
       drop_if_exists table("posts")
       drop_if_exists index("posts", [:name]), mode: :cascade
       drop_if_exists table("posts"), mode: :cascade
 
   ## Options
 
     * `:mode` - when set to `:cascade`, automatically drop objects that depend
       on the index, and in turn all objects that depend on those objects
       on the table. Default is `:restrict`
 
   """
   def drop_if_exists(%{} = index_or_table, opts \\ []) when is_list(opts) do
     Runner.execute {:drop_if_exists, __prefix__(index_or_table), Keyword.get(opts, :mode, :restrict)}
 
     index_or_table
   end
 
   @doc """
   Returns a table struct that can be given to `create/2`, `alter/2`, `drop/1`,
   etc.
 
   ## Examples
 
       create table("products") do
         add :name, :string
         add :price, :decimal
       end
 
       drop table("products")
 
       create table("products", primary_key: false) do
         add :name, :string
         add :price, :decimal
       end
 
   ## Options
 
     * `:primary_key` - when `false`, a primary key field is not generated on table
       creation. Alternatively, a keyword list in the same style of the
       `:migration_primary_key` repository configuration can be supplied
       to control the generation of the primary key field. The keyword list
       must include `:name` and `:type`. See `add/3` for further options.
     * `:engine` - customizes the table storage for supported databases. For MySQL,
       the default is InnoDB.
     * `:prefix` - the prefix for the table. This prefix will automatically be used
       for all constraints and references defined for this table unless explicitly
       overridden in said constraints/references.
     * `:comment` - adds a comment to the table.
     * `:options` - provide custom options that will be appended after the generated
       statement. For example, "WITH", "INHERITS", or "ON COMMIT" clauses.
 
   """
   def table(name, opts \\ [])
 
   def table(name, opts) when is_atom(name) do
     table(Atom.to_string(name), opts)
   end
 
   def table(name, opts) when is_binary(name) and is_list(opts) do
     struct(%Table{name: name}, opts)
   end
 
   @doc ~S"""
   Returns an index struct that can be given to `create/1`, `drop/1`, etc.
 
   Expects the table name as the first argument and the index field(s) as
   the second. The fields can be atoms, representing columns, or strings,
   representing expressions that are sent as-is to the database.
 
   ## Options
 
     * `:name` - the name of the index. Defaults to "#{table}_#{column}_index".
     * `:unique` - indicates whether the index should be unique. Defaults to
       `false`.
     * `:concurrently` - indicates whether the index should be created/dropped
       concurrently.
     * `:using` - configures the index type.
     * `:prefix` - specify an optional prefix for the index.
     * `:where` - specify conditions for a partial index.
     * `:include` - specify fields for a covering index. This is not supported
       by all databases. For more information on PostgreSQL support, please
       [read the official docs](https://www.postgresql.org/docs/current/indexes-index-only-scans.html).
     * `:nulls_distinct` - specify whether null values should be considered
       distinct for a unique index. Defaults to `nil`, which will not add the
       parameter to the generated SQL and thus use the database default.
       This option is currently only supported by PostgreSQL 15+.
       For MySQL, it is always false. For MSSQL, it is always true.
       See the dedicated section on this option for more information.
     * `:comment` - adds a comment to the index.
 
   ## Adding/dropping indexes concurrently
 
   PostgreSQL supports adding/dropping indexes concurrently (see the
   [docs](http://www.postgresql.org/docs/current/static/sql-createindex.html)).
   However, this feature does not work well with the transactions used by
   Ecto to guarantee integrity during migrations.
 
   You can address this with two changes:
 
     1. Change your repository to use PG advisory locks as the migration lock.
        Note this may not be supported by Postgres-like databases and proxies.
 
     2. Disable DDL transactions. Doing this removes the guarantee that all of
       the changes in the migration will happen at once, so you will want to
       keep it short.
 
   If the database adapter supports several migration lock strategies, such as
   Postgrex, then review those strategies and consider using a strategy that
   utilizes advisory locks to faciliate running migrations one at a time even
   across multiple nodes. For example:
 
       # Config the Repo (PostgreSQL example)
       config MyApp.Repo, migration_lock: :pg_advisory_lock
 
       # Migrate with your concurrent operation
       defmodule MyRepo.Migrations.CreateIndexes do
         use Ecto.Migration
         @disable_ddl_transaction true
 
         def change do
           create index("posts", [:slug], concurrently: true)
         end
       end
 
   ## Index types
 
   When creating an index, the index type can be specified with the `:using`
   option. The `:using` option can be an atom or a string, and its value is
   passed to the generated `USING` clause as-is.
 
   For example, PostgreSQL supports several index types like B-tree (the
   default), Hash, GIN, and GiST. More information on index types can be found
   in the [PostgreSQL docs](http://www.postgresql.org/docs/current/indexes-types.html).
 
   ## Partial indexes
 
   Databases like PostgreSQL and MSSQL support partial indexes.
 
   A partial index is an index built over a subset of a table. The subset
   is defined by a conditional expression using the `:where` option.
   The `:where` option can be an atom or a string; its value is passed
   to the generated `WHERE` clause as-is.
 
   More information on partial indexes can be found in the [PostgreSQL
   docs](http://www.postgresql.org/docs/current/indexes-partial.html).
 
   ## The `:nulls_distinct` option
 
   A unique index does not prevent multiple null values by default in most databases.
 
   For example, imagine we have a "products" table and need to guarantee that
   sku's are unique within their category, but the category is optional.
   Creating a regular unique index over the sku and category_id fields with:
 
       create index("products", [:sku, :category_id], unique: true)
 
   will allow products with the same sku to be inserted if their category_id is `nil`.
   The `:nulls_distinct` option can be used to change this behavior by considering
   null values as equal, i.e. not distinct:
 
       create index("products", [:sku, :category_id], unique: true, nulls_distinct: false)
 
   This option is currently only supported by PostgreSQL 15+.
   As a workaround for older PostgreSQL versions and other databases, an
   additional partial unique index for the sku can be created:
 
       create index("products", [:sku, :category_id], unique: true)
       create index("products", [:sku], unique: true, where: "category_id IS NULL")
 
   ## Examples
 
       # With no name provided, the name of the below index defaults to
       # products_category_id_sku_index
       create index("products", [:category_id, :sku], unique: true)
 
       # The name can also be set explicitly
       create index("products", [:category_id, :sku], name: :my_special_name)
 
       # Indexes can be added concurrently
       create index("products", [:category_id, :sku], concurrently: true)
 
       # The index type can be specified
       create index("products", [:name], using: :hash)
 
       # Partial indexes are created by specifying a :where option
       create index("products", [:user_id], where: "price = 0", name: :free_products_index)
 
       # Covering indexes are created by specifying a :include option
       create index("products", [:user_id], include: [:category_id])
 
   Indexes also support custom expressions. Some databases may require the
   index expression to be written between parentheses:
 
       # Create an index on a custom expression
       create index("products", ["(lower(name))"], name: :products_lower_name_index)
 
       # Create a tsvector GIN index on PostgreSQL
       create index("products", ["(to_tsvector('english', name))"],
                    name: :products_name_vector, using: "GIN")
   """
   def index(table, columns, opts \\ [])
 
   def index(table, columns, opts) when is_atom(table) do
     index(Atom.to_string(table), columns, opts)
   end
 
   def index(table, column, opts) when is_binary(table) and is_atom(column) do
     index(table, [column], opts)
   end
 
   def index(table, columns, opts) when is_binary(table) and is_list(columns) and is_list(opts) do
     validate_index_opts!(opts)
     index = struct(%Index{table: table, columns: columns}, opts)
     %{index | name: index.name || default_index_name(index)}
   end
 
   @doc """
   Shortcut for creating a unique index.
 
   See `index/3` for more information.
   """
   def unique_index(table, columns, opts \\ [])
 
   def unique_index(table, columns, opts) when is_list(opts) do
     index(table, columns, [unique: true] ++ opts)
   end
 
   defp default_index_name(index) do
     [index.table, index.columns, "index"]
     |> List.flatten
     |> Enum.map(&to_string(&1))
     |> Enum.map(&String.replace(&1, ~r"[^\w_]", "_"))
     |> Enum.map(&String.replace_trailing(&1, "_", ""))
     |> Enum.join("_")
     |> String.to_atom
   end
 
   @doc """
   Executes arbitrary SQL, anonymous function or a keyword command.
 
   The argument is typically a string, containing the SQL command to be executed.
   Keyword commands exist for non-SQL adapters and are not used in most situations.
 
   Supplying an anonymous function does allow for arbitrary code to execute as
   part of the migration. This is most often used in combination with `repo/0`
   by library authors who want to create high-level migration helpers.
 
   Reversible commands can be defined by calling `execute/2`.
 
   ## Examples
 
       execute "CREATE EXTENSION postgres_fdw"
 
       execute create: "posts", capped: true, size: 1024
 
       execute(fn -> repo().query!("SELECT $1::integer + $2", [40, 2], [log: :info]) end)
 
       execute(fn -> repo().update_all("posts", set: [published: true]) end)
   """
   def execute(command) when is_binary(command) or is_function(command, 0) or is_list(command) do
     Runner.execute command
   end
 
   @doc """
   Executes reversible SQL commands.
 
   This is useful for database-specific functionality that does not
   warrant special support in Ecto, for example, creating and dropping
   a PostgreSQL extension. The `execute/2` form avoids having to define
   separate `up/0` and `down/0` blocks that each contain an `execute/1`
   expression.
 
   The allowed parameters are explained in `execute/1`.
 
   ## Examples
 
       defmodule MyApp.MyMigration do
         use Ecto.Migration
 
         def change do
           execute "CREATE EXTENSION postgres_fdw", "DROP EXTENSION postgres_fdw"
           execute(&execute_up/0, &execute_down/0)
         end
 
         defp execute_up, do: repo().query!("select 'Up query …';", [], [log: :info])
         defp execute_down, do: repo().query!("select 'Down query …';", [], [log: :info])
       end
   """
   def execute(up, down) when (is_binary(up) or is_function(up, 0) or is_list(up)) and
                              (is_binary(down) or is_function(down, 0) or is_list(down)) do
     Runner.execute %Command{up: up, down: down}
   end
 
   @doc """
   Gets the migrator direction.
   """
   @spec direction :: :up | :down
   def direction do
     Runner.migrator_direction()
   end
 
   @doc """
   Gets the migrator repo.
   """
   @spec repo :: Ecto.Repo.t
   def repo do
     Runner.repo()
   end
 
   @doc """
   Gets the migrator prefix.
   """
   def prefix do
     Runner.prefix()
   end
 
   @doc """
   Adds a column when creating or altering a table.
 
   This function also accepts Ecto primitive types as column types
   that are normalized by the database adapter. For example,
   `:string` is converted to `:varchar`, `:binary` to `:bits` or `:blob`,
   and so on.
 
   However, the column type is not always the same as the type used in your
   schema. For example, a schema that has a `:string` field can be supported by
   columns of type `:char`, `:varchar`, `:text`, and others. For this reason,
   this function also accepts `:text` and other type annotations that are native
   to the database. These are passed to the database as-is.
 
   To sum up, the column type may be either an Ecto primitive type,
   which is normalized in cases where the database does not understand it,
   such as `:string` or `:binary`, or a database type which is passed as-is.
   Custom Ecto types like `Ecto.UUID` are not supported because
   they are application-level concerns and may not always map to the database.
 
   Note: It may be necessary to quote case-sensitive, user-defined type names.
   For example, PostgreSQL normalizes all identifiers to lower case unless
   they are wrapped in double quotes. To ensure a case-sensitive type name
   is sent properly, it must be defined `:'"LikeThis"'` or `:"\"LikeThis\""`.
   This is not necessary for column names because Ecto quotes them automatically.
   Type names are not automatically quoted because they may be expressions such
   as `varchar(255)`.
 
   ## Examples
 
       create table("posts") do
         add :title, :string, default: "Untitled"
       end
 
       alter table("posts") do
         add :summary, :text               # Database type
         add :object,  :map                # Elixir type which is handled by the database
         add :custom, :'"UserDefinedType"' # A case-sensitive, user-defined type name
       end
 
   ## Options
 
     * `:primary_key` - when `true`, marks this field as the primary key.
       If multiple fields are marked, a composite primary key will be created.
     * `:default` - the column's default value. It can be a string, number, empty
       list, list of strings, list of numbers, or a fragment generated by
       `fragment/1`.
     * `:null` - determines whether the column accepts null values. When not specified,
       the database will use its default behaviour (which is to treat the column as nullable
       in most databases).
     * `:size` - the size of the type (for example, the number of characters).
       The default is no size, except for `:string`, which defaults to `255`.
     * `:precision` - the precision for a numeric type. Required when `:scale` is
       specified.
     * `:scale` - the scale of a numeric type. Defaults to `0`.
     * `:comment` - adds a comment to the added column.
     * `:after` - positions field after the specified one. Only supported on MySQL,
       it is ignored by other databases.
 
   """
   def add(column, type, opts \\ []) when is_atom(column) and is_list(opts) do
     validate_precision_opts!(opts, column)
     validate_type!(type)
     Runner.subcommand {:add, column, type, opts}
   end
 
   @doc """
   Adds a column if it not exists yet when altering a table.
 
   If the `type` value is a `%Reference{}`, it is used to add a constraint.
 
   `type` and `opts` are exactly the same as in `add/3`.
 
   This command is not reversible as Ecto does not know about column existence before the creation attempt.
 
   ## Examples
 
       alter table("posts") do
         add_if_not_exists :title, :string, default: ""
       end
 
   """
   def add_if_not_exists(column, type, opts \\ []) when is_atom(column) and is_list(opts) do
     validate_precision_opts!(opts, column)
     validate_type!(type)
     Runner.subcommand {:add_if_not_exists, column, type, opts}
   end
 
   @doc """
   Renames a table.
 
   ## Examples
 
       rename table("posts"), to: table("new_posts")
   """
   def rename(%Table{} = table_current, to: %Table{} = table_new) do
     Runner.execute {:rename, __prefix__(table_current), __prefix__(table_new)}
     table_new
   end
 
   @doc """
   Renames a column.
 
   Note that this occurs outside of the `alter` statement.
 
   ## Examples
 
       rename table("posts"), :title, to: :summary
   """
   def rename(%Table{} = table, current_column, to: new_column) when is_atom(current_column) and is_atom(new_column) do
     Runner.execute {:rename, __prefix__(table), current_column, new_column}
     table
   end
 
   @doc """
   Generates a fragment to be used as a default value.
 
   ## Examples
 
       create table("posts") do
         add :inserted_at, :naive_datetime, default: fragment("now()")
       end
   """
   def fragment(expr) when is_binary(expr) do
     {:fragment, expr}
   end
 
   @doc """
   Adds `:inserted_at` and `:updated_at` timestamp columns.
 
   Those columns are of `:naive_datetime` type and by default cannot be null. A
   list of `opts` can be given to customize the generated fields.
 
   Following options will override the repo configuration specified by
   `:migration_timestamps` option.
 
   ## Options
 
     * `:inserted_at` - the name of the column for storing insertion times.
       Setting it to `false` disables the column.
     * `:updated_at` - the name of the column for storing last-updated-at times.
       Setting it to `false` disables the column.
     * `:type` - the type of the `:inserted_at` and `:updated_at` columns.
       Defaults to `:naive_datetime`.
     * `:default` - the columns' default value. It can be a string, number, empty
       list, list of strings, list of numbers, or a fragment generated by
       `fragment/1`.
 
   """
   def timestamps(opts \\ []) when is_list(opts) do
     opts = Keyword.merge(Runner.repo_config(:migration_timestamps, []), opts)
     opts = Keyword.put_new(opts, :null, false)
 
     {type, opts} = Keyword.pop(opts, :type, :naive_datetime)
     {inserted_at, opts} = Keyword.pop(opts, :inserted_at, :inserted_at)
     {updated_at, opts} = Keyword.pop(opts, :updated_at, :updated_at)
 
     if inserted_at != false, do: add(inserted_at, type, opts)
     if updated_at != false, do: add(updated_at, type, opts)
   end
 
   @doc """
   Modifies the type of a column when altering a table.
 
   This command is not reversible unless the `:from` option is provided.
   When the `:from` option is set, the adapter will try to drop
   the corresponding foreign key constraints before modifying the type.
   Generally speaking, you want to pass the type and each option
   you are modifying to `:from`, so the column can be rolled back properly.
   However, note that `:from` cannot be be used to modify primary keys,
   as those are generally trickier to revert.
 
   See `add/3` for more information on supported types.
 
   If you want to modify a column without changing its type,
   such as adding or dropping a null constraints, consider using
   the `execute/2` command with the relevant SQL command instead
   of `modify/3`, if supported by your database. This may avoid
   redundant type updates and be more efficient, as an unnecessary
   type update can lock the table, even if the type actually
   doesn't change.
 
   ## Examples
 
       alter table("posts") do
         modify :title, :text
       end
 
       # Self rollback when using the :from option
       alter table("posts") do
         modify :title, :text, from: :string
       end
 
       # Modify column with rollback options
       alter table("posts") do
         modify :title, :text, null: false, from: {:string, null: true}
       end
 
   ## Options
 
     * `:null` - determines whether the column accepts null values. If this option is
       not set, the nullable behaviour of the underlying column is not modified.
     * `:default` - changes the default value of the column.
     * `:from` - specifies the current type and options of the column.
     * `:size` - specifies the size of the type (for example, the number of characters).
       The default is no size.
     * `:precision` - the precision for a numeric type. Required when `:scale` is
       specified.
     * `:scale` - the scale of a numeric type. Defaults to `0`.
     * `:comment` - adds a comment to the modified column.
   """
   def modify(column, type, opts \\ []) when is_atom(column) and is_list(opts) do
     validate_precision_opts!(opts, column)
     validate_type!(type)
     Runner.subcommand {:modify, column, type, opts}
   end
 
   @doc """
   Removes a column when altering a table.
 
   This command is not reversible as Ecto does not know what type it should add
   the column back as. See `remove/3` as a reversible alternative.
 
   ## Examples
 
       alter table("posts") do
         remove :title
       end
 
   """
   def remove(column) when is_atom(column) do
     Runner.subcommand {:remove, column}
   end
 
   @doc """
   Removes a column in a reversible way when altering a table.
 
   `type` and `opts` are exactly the same as in `add/3`, and
   they are used when the command is reversed.
 
   If the `type` value is a `%Reference{}`, it is used to remove the constraint.
 
   ## Examples
 
       alter table("posts") do
         remove :title, :string, default: ""
       end
 
   """
   def remove(column, type, opts \\ []) when is_atom(column) do
     validate_type!(type)
     Runner.subcommand {:remove, column, type, opts}
   end
 
   @doc """
   Removes a column only if the column exists when altering the constraint if the reference type is passed
   once it only has the constraint name on reference structure.
 
   This command is not reversible as Ecto does not know about column existence before the removal attempt.
 
   ## Examples
 
       alter table("posts") do
         remove_if_exists :title, :string
       end
 
   """
   def remove_if_exists(column, type) when is_atom(column) do
     validate_type!(type)
     Runner.subcommand {:remove_if_exists, column, type}
   end
 
   @doc ~S"""
   Defines a foreign key.
 
   By default it assumes you are linking to the referenced table
   via its primary key with name `:id`. If you are using a non-default
   key setup (e.g. using `uuid` type keys) you must ensure you set the
   options, such as `:name` and `:type`, to match your target key.
 
   ## Examples
 
       create table("products") do
         add :group_id, references("groups")
       end
 
       create table("categories") do
         add :group_id, :integer
         # A composite foreign that points from categories (product_id, group_id)
         # to products (id, group_id)
         add :product_id, references("products", with: [group_id: :group_id])
       end
 
   ## Options
 
     * `:name` - The name of the underlying reference, which defaults to
       "#{table}_#{column}_fkey".
     * `:column` - The column name in the referenced table, which defaults to `:id`.
     * `:prefix` - The prefix for the reference. Defaults to the prefix
       defined by the block's `table/2` struct (the "products" table in
       the example above), or `nil`.
     * `:type` - The foreign key type, which defaults to `:bigserial`.
     * `:on_delete` - What to do if the referenced entry is deleted. May be
       `:nothing` (default), `:delete_all`, `:nilify_all`, or `:restrict`.
     * `:on_update` - What to do if the referenced entry is updated. May be
       `:nothing` (default), `:update_all`, `:nilify_all`, or `:restrict`.
     * `:validate` - Whether or not to validate the foreign key constraint on
        creation or not. Only available in PostgreSQL, and should be followed by
        a command to validate the foreign key in a following migration if false.
     * `:with` - defines additional keys to the foreign key in order to build
       a composite primary key
     * `:match` - select if the match is `:simple`, `:partial`, or `:full`. This is
       [supported only by PostgreSQL](https://www.postgresql.org/docs/current/sql-createtable.html)
       at the moment.
 
   """
   def references(table, opts \\ [])
 
   def references(table, opts) when is_atom(table) do
     references(Atom.to_string(table), opts)
   end
 
   def references(table, opts) when is_binary(table) and is_list(opts) do
     opts = Keyword.merge(foreign_key_repo_opts(), opts)
     reference = struct(%Reference{table: table}, opts)
 
     unless reference.on_delete in [:nothing, :delete_all, :nilify_all, :restrict] do
       raise ArgumentError, "unknown :on_delete value: #{inspect reference.on_delete}"
     end
 
     unless reference.on_update in [:nothing, :update_all, :nilify_all, :restrict] do
       raise ArgumentError, "unknown :on_update value: #{inspect reference.on_update}"
     end
 
     reference
   end
 
   defp foreign_key_repo_opts() do
     case Runner.repo_config(:migration_primary_key, []) do
       false -> []
       opts -> opts
     end
     |> Keyword.take([:type])
     |> Keyword.merge(Runner.repo_config(:migration_foreign_key, []))
   end
 
   @doc ~S"""
   Defines a constraint (either a check constraint or an exclusion constraint)
   to be evaluated by the database when a row is inserted or updated.
 
   ## Examples
 
       create constraint("users", :price_must_be_positive, check: "price > 0")
       create constraint("size_ranges", :no_overlap, exclude: ~s|gist (int4range("from", "to", '[]') WITH &&)|)
       drop   constraint("products", "price_must_be_positive")
 
   ## Options
 
     * `:check` - A check constraint expression. Required when creating a check constraint.
     * `:exclude` - An exclusion constraint expression. Required when creating an exclusion constraint.
     * `:prefix` - The prefix for the table.
     * `:validate` - Whether or not to validate the constraint on creation (true by default). Only
        available in PostgreSQL, and should be followed by a command to validate the new constraint in
        a following migration if false.
     * `:comment` - adds a comment to the constraint.
 
   """
   def constraint(table, name, opts \\ [])
 
   def constraint(table, name, opts) when is_atom(table) do
     constraint(Atom.to_string(table), name, opts)
   end
 
   def constraint(table, name, opts) when is_binary(table) and is_list(opts) do
     struct(%Constraint{table: table, name: name}, opts)
   end
 
   @doc "Executes queue migration commands."
   defmacro flush do
     quote do
       if direction() == :down and not function_exported?(__MODULE__, :down, 0) do
         raise "calling flush() inside change when doing rollback is not supported."
       else
         Runner.flush()
       end
     end
   end
 
   # Validation helpers
   defp validate_type!(:datetime) do
     raise ArgumentError, "the :datetime type in migrations is not supported, " <>
                          "please use :utc_datetime or :naive_datetime instead"
   end
 
   defp validate_type!(type) when is_atom(type) do
     case Atom.to_string(type) do
       "Elixir." <> _ ->
         raise ArgumentError,
           "#{inspect type} is not a valid database type, " <>
           "please use an atom like :string, :text and so on"
       _ ->
         :ok
     end
   end
 
   defp validate_type!({type, subtype}) when is_atom(type) and is_atom(subtype) do
     validate_type!(subtype)
   end
 
   defp validate_type!({type, subtype}) when is_atom(type) and is_tuple(subtype) do
     for t <- Tuple.to_list(subtype), do: validate_type!(t)
   end
 
   defp validate_type!(%Reference{} = reference) do
     reference
   end
 
   defp validate_type!(type) do
     raise ArgumentError, """
     invalid migration type: #{inspect(type)}. Expected one of:
 
       * an atom, such as :string
       * a quoted atom, such as :"integer unsigned"
       * an Ecto.Type, such as Ecto.UUID
       * a tuple of the above, such as {:array, :integer} or {:array, Ecto.UUID}
       * a reference, such as references(:users)
 
     All Ecto types are allowed and properly translated.
     All other types are sent to the database as is.
     """
   end
 
   defp validate_index_opts!(opts) when is_list(opts) do
     if opts[:nulls_distinct] != nil and opts[:unique] != true do
       raise ArgumentError, "the `nulls_distinct` option can only be used with unique indexes"
     end
 
     case Keyword.get_values(opts, :where) do
       [_, _ | _] ->
         raise ArgumentError,
               "only one `where` keyword is supported when declaring a partial index. " <>
                 "To specify multiple conditions, write a single WHERE clause using AND between them"
 
       _ ->
         :ok
     end
   end
 
   defp validate_index_opts!(opts), do: opts
 
   defp validate_precision_opts!(opts, column) when is_list(opts) do
     if opts[:scale] && !opts[:precision] do
       raise ArgumentError, "column #{Atom.to_string(column)} is missing precision option"
     end
   end
 
   @doc false
   def __prefix__(%{prefix: prefix} = index_or_table) do
     runner_prefix = Runner.prefix()
 
     cond do
       is_nil(prefix) ->
         prefix = runner_prefix || Runner.repo_config(:migration_default_prefix, nil)
         %{index_or_table | prefix: prefix}
       is_nil(runner_prefix) or runner_prefix == to_string(prefix) ->
         index_or_table
       true ->
         raise Ecto.MigrationError,  message:
           "the :prefix option `#{prefix}` does not match the migrator prefix `#{runner_prefix}`"
     end
   end
 
   @doc false
   def __primary_key__(table) do
     case table.primary_key do
       false -> false
 
       true ->
         case Runner.repo_config(:migration_primary_key, []) do
           false -> false
           opts when is_list(opts) -> pk_opts_to_tuple(opts)
         end
 
       opts when is_list(opts) -> pk_opts_to_tuple(opts)
 
       _ ->
         raise ArgumentError, ":primary_key option must be either a boolean or a keyword list of options"
     end
   end
 
   defp pk_opts_to_tuple(opts) do
     opts = Keyword.put(opts, :primary_key, true)
     {name, opts} = Keyword.pop(opts, :name, :id)
     {type, opts} = Keyword.pop(opts, :type, :bigserial)
     {name, type, opts}
   end
 end