[ci skip] Rename: Rails Database Migration to Active Record Migration.

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-Active Record Migrations
-========================
-
-Migrations are a feature of Active Record that allows you to evolve your
-database schema over time. Rather than write schema modifications in pure SQL,
-migrations allow you to use an easy Ruby DSL to describe changes to your
-tables.
-
-After reading this guide, you will know:
-
-* The generators you can use to create them.
-* The methods Active Record provides to manipulate your database.
-* The Rake tasks that manipulate migrations and your schema.
-* How migrations relate to `schema.rb`.
-
---------------------------------------------------------------------------------
-
-Migration Overview
-------------------
-
-Migrations are a convenient way to
-[alter your database schema over time](http://en.wikipedia.org/wiki/Schema_migration)
-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:
-
-```ruby
-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.
-
-NOTE: 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`:
-
-```ruby
-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`:
-
-```ruby
-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
-```
-
-Creating a Migration
---------------------
-
-### 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:
-
-```bash
-$ bin/rails generate migration AddPartNumberToProducts
-```
-
-This will create an empty but appropriately named migration:
-
-```ruby
-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.
-
-```bash
-$ bin/rails generate migration AddPartNumberToProducts part_number:string
-```
-
-will generate
-
-```ruby
-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:
-
-```bash
-$ bin/rails generate migration AddPartNumberToProducts part_number:string:index
-```
-
-will generate
-
-```ruby
-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:
-
-```bash
-$ bin/rails generate migration RemovePartNumberFromProducts part_number:string
-```
-
-generates
-
-```ruby
-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:
-
-```bash
-$ bin/rails generate migration AddDetailsToProducts part_number:string price:decimal
-```
-
-generates
-
-```ruby
-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:
-
-```bash
-$ bin/rails generate migration CreateProducts name:string part_number:string
-```
-
-generates
-
-```ruby
-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:
-
-```bash
-$ bin/rails generate migration AddUserRefToProducts user:references
-```
-
-generates
-
-```ruby
-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:
-
-```bash
-$ bin/rails g migration CreateJoinTableCustomerProduct customer product
-```
-
-will produce the following migration:
-
-```ruby
-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
-```
-
-### 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:
-
-```bash
-$ bin/rails generate model Product name:string description:text
-```
-
-will create a migration that looks like this
-
-```ruby
-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.
-
-### 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 the `string/text/binary/integer` fields.
-* `precision`    Defines the precision for the `decimal` fields, representing the total number of digits in the number.
-* `scale`        Defines the scale for the `decimal` fields, representing the number of digits after the decimal point.
-* `polymorphic`  Adds a `type` column for `belongs_to` associations.
-* `null`         Allows or disallows `NULL` values in the column.
-* `default`      Allows to set a default value on the column. NOTE: If using a dynamic value (such as date), the default will only be calculated the first time (e.g. on the date the migration is applied.)
-
-NOTE: `null` and `default` cannot be specified via command line.
-
-For instance, running:
-
-```bash
-$ bin/rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic}
-```
-
-will produce a migration that looks like this
-
-```ruby
-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
-```
-
-Writing a Migration
--------------------
-
-Once you have created your migration using one of the generators it's time to
-get to work!
-
-### 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
-
-```ruby
-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:
-
-```ruby
-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`).
-
-### Creating a Join Table
-
-Migration method `create_join_table` creates a HABTM join table. A typical use
-would be:
-
-```ruby
-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. This can be overridden by specifying the `:column_options`
-option.
-
-```ruby
-create_join_table :products, :categories, column_options: {null: true}
-```
-
-will create the `product_id` and `category_id` with the `:null` option as
-`true`.
-
-You can pass the option `:table_name` when you want to customize the table
-name. For example:
-
-```ruby
-create_join_table :products, :categories, table_name: :categorization
-```
-
-will create a `categorization` table.
-
-`create_join_table` also accepts a block, which you can use to add indices
-(which are not created by default) or additional columns:
-
-```ruby
-create_join_table :products, :categories do |t|
-  t.index :product_id
-  t.index :category_id
-end
-```
-
-### 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:
-
-```ruby
-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.
-
-### Changing Columns
-
-Like the `remove_column` and `add_column` Rails provides the `change_column`
-migration method.
-
-```ruby
-change_column :products, :part_number, :text
-```
-
-This changes the column `part_number` on products table to be a `:text` field.
-
-Besides `change_column`, the `change_column_null` and `change_column_default`
-methods are used specifically to change the null and default values of a
-column.
-
-```ruby
-change_column_null :products, :name, false
-change_column_default :products, :approved, false
-```
-
-This sets `:name` field on products to a `NOT NULL` column and the default
-value of the `:approved` field to false.
-
-### 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:
-
-```ruby
-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`](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html)
-(which provides the methods available in the `change`, `up` and `down` methods),
-[`ActiveRecord::ConnectionAdapters::TableDefinition`](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/TableDefinition.html)
-(which provides the methods available on the object yielded by `create_table`)
-and
-[`ActiveRecord::ConnectionAdapters::Table`](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/Table.html)
-(which provides the methods available on the object yielded by `change_table`).
-
-### 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.
-
-### 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:
-
-```ruby
-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
-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.
-
-### 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:
-
-```ruby
-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.
-
-### Reverting Previous Migrations
-
-You can use Active Record's ability to rollback migrations using the `revert` method:
-
-```ruby
-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:
-
-```ruby
-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`.
-
-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` task 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:
-
-```bash
-$ bin/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.
-
-### 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:
-
-```bash
-$ bin/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:
-
-```bash
-$ bin/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:
-
-```bash
-$ bin/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.
-
-### Setup the Database
-
-The `rake db:setup` task will create the database, load the schema and initialize
-it with the seed data.
-
-### Resetting the Database
-
-The `rake db:reset` task will drop the database and set it up again. This is
-functionally equivalent to `rake db:drop db:setup`.
-
-NOTE: 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](#schema-dumping-and-you) section.
-
-### 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:
-
-```bash
-$ bin/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.
-
-### 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:
-
-```bash
-$ bin/rake db:migrate RAILS_ENV=test
-```
-
-### 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
-
-```bash
-==  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:
-
-```ruby
-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
-
-```bash
-==  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.
-
-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](#reverting-previous-migrations) above).
-
-Schema Dumping and You
-----------------------
-
-### 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](https://github.com/ctran/annotate_models) gem automatically
-adds and updates comments at the top of each model summarizing the schema if
-you desire that functionality.
-
-### 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:
-
-```ruby
-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.
-
-### 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.
-
-`db/schema.rb` contains the current version number of the database. This
-ensures conflicts are going to happen in the case of a merge where both
-branches touched the schema. When that happens, solve conflicts manually,
-keeping the highest version number of the two.
-
-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
-can also use a gem like
-[foreigner](https://github.com/matthuhiggins/foreigner) which adds foreign key
-support to Active Record (including support for dumping foreign keys in
-`db/schema.rb`).
-
-Migrations and Seed Data
-------------------------
-
-Some people use migrations to add data to the database:
-
-```ruby
-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`:
-
-```ruby
-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.