1 Migration Overview
Migrations are a convenient way to alter your database schema over time in a consistent and easy way. They use a Ruby DSL so that you don't have to write SQL by hand, allowing your schema and changes to be database independent.
You can think of each migration as being a new 'version' of the database. A
schema starts off with nothing in it, and each migration modifies it to add or
remove tables, columns, or entries. Active Record knows how to update your
schema along this timeline, bringing it from whatever point it is in the
history to the latest version. Active Record will also update your
db/schema.rb
file to match the up-to-date structure of your database.
Here's an example of a migration:
class CreateProducts < ActiveRecord::Migration def change create_table :products do |t| t.string :name t.text :description t.timestamps end end end
This migration adds a table called products
with a string column called
name
and a text column called description
. A primary key column called id
will also be added implicitly, as it's the default primary key for all Active
Record models. The timestamps
macro adds two columns, created_at
and
updated_at
. These special columns are automatically managed by Active Record
if they exist.
Note that we define the change that we want to happen moving forward in time. Before this migration is run, there will be no table. After, the table will exist. Active Record knows how to reverse this migration as well: if we roll this migration back, it will remove the table.
On databases that support transactions with statements that change the schema, migrations are wrapped in a transaction. If the database does not support this then when a migration fails the parts of it that succeeded will not be rolled back. You will have to rollback the changes that were made by hand.
There are certain queries that can't run inside a transaction. If your
adapter supports DDL transactions you can use disable_ddl_transaction!
to
disable them for a single migration.
If you wish for a migration to do something that Active Record doesn't know how
to reverse, you can use reversible
:
class ChangeProductsPrice < ActiveRecord::Migration def change reversible do |dir| change_table :products do |t| dir.up { t.change :price, :string } dir.down { t.change :price, :integer } end end end end
Alternatively, you can use up
and down
instead of change
:
class ChangeProductsPrice < ActiveRecord::Migration def up change_table :products do |t| t.change :price, :string end end def down change_table :products do |t| t.change :price, :integer end end end
2 Creating a Migration
2.1 Creating a Standalone Migration
Migrations are stored as files in the db/migrate
directory, one for each
migration class. The name of the file is of the form
YYYYMMDDHHMMSS_create_products.rb
, that is to say a UTC timestamp
identifying the migration followed by an underscore followed by the name
of the migration. The name of the migration class (CamelCased version)
should match the latter part of the file name. For example
20080906120000_create_products.rb
should define class CreateProducts
and
20080906120001_add_details_to_products.rb
should define
AddDetailsToProducts
. Rails uses this timestamp to determine which migration
should be run and in what order, so if you're copying a migration from another
application or generate a file yourself, be aware of its position in the order.
Of course, calculating timestamps is no fun, so Active Record provides a generator to handle making it for you:
$ rails generate migration AddPartNumberToProducts
This will create an empty but appropriately named migration:
class AddPartNumberToProducts < ActiveRecord::Migration def change end end
If the migration name is of the form "AddXXXToYYY" or "RemoveXXXFromYYY" and is
followed by a list of column names and types then a migration containing the
appropriate add_column
and remove_column
statements will be created.
$ rails generate migration AddPartNumberToProducts part_number:string
will generate
class AddPartNumberToProducts < ActiveRecord::Migration def change add_column :products, :part_number, :string end end
If you'd like to add an index on the new column, you can do that as well:
$ rails generate migration AddPartNumberToProducts part_number:string:index
will generate
class AddPartNumberToProducts < ActiveRecord::Migration def change add_column :products, :part_number, :string add_index :products, :part_number end end
Similarly, you can generate a migration to remove a column from the command line:
$ rails generate migration RemovePartNumberFromProducts part_number:string
generates
class RemovePartNumberFromProducts < ActiveRecord::Migration def change remove_column :products, :part_number, :string end end
You are not limited to one magically generated column. For example
$ rails generate migration AddDetailsToProducts part_number:string price:decimal
generates
class AddDetailsToProducts < ActiveRecord::Migration def change add_column :products, :part_number, :string add_column :products, :price, :decimal end end
If the migration name is of the form "CreateXXX" and is followed by a list of column names and types then a migration creating the table XXX with the columns listed will be generated. For example:
$ rails generate migration CreateProducts name:string part_number:string
generates
class CreateProducts < ActiveRecord::Migration def change create_table :products do |t| t.string :name t.string :part_number end end end
As always, what has been generated for you is just a starting point. You can add
or remove from it as you see fit by editing the
db/migrate/YYYYMMDDHHMMSS_add_details_to_products.rb
file.
Also, the generator accepts column type as references
(also available as
belongs_to
). For instance
$ rails generate migration AddUserRefToProducts user:references
generates
class AddUserRefToProducts < ActiveRecord::Migration def change add_reference :products, :user, index: true end end
This migration will create a user_id
column and appropriate index.
There is also a generator which will produce join tables if JoinTable
is part of the name:
rails g migration CreateJoinTableCustomerProduct customer product
will produce the following migration:
class CreateJoinTableCustomerProduct < ActiveRecord::Migration def change create_join_table :customers, :products do |t| # t.index [:customer_id, :product_id] # t.index [:product_id, :customer_id] end end end
2.2 Model Generators
The model and scaffold generators will create migrations appropriate for adding a new model. This migration will already contain instructions for creating the relevant table. If you tell Rails what columns you want, then statements for adding these columns will also be created. For example, running
$ rails generate model Product name:string description:text
will create a migration that looks like this
class CreateProducts < ActiveRecord::Migration def change create_table :products do |t| t.string :name t.text :description t.timestamps end end end
You can append as many column name/type pairs as you want.
2.3 Supported Type Modifiers
You can also specify some options just after the field type between curly braces. You can use the following modifiers:
-
limit
Sets the maximum size of thestring/text/binary/integer
fields -
precision
Defines the precision for thedecimal
fields -
scale
Defines the scale for thedecimal
fields -
polymorphic
Adds atype
column forbelongs_to
associations -
null
Allows or disallowsNULL
values in the column.
For instance, running
$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic}
will produce a migration that looks like this
class AddDetailsToProducts < ActiveRecord::Migration def change add_column :products, :price, :decimal, precision: 5, scale: 2 add_reference :products, :supplier, polymorphic: true, index: true end end
3 Writing a Migration
Once you have created your migration using one of the generators it's time to get to work!
3.1 Creating a Table
The create_table
method is one of the most fundamental, but most of the time,
will be generated for you from using a model or scaffold generator. A typical
use would be
create_table :products do |t| t.string :name end
which creates a products
table with a column called name
(and as discussed
below, an implicit id
column).
By default, create_table
will create a primary key called id
. You can change
the name of the primary key with the :primary_key
option (don't forget to
update the corresponding model) or, if you don't want a primary key at all, you
can pass the option id: false
. If you need to pass database specific options
you can place an SQL fragment in the :options
option. For example,
create_table :products, options: "ENGINE=BLACKHOLE" do |t| t.string :name, null: false end
will append ENGINE=BLACKHOLE
to the SQL statement used to create the table
(when using MySQL, the default is ENGINE=InnoDB
).
3.2 Creating a Join Table
Migration method create_join_table
creates a HABTM join table. A typical use
would be
create_join_table :products, :categories
which creates a categories_products
table with two columns called
category_id
and product_id
. These columns have the option :null
set to
false
by default.
You can pass the option :table_name
when you want to customize the table
name. For example,
create_join_table :products, :categories, table_name: :categorization
will create a categorization
table.
By default, create_join_table
will create two columns with no options, but
you can specify these options using the :column_options
option. For example,
create_join_table :products, :categories, column_options: {null: true}
will create the product_id
and category_id
with the :null
option as
true
.
create_join_table
also accepts a block, which you can use to add indices
(which are not created by default) or additional columns:
create_join_table :products, :categories do |t| t.index :product_id t.index :category_id end
3.3 Changing Tables
A close cousin of create_table
is change_table
, used for changing existing
tables. It is used in a similar fashion to create_table
but the object
yielded to the block knows more tricks. For example
change_table :products do |t| t.remove :description, :name t.string :part_number t.index :part_number t.rename :upccode, :upc_code end
removes the description
and name
columns, creates a part_number
string
column and adds an index on it. Finally it renames the upccode
column.
3.4 When Helpers aren't Enough
If the helpers provided by Active Record aren't enough you can use the execute
method to execute arbitrary SQL:
Product.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1')
For more details and examples of individual methods, check the API documentation.
In particular the documentation for
ActiveRecord::ConnectionAdapters::SchemaStatements
(which provides the methods available in the change
, up
and down
methods),
ActiveRecord::ConnectionAdapters::TableDefinition
(which provides the methods available on the object yielded by create_table
)
and
ActiveRecord::ConnectionAdapters::Table
(which provides the methods available on the object yielded by change_table
).
3.5 Using the change
Method
The change
method is the primary way of writing migrations. It works for the
majority of cases, where Active Record knows how to reverse the migration
automatically. Currently, the change
method supports only these migration
definitions:
add_column
add_index
add_reference
add_timestamps
create_table
create_join_table
-
drop_table
(must supply a block) -
drop_join_table
(must supply a block) remove_timestamps
rename_column
rename_index
remove_reference
rename_table
change_table
is also reversible, as long as the block does not call change
,
change_default
or remove
.
If you're going to need to use any other methods, you should use reversible
or write the up
and down
methods instead of using the change
method.
3.6 Using reversible
Complex migrations may require processing that Active Record doesn't know how
to reverse. You can use reversible
to specify what to do when running a
migration what else to do when reverting it. For example,
class ExampleMigration < ActiveRecord::Migration def change create_table :products do |t| t.references :category end reversible do |dir| dir.up do #add a foreign key execute <<-SQL ALTER TABLE products ADD CONSTRAINT fk_products_categories FOREIGN KEY (category_id) REFERENCES categories(id) SQL end dir.down do execute <<-SQL ALTER TABLE products DROP FOREIGN KEY fk_products_categories SQL end end add_column :users, :home_page_url, :string rename_column :users, :email, :email_address end
Using reversible
will ensure that the instructions are executed in the
right order too. If the previous example migration is reverted,
the down
block will be run after the home_page_url
column is removed and
right before the table products
is dropped.
Sometimes your migration will do something which is just plain irreversible; for
example, it might destroy some data. In such cases, you can raise
ActiveRecord::IrreversibleMigration
in your down
block. If someone tries
to revert your migration, an error message will be displayed saying that it
can't be done.
3.7 Using the up
/down
Methods
You can also use the old style of migration using up
and down
methods
instead of the change
method.
The up
method should describe the transformation you'd like to make to your
schema, and the down
method of your migration should revert the
transformations done by the up
method. In other words, the database schema
should be unchanged if you do an up
followed by a down
. For example, if you
create a table in the up
method, you should drop it in the down
method. It
is wise to reverse the transformations in precisely the reverse order they were
made in the up
method. The example in the reversible
section is equivalent to:
class ExampleMigration < ActiveRecord::Migration def up create_table :products do |t| t.references :category end # add a foreign key execute <<-SQL ALTER TABLE products ADD CONSTRAINT fk_products_categories FOREIGN KEY (category_id) REFERENCES categories(id) SQL add_column :users, :home_page_url, :string rename_column :users, :email, :email_address end def down rename_column :users, :email_address, :email remove_column :users, :home_page_url execute <<-SQL ALTER TABLE products DROP FOREIGN KEY fk_products_categories SQL drop_table :products end end
If your migration is irreversible, you should raise
ActiveRecord::IrreversibleMigration
from your down
method. If someone tries
to revert your migration, an error message will be displayed saying that it
can't be done.
3.8 Reverting Previous Migrations
You can use Active Record's ability to rollback migrations using the revert
method:
require_relative '2012121212_example_migration' class FixupExampleMigration < ActiveRecord::Migration def change revert ExampleMigration create_table(:apples) do |t| t.string :variety end end end
The revert
method also accepts a block of instructions to reverse.
This could be useful to revert selected parts of previous migrations.
For example, let's imagine that ExampleMigration
is committed and it
is later decided it would be best to serialize the product list instead.
One could write:
class SerializeProductListMigration < ActiveRecord::Migration def change add_column :categories, :product_list reversible do |dir| dir.up do # transfer data from Products to Category#product_list end dir.down do # create Products from Category#product_list end end revert do # copy-pasted code from ExampleMigration create_table :products do |t| t.references :category end reversible do |dir| dir.up do #add a foreign key execute <<-SQL ALTER TABLE products ADD CONSTRAINT fk_products_categories FOREIGN KEY (category_id) REFERENCES categories(id) SQL end dir.down do execute <<-SQL ALTER TABLE products DROP FOREIGN KEY fk_products_categories SQL end end # The rest of the migration was ok end end end
The same migration could also have been written without using revert
but this would have involved a few more steps: reversing the order
of create_table
and reversible
, replacing create_table
by drop_table
, and finally replacing up
by down
and vice-versa.
This is all taken care of by revert
.
4 Running Migrations
Rails provides a set of Rake tasks to run certain sets of migrations.
The very first migration related Rake task you will use will probably be
rake db:migrate
. In its most basic form it just runs the change
or up
method for all the migrations that have not yet been run. If there are
no such migrations, it exits. It will run these migrations in order based
on the date of the migration.
Note that running the db:migrate
also invokes the db:schema:dump
task, which
will update your db/schema.rb
file to match the structure of your database.
If you specify a target version, Active Record will run the required migrations (change, up, down) until it has reached the specified version. The version is the numerical prefix on the migration's filename. For example, to migrate to version 20080906120000 run
$ rake db:migrate VERSION=20080906120000
If version 20080906120000 is greater than the current version (i.e., it is
migrating upwards), this will run the change
(or up
) method
on all migrations up to and
including 20080906120000, and will not execute any later migrations. If
migrating downwards, this will run the down
method on all the migrations
down to, but not including, 20080906120000.
4.1 Rolling Back
A common task is to rollback the last migration. For example, if you made a mistake in it and wish to correct it. Rather than tracking down the version number associated with the previous migration you can run
$ rake db:rollback
This will rollback the latest migration, either by reverting the change
method or by running the down
method. If you need to undo
several migrations you can provide a STEP
parameter:
$ rake db:rollback STEP=3
will revert the last 3 migrations.
The db:migrate:redo
task is a shortcut for doing a rollback and then migrating
back up again. As with the db:rollback
task, you can use the STEP
parameter
if you need to go more than one version back, for example
$ rake db:migrate:redo STEP=3
Neither of these Rake tasks do anything you could not do with db:migrate
. They
are simply more convenient, since you do not need to explicitly specify the
version to migrate to.
4.2 Resetting the Database
The rake db:reset
task will drop the database, recreate it and load the
current schema into it.
This is not the same as running all the migrations. It will only use the contents of the current schema.rb file. If a migration can't be rolled back, 'rake db:reset' may not help you. To find out more about dumping the schema see 'schema dumping and you.'
4.3 Running Specific Migrations
If you need to run a specific migration up or down, the db:migrate:up
and
db:migrate:down
tasks will do that. Just specify the appropriate version and
the corresponding migration will have its change
, up
or down
method
invoked, for example,
$ rake db:migrate:up VERSION=20080906120000
will run the 20080906120000 migration by running the change
method (or the
up
method). This task will
first check whether the migration is already performed and will do nothing if
Active Record believes that it has already been run.
4.4 Running Migrations in Different Environments
By default running rake db:migrate
will run in the development
environment.
To run migrations against another environment you can specify it using the
RAILS_ENV
environment variable while running the command. For example to run
migrations against the test
environment you could run:
$ rake db:migrate RAILS_ENV=test
4.5 Changing the Output of Running Migrations
By default migrations tell you exactly what they're doing and how long it took. A migration creating a table and adding an index might produce output like this
== CreateProducts: migrating ================================================= -- create_table(:products) -> 0.0028s == CreateProducts: migrated (0.0028s) ========================================
Several methods are provided in migrations that allow you to control all this:
Method | Purpose |
---|---|
suppress_messages | Takes a block as an argument and suppresses any output generated by the block. |
say | Takes a message argument and outputs it as is. A second boolean argument can be passed to specify whether to indent or not. |
say_with_time | Outputs text along with how long it took to run its block. If the block returns an integer it assumes it is the number of rows affected. |
For example, this migration
class CreateProducts < ActiveRecord::Migration def change suppress_messages do create_table :products do |t| t.string :name t.text :description t.timestamps end end say "Created a table" suppress_messages {add_index :products, :name} say "and an index!", true say_with_time 'Waiting for a while' do sleep 10 250 end end end
generates the following output
== CreateProducts: migrating ================================================= -- Created a table -> and an index! -- Waiting for a while -> 10.0013s -> 250 rows == CreateProducts: migrated (10.0054s) =======================================
If you want Active Record to not output anything, then running rake db:migrate
VERBOSE=false
will suppress all output.
5 Changing Existing Migrations
Occasionally you will make a mistake when writing a migration. If you have
already run the migration then you cannot just edit the migration and run the
migration again: Rails thinks it has already run the migration and so will do
nothing when you run rake db:migrate
. You must rollback the migration (for
example with rake db:rollback
), edit your migration and then run
rake db:migrate
to run the corrected version.
In general, editing existing migrations is not a good idea. You will be creating extra work for yourself and your co-workers and cause major headaches if the existing version of the migration has already been run on production machines. Instead, you should write a new migration that performs the changes you require. Editing a freshly generated migration that has not yet been committed to source control (or, more generally, which has not been propagated beyond your development machine) is relatively harmless.
The revert
method can be helpful when writing a new migration to undo
previous migrations in whole or in part
(see Reverting Previous Migrations above).
6 Using Models in Your Migrations
When creating or updating data in a migration it is often tempting to use one of your models. After all, they exist to provide easy access to the underlying data. This can be done, but some caution should be observed.
For example, problems occur when the model uses database columns which are (1) not currently in the database and (2) will be created by this or a subsequent migration.
Consider this example, where Alice and Bob are working on the same code base
which contains a Product
model:
Bob goes on vacation.
Alice creates a migration for the products
table which adds a new column and
initializes it:
# db/migrate/20100513121110_add_flag_to_product.rb class AddFlagToProduct < ActiveRecord::Migration def change add_column :products, :flag, :boolean reversible do |dir| dir.up { Product.update_all flag: false } end end end
She also adds a validation to the Product
model for the new column:
# app/models/product.rb class Product < ActiveRecord::Base validates :flag, presence: true end
Alice adds a second migration which adds another column to the products
table and initializes it:
# db/migrate/20100515121110_add_fuzz_to_product.rb class AddFuzzToProduct < ActiveRecord::Migration def change add_column :products, :fuzz, :string reversible do |dir| dir.up { Product.update_all fuzz: 'fuzzy' } end end end
She also adds a validation to the Product
model for the new column:
# app/models/product.rb class Product < ActiveRecord::Base validates :flag, :fuzz, presence: true end
Both migrations work for Alice.
Bob comes back from vacation and:
- Updates the source - which contains both migrations and the latest version of the Product model.
- Runs outstanding migrations with
rake db:migrate
, which includes the one that updates theProduct
model.
The migration crashes because when the model attempts to save, it tries to validate the second added column, which is not in the database when the first migration runs:
rake aborted! An error has occurred, this and all later migrations canceled: undefined method `fuzz' for #<Product:0x000001049b14a0>
A fix for this is to create a local model within the migration. This keeps Rails from running the validations, so that the migrations run to completion.
When using a local model, it's a good idea to call
Product.reset_column_information
to refresh the Active Record cache for the
Product
model prior to updating data in the database.
If Alice had done this instead, there would have been no problem:
# db/migrate/20100513121110_add_flag_to_product.rb class AddFlagToProduct < ActiveRecord::Migration class Product < ActiveRecord::Base end def change add_column :products, :flag, :boolean Product.reset_column_information reversible do |dir| dir.up { Product.update_all flag: false } end end end
# db/migrate/20100515121110_add_fuzz_to_product.rb class AddFuzzToProduct < ActiveRecord::Migration class Product < ActiveRecord::Base end def change add_column :products, :fuzz, :string Product.reset_column_information reversible do |dir| dir.up { Product.update_all fuzz: 'fuzzy' } end end end
There are other ways in which the above example could have gone badly.
For example, imagine that Alice creates a migration that selectively
updates the description
field on certain products. She runs the
migration, commits the code, and then begins working on the next feature,
which is to add a new column fuzz
to the products table.
She creates two migrations for this new feature, one which adds the new
column, and a second which selectively updates the fuzz
column based on
other product attributes.
These migrations run just fine, but when Bob comes back from his vacation
and calls rake db:migrate
to run all the outstanding migrations, he gets a
subtle bug: The descriptions have defaults, and the fuzz
column is present,
but fuzz
is nil
on all products.
The solution is again to use Product.reset_column_information
before
referencing the Product model in a migration, ensuring the Active Record's
knowledge of the table structure is current before manipulating data in those
records.
7 Schema Dumping and You
7.1 What are Schema Files for?
Migrations, mighty as they may be, are not the authoritative source for your
database schema. That role falls to either db/schema.rb
or an SQL file which
Active Record generates by examining the database. They are not designed to be
edited, they just represent the current state of the database.
There is no need (and it is error prone) to deploy a new instance of an app by replaying the entire migration history. It is much simpler and faster to just load into the database a description of the current schema.
For example, this is how the test database is created: the current development
database is dumped (either to db/schema.rb
or db/structure.sql
) and then
loaded into the test database.
Schema files are also useful if you want a quick look at what attributes an Active Record object has. This information is not in the model's code and is frequently spread across several migrations, but the information is nicely summed up in the schema file. The annotate_models gem automatically adds and updates comments at the top of each model summarizing the schema if you desire that functionality.
7.2 Types of Schema Dumps
There are two ways to dump the schema. This is set in config/application.rb
by the config.active_record.schema_format
setting, which may be either :sql
or :ruby
.
If :ruby
is selected then the schema is stored in db/schema.rb
. If you look
at this file you'll find that it looks an awful lot like one very big
migration:
ActiveRecord::Schema.define(version: 20080906171750) do create_table "authors", force: true do |t| t.string "name" t.datetime "created_at" t.datetime "updated_at" end create_table "products", force: true do |t| t.string "name" t.text "description" t.datetime "created_at" t.datetime "updated_at" t.string "part_number" end end
In many ways this is exactly what it is. This file is created by inspecting the
database and expressing its structure using create_table
, add_index
, and so
on. Because this is database-independent, it could be loaded into any database
that Active Record supports. This could be very useful if you were to
distribute an application that is able to run against multiple databases.
There is however a trade-off: db/schema.rb
cannot express database specific
items such as foreign key constraints, triggers, or stored procedures. While in
a migration you can execute custom SQL statements, the schema dumper cannot
reconstitute those statements from the database. If you are using features like
this, then you should set the schema format to :sql
.
Instead of using Active Record's schema dumper, the database's structure will
be dumped using a tool specific to the database (via the db:structure:dump
Rake task) into db/structure.sql
. For example, for PostgreSQL, the pg_dump
utility is used. For MySQL, this file will contain the output of SHOW CREATE
TABLE
for the various tables.
Loading these schemas is simply a question of executing the SQL statements they
contain. By definition, this will create a perfect copy of the database's
structure. Using the :sql
schema format will, however, prevent loading the
schema into a RDBMS other than the one used to create it.
7.3 Schema Dumps and Source Control
Because schema dumps are the authoritative source for your database schema, it is strongly recommended that you check them into source control.
8 Active Record and Referential Integrity
The Active Record way claims that intelligence belongs in your models, not in the database. As such, features such as triggers or foreign key constraints, which push some of that intelligence back into the database, are not heavily used.
Validations such as validates :foreign_key, uniqueness: true
are one way in
which models can enforce data integrity. The :dependent
option on
associations allows models to automatically destroy child objects when the
parent is destroyed. Like anything which operates at the application level,
these cannot guarantee referential integrity and so some people augment them
with foreign key constraints in the database.
Although Active Record does not provide any tools for working directly with
such features, the execute
method can be used to execute arbitrary SQL. You
could also use some gem like
foreigner which add foreign key
support to Active Record (including support for dumping foreign keys in
db/schema.rb
).
9 Migrations and Seed Data
Some people use migrations to add data to the database:
class AddInitialProducts < ActiveRecord::Migration def up 5.times do |i| Product.create(name: "Product ##{i}", description: "A product.") end end def down Product.delete_all end end
However, Rails has a 'seeds' feature that should be used for seeding a database
with initial data. It's a really simple feature: just fill up db/seeds.rb
with some Ruby code, and run rake db:seed
:
5.times do |i| Product.create(name: "Product ##{i}", description: "A product.") end
This is generally a much cleaner way to set up the database of a blank application.
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