Ecto

v2.1.4

Ecto v2.1.4 Ecto.Migration

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.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

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.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

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

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 :bits or :blob, and so on.

Similarly, you can pass any field type supported by your database as long as it maps to an Ecto type. For instance, you can use :text, :varchar or :char in your migrations as add :field_name, :text. In your Ecto schema, they will all map to the same :string type.

Remember, atoms can containing arbitrary characters by enclosing in double quotes the characters following the colon. So, if you want to use field type with your database specific options, you can pass atoms containing these options like :"int unsigned", :"time without time zone".

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 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.

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).

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.

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. At the moment there is support only for Postgres.

def up do
  create index(:posts, [:name], comment: "Index Comment")
  create constraint(:products, "price_must_be_positive", check: "price > 0", comment: "Index Comment")
  create table(:weather, prefix: "north_america", comment: "Table Comment") do
    add :city, :string, size: 40, comment: "Column Comment"
    timestamps()
  end
end

Schema Migrations table

Version numbers of migrations will be saved in schema_migrations table. But you can configure the table via:

config :app, App.Repo, migration_source: "my_migrations"

Summary

Functions

add(column, type, opts \\ [])

Adds a column when creating or altering a table

alter(object, list)

Alters a table

constraint(table, name, opts \\ [])

Defines a constraint (either a check constraint or an exclusion constraint) to be evaluated by the database when a row is inserted or updated

create(index)

Creates one of the following

create(object, list)

Creates a table

create_if_not_exists(index)

Creates an index or a table with only :id field if one does not yet exist

create_if_not_exists(object, list)

Creates a table if it does not exist

direction()

Gets the migrator direction

drop(index_or_table_or_constraint)

Drops one of the following

drop_if_exists(index_or_table)

Drops a table or index if it exists

execute(command)

Executes arbitrary SQL or a keyword command in NoSQL databases

flush()

Executes queue migration commands

fragment(expr)

Generates a fragment to be used as default value

index(table, columns, opts \\ [])

Returns an index struct that can be used on create, drop, etc

modify(column, type, opts \\ [])

Modifies the type of column when altering a table

prefix()

Gets the migrator prefix

references(table, opts \\ [])

Defines a foreign key

remove(column)

Removes a column when altering a table

rename(table_current, list)

Renames a table

rename(table, current_column, list)

Renames a column outside of the alter statement

table(name, opts \\ [])

Returns a table struct that can be given on create, alter, etc

timestamps(opts \\ [])

Adds :inserted_at and :updated_at timestamps columns

unique_index(table, columns, opts \\ [])

Shortcut for creating a unique index

Functions

add(column, type, opts \\ [])

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, :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 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.UUID, 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,  :map  # Elixir type which is handled by the database
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
alter(object, list) (macro)

Alters a table.

Examples 

alter table(:posts) do
  add :summary, :text
  modify :title, :text
  remove :views
end
constraint(table, name, opts \\ [])

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 - The expression to evaluate on a row. Required when creating.
  • :name - The name of the constraint - required.
create(index)

Creates one of the following:

  • an index
  • a table with only an :id field
  • a constraint

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)
create constraint(:products, "price_must_be_positive", check: "price > 0")
create(object, list) (macro)

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
create_if_not_exists(index)

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)
create_if_not_exists(object, list) (macro)

Creates a table if it does not exist.

Works just like create/2 but does not raise an error when table already exists.

direction() 
direction() :: :up | :down

Gets the migrator direction.

drop(index_or_table_or_constraint)

Drops one of the following:

  • an index
  • a table
  • a constraint

Examples 

drop index(:posts, [:name])
drop table(:posts)
drop constraint(:products, name: "price_must_be_positive")
drop_if_exists(index_or_table)

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)
execute(command)

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
flush()

Executes queue migration commands.

Reverses the order commands are executed when doing a rollback on a change/0 function and resets commands queue.

fragment(expr)

Generates a fragment to be used as default value.

Examples 

create table(:posts) do
  add :inserted_at, :naive_datetime, default: fragment("now()")
end
index(table, columns, opts \\ [])

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
  • :where - the conditions for a partial index

Adding/dropping indexes concurrently

PostgreSQL supports adding/dropping indexes concurrently (see the docs). 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.

Partial indexes

Databases like PostgreSQL and MSSQL supports 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 WHERE clause as is.

More information on partial indexes can be found in the PostgreSQL docs.

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)

# Create a partial index
create index(:products, [:user_id], where: "price = 0", name: :free_products_index)
modify(column, type, opts \\ [])

Modifies the type of column when altering a table.

This command is not reversible as Ecto does not know what is the current type to revert it back to.

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 numeric type. Default is no precision.
  • :scale - the scale of a numeric type. Default is 0 scale.
prefix()

Gets the migrator prefix.

references(table, opts \\ [])

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 referenced entry is deleted. May be :nothing, :delete_all or :nilify_all. Defaults to :nothing.
  • :on_update - What to perform if the referenced entry is updated. May be :nothing, :update_all or :nilify_all. Defaults to :nothing.
remove(column)

Removes a column when altering a table.

This command is not reversible as Ecto does not know what is the current type to add the column back as.

Examples 

alter table(:posts) do
  remove :title
end
rename(table_current, list)

Renames a table.

Examples 

rename table(:posts), to: table(:new_posts)
rename(table, current_column, list)

Renames a column outside of the alter statement.

Examples 

rename table(:posts), :title, to: :summary
table(name, opts \\ [])

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
  • :prefix - the prefix for the table
  • :options - provide custom options that will be appended after generated statement, for example “WITH”, “INHERITS” or “ON COMMIT” clauses
timestamps(opts \\ [])

Adds :inserted_at and :updated_at timestamps columns.

Those columns are of :naive_datetime type, and by default cannot be null. opts can be given to customize the generated fields.

Options

  • :inserted_at - the name of the column for insertion times, providing false disables column
  • :updated_at - the name of the column for update times, providing false disables column
  • :type - column type, defaults to :naive_datetime
unique_index(table, columns, opts \\ [])

Shortcut for creating a unique index.

See index/3 for more information.