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.

  Here is an example:

      defmodule MyRepo.Migrations.CreatePosts 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

  Note migrations have an `up/0` and `down/0` instructions, where
  `up/0` is used to update your database and `down/0` rolls back
  the prompted changes.

  Ecto provides some mix tasks to help developers work with migrations:

    * `mix ecto.gen.migration add_weather_table` - generates a
      migration that the user can fill in with particular commands
    * `mix ecto.migrate` - migrates a repository
    * `mix ecto.rollback` - rolls back a particular migration

  Run the `mix help COMMAND` for more information.

  ## Change

  Migrations can also be automatically reversible by implementing
  `change/0` instead of `up/0` and `down/0`. For example, the
  migration above can be written as:

      defmodule MyRepo.Migrations.CreatePosts 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

  Notice not all commands are reversible though. Trying to rollback
  a non-reversible command will raise an `Ecto.MigrationError`.

  ## Prefixes

  Migrations support specifying a table prefix or index prefix which will target either a schema
  if using Postgres, or a different database if using MySQL. If no prefix is
  provided, the default schema or database is used.
  Any reference declated in table migration refer by default table with same prefix declared for table.
  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.

  For both MySQL and Postgres with a prefixed table, you must use the same prefix for the index field to ensure 
  you index the prefix qualified table. 

  ## Transactions

  By default, Ecto runs all migrations inside a transaction. That's not always
  ideal: for example, PostgreSQL allows to create/drop indexes concurrently but
  only outside of any transaction (see the [PostgreSQL
  docs](http://www.postgresql.org/docs/9.2/static/sql-createindex.html#SQL-CREATEINDEX-CONCURRENTLY)).

  Migrations can be forced to run outside a transaction by setting the
  `@disable_ddl_transaction` module attribute to `true`:

      defmodule MyRepo.Migrations.CreateIndexes do
        use Ecto.Migration
        @disable_ddl_transaction true

        def change do
          create index(:posts, [:slug], concurrently: true)
        end
      end

  Since running migrations outside a transaction can be dangerous, consider
  performing very few operations in such migrations.

  See the `index/3` function for more information on creating/dropping indexes
  concurrently.

  """

  defmodule Index do
    @moduledoc """
    Defines an index struct used in migrations.
    """
    defstruct table: nil,
              prefix: nil,
              name: nil,
              columns: [],
              unique: false,
              concurrently: false,
              using: nil

    @type t :: %__MODULE__{
      table: atom,
      prefix: atom,
      name: atom,
      columns: [atom | String.t],
      unique: boolean,
      concurrently: boolean,
      using: atom | String.t
    }
  end

  defmodule Table do
    @moduledoc """
    Defines a table struct used in migrations.
    """
    defstruct name: nil, prefix: nil, primary_key: true, engine: nil, options: nil
    @type t :: %__MODULE__{name: atom, prefix: atom, primary_key: boolean, engine: atom}
  end

  defmodule Reference do
    @moduledoc """
    Defines a reference struct used in migrations.
    """
    defstruct name: nil,
              table: nil,
              column: :id,
              type: :serial,
              on_delete: :nothing

    @type t :: %__MODULE__{
      table: atom,
      column: atom,
      type: atom,
      on_delete: atom
    }
  end

  alias Ecto.Migration.Runner

  @doc false
  defmacro __using__(_) do
    quote location: :keep do
      import Ecto.Migration
      @disable_ddl_transaction 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]
    end
  end

  @doc """
  Creates a table.

  By default, the table will also include a primary_key of name `:id`
  and type `:serial`. Check `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
    do_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 table
  already exists.
  """
  defmacro create_if_not_exists(object, do: block) do
    do_create(object, :create_if_not_exists, block)
  end

  defp do_create(object, command, block) do
    quote do
      table = %Table{} = unquote(object)
      Runner.start_command({unquote(command), Ecto.Migration.__prefix__(table)})

      if table.primary_key do
        add(:id, :serial, primary_key: true)
      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 an index or a table with only `:id` field.

  When reversing (in `change` running backward) 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)

  """
  def create(%Index{} = index) do
    Runner.execute {:create, __prefix__(index)}
  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 table.primary_key do
        [{:add, :id, :serial, primary_key: true}]
      else
        []
      end
      
    Runner.execute {command, __prefix__(table), columns}
  end

  @doc """
  Drops a table or index.

  ## Examples

      drop index(:posts, [:name])
      drop table(:posts)

  """
  def drop(%{} = index_or_table) do
    Runner.execute {:drop, __prefix__(index_or_table)}
    index_or_table
  end

  @doc """
  Drops a table or index if it exists.

  Does not raise an error if table or index does not exist.

  ## Examples

      drop_if_exists index(:posts, [:name])
      drop_if_exists table(:posts)

  """
  def drop_if_exists(%{} = index_or_table) do
    Runner.execute {:drop_if_exists, __prefix__(index_or_table)}
    index_or_table
  end

  @doc """
  Returns a table struct that can be given on create, alter, 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, does not generate primary key on table creation
    * `:engine` - customizes the table storage for supported databases. For MySQL,
      the default is InnoDB
    * `:options` - provide custom options that will be appended after generated
      statement, for example "WITH", "INHERITS" or "ON COMMIT" clauses

  """
  def table(name, opts \\ []) when is_atom(name) do
    struct(%Table{name: name}, opts)
  end

  @doc ~S"""
  Returns an index struct that can be used on `create`, `drop`, etc.

  Expects the table name as first argument and the index fields as
  second. The field can be an atom, representing a column, or a
  string representing an expression that is sent as is to the database.

  Indexes are non-unique by default.

  ## Options

    * `:name` - the name of the index. Defaults to "#{table}_#{column}_index"
    * `:unique` - if the column(s) is unique or not
    * `:concurrently` - if the index should be created/dropped concurrently
    * `:using` - configures the index type
    * `:prefix` - prefix for the index

  ## Adding/dropping indexes concurrently

  PostgreSQL supports adding/dropping indexes concurrently (see the
  [docs](http://www.postgresql.org/docs/9.4/static/sql-createindex.html)).
  In order to take advantage of this, the `:concurrently` option needs to be set
  to `true` when the index is created/dropped.

  **Note**: in order for the `:concurrently` option to work, the migration must
  not be run inside a transaction. See the `Ecto.Migration` docs for more
  information on running migrations outside of a transaction.

  ## Index types

  PostgreSQL supports several index types like B-tree, Hash or GiST. When
  creating an index, the index type defaults to B-tree, but it can be specified
  with the `:using` option. The `:using` option can be an atom or a string; its
  value is passed to the `USING` clause as is.

  More information on index types can be found in the [PostgreSQL
  docs](http://www.postgresql.org/docs/9.4/static/indexes-types.html).

  ## Examples

      # Without a name, index defaults to products_category_id_sku_index
      create index(:products, [:category_id, :sku], unique: true)

      # Name can be given explicitly though
      drop 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)

      # Create an index on custom expressions
      create index(:products, ["lower(name)"], name: :products_lower_name_index)

  """
  def index(table, columns, opts \\ []) when is_atom(table) and is_list(columns) do
    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 \\ []) when is_atom(table) and is_list(columns) do
    index(table, columns, [unique: true] ++ opts)
  end

  defp default_index_name(index) do
    [index.table, index.columns, "index"]
    |> List.flatten
    |> Enum.join("_")
    |> String.replace(~r"[^\w_]", "_")
    |> String.replace("__", "_")
    |> String.to_atom
  end

  @doc """
  Executes arbitrary SQL or a keyword command in NoSQL databases.

  ## Examples

      execute "UPDATE posts SET published_at = NULL"

      execute create: "posts", capped: true, size: 1024

  """
  def execute(command) when is_binary(command) or is_list(command) do
    Runner.execute command
  end

  @doc """
  Gets the migrator direction.
  """
  @spec direction :: :up | :down
  def direction do
    Runner.migrator_direction
  end

  @doc """
  Adds a column when creating or altering a table.

  This function also accepts Ecto primitive types as column types
  and they are normalized by the database adapter. For example,
  `string` is converted to varchar, `datetime` to the underlying
  datetime or timestamp type, `binary` to bits or blob, and so on.

  However, the column type is not always the same as the type in your
  model. For example, a model that has a `string` field, can be
  supported by columns of types `char`, `varchar`, `text` and others.
  For this reason, this function also accepts `text` and other columns,
  which are sent as is to the underlying database.

  To sum up, the column type may be either an Ecto primitive type,
  which is normalized in cases the database does not understand it,
  like `string` or `binary`, or a database type which is passed as is.
  Custom Ecto types, like `Ecto.Datetime`, are not supported because
  they are application level concern and may not always map to the
  database.

  ## Examples

      create table(:posts) do
        add :title, :string, default: "Untitled"
      end

      alter table(:posts) do
        add :summary, :text # Database type
        add :object,  :json
      end

  ## Options

    * `:primary_key` - when true, marks this field as the primary key
    * `:default` - the column's default value. can be a string, number
      or a fragment generated by `fragment/1`
    * `:null` - when `false`, the column does not allow null values
    * `:size` - the size of the type (for example the numbers of characters).
      Default is no size, except for `:string` that defaults to 255.
    * `:precision` - the precision for a numeric type. Default is no precision
    * `:scale` - the scale of a numeric type. Default is 0 scale

  """
  def add(column, type, opts \\ []) when is_atom(column) do
    validate_type!(type)
    Runner.subcommand {:add, 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 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 default value.

  ## Examples

      create table(:posts) do
        add :inserted_at, :datetime, default: fragment("now()")
      end
  """
  def fragment(expr) when is_binary(expr) do
    {:fragment, expr}
  end

  @doc """
  Adds `:inserted_at` and `:updated_at` timestamps columns.

  Those columns are of `:datetime` type and by default cannot
  be null. `opts` can be given to customize the generated
  fields.
  """
  def timestamps(opts \\ []) do
    opts = Keyword.put_new(opts, :null, false)
    add(:inserted_at, :datetime, opts)
    add(:updated_at, :datetime, opts)
  end

  @doc """
  Modifies the type of column when altering a table.

  See `add/3` for more information on supported types.

  ## Examples

      alter table(:posts) do
        modify :title, :text
      end

  ## Options

    * `:null` - sets to null or not null
    * `:default` - changes the default
    * `:size` - the size of the type (for example the numbers of characters). Default is no size.
    * `:precision` - the precision for a numberic type. Default is no precision.
    * `:scale` - the scale of a numberic type. Default is 0 scale.
  """
  def modify(column, type, opts \\ []) when is_atom(column) do
    Runner.subcommand {:modify, column, type, opts}
  end

  @doc """
  Removes a column when altering a table.

  ## Examples

      alter table(:posts) do
        remove :title
      end

  """
  def remove(column) when is_atom(column) do
    Runner.subcommand {:remove, column}
  end

  @doc ~S"""
  Defines a foreign key.

  ## Examples

      create table(:products) do
        add :group_id, references(:groups)
      end

  ## Options

    * `:name` - The name of the underlying reference,
      defaults to "#{table}_#{column}_fkey"
    * `:column` - The foreign key column, default is `:id`
    * `:type`   - The foreign key type, default is `:serial`
    * `:on_delete` - What to perform if the entry is deleted.
      May be `:nothing`, `:delete_all` or `:nilify_all`.
      Defaults to `:nothing`.

  """
  def references(table, opts \\ []) when is_atom(table) do
    reference = struct(%Reference{table: table}, opts)

    unless reference.on_delete in [:nothing, :delete_all, :nilify_all] do
      raise ArgumentError, "unknown :on_delete value: #{inspect reference.on_delete}"
    end

    reference
  end

  @doc """
  Executes queue migration commands.

  Reverses the order commands are executed when doing a rollback
  on a change/0 function and resets commands queue.
  """
  def flush do
    Runner.flush
  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!(%Reference{} = reference) do
    reference
  end

  @doc false
  def __prefix__(%{prefix: prefix} = index_or_table) do
    runner_prefix = Runner.prefix()

    cond do
      is_nil(prefix) ->
        %{index_or_table | prefix: runner_prefix}
      is_nil(runner_prefix) or runner_prefix == prefix ->
        index_or_table
      true ->
        raise Ecto.MigrationError,  message:
          "the :prefix option `#{inspect prefix}` does match the migrator prefix `#{inspect runner_prefix}`"
    end
  end
end