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+Migrations
+==========
+
+Migrations are a convenient way for you to alter your database in a structured
+and organized manner. You could edit fragments of SQL by hand but you would then
+be responsible for telling other developers that they need to go and run them.
+You'd also have to keep track of which changes need to be run against the
+production machines next time you deploy.
+
+Active Record tracks which migrations have already been run so all you have to
+do is update your source and run `rake db:migrate`. Active Record will work out
+which migrations should be run. Active Record will also update your `db/schema.rb` file to match the up-to-date structure of your database.
+
+Migrations also allow you to describe these transformations using Ruby. The
+great thing about this is that (like most of Active Record's functionality) it
+is database independent: you don't need to worry about the precise syntax of
+`CREATE TABLE` any more than you worry about variations on `SELECT *` (you can
+drop down to raw SQL for database specific features). For example, you could use
+SQLite3 in development, but MySQL in production.
+
+In this guide, you'll learn all about migrations including:
+
+* The generators you can use to create them
+* The methods Active Record provides to manipulate your database
+* The Rake tasks that manipulate them
+* How they relate to `schema.rb`
+
+--------------------------------------------------------------------------------
+
+Anatomy of a Migration
+----------------------
+
+Before we dive into the details of a migration, here are a few examples of the
+sorts of things you can do:
+
+```ruby
+class CreateProducts < ActiveRecord::Migration
+  def up
+    create_table :products do |t|
+      t.string :name
+      t.text :description
+
+      t.timestamps
+    end
+  end
+
+  def down
+    drop_table :products
+  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, however since this is the default we do not need to explicitly specify it.
+The timestamp columns `created_at` and `updated_at` which Active Record
+populates automatically will also be added. Reversing this migration is as
+simple as dropping the table.
+
+Migrations are not limited to changing the schema. You can also use them to fix
+bad data in the database or populate new fields:
+
+```ruby
+class AddReceiveNewsletterToUsers < ActiveRecord::Migration
+  def up
+    change_table :users do |t|
+      t.boolean :receive_newsletter, :default => false
+    end
+    User.update_all :receive_newsletter => true
+  end
+
+  def down
+    remove_column :users, :receive_newsletter
+  end
+end
+```
+
+NOTE: Some [caveats](#using-models-in-your-migrations) apply to using models in
+your migrations.
+
+This migration adds a `receive_newsletter` column to the `users` table. We want
+it to default to `false` for new users, but existing users are considered to
+have already opted in, so we use the User model to set the flag to `true` for
+existing users.
+
+### Using the change method
+
+Rails 3.1 and up makes migrations smarter by providing a `change` method.
+This method is preferred for writing constructive migrations (adding columns or
+tables). The migration knows how to migrate your database and reverse it when
+the migration is rolled back without the need to write a separate `down` method.
+
+```ruby
+class CreateProducts < ActiveRecord::Migration
+  def change
+    create_table :products do |t|
+      t.string :name
+      t.text :description
+
+      t.timestamps
+    end
+  end
+end
+```
+
+### Migrations are Classes
+
+A migration is a subclass of `ActiveRecord::Migration` that implements
+two methods: `up` (perform the required transformations) and `down` (revert
+them).
+
+Active Record provides methods that perform common data definition tasks in a
+database independent way (you'll read about them in detail later):
+
+* `add_column`
+* `add_reference`
+* `add_index`
+* `change_column`
+* `change_table`
+* `create_table`
+* `create_join_table`
+* `drop_table`
+* `remove_column`
+* `remove_index`
+* `rename_column`
+* `remove_reference`
+
+If you need to perform tasks specific to your database (e.g., create a
+[foreign key](#active-record-and-referential-integrity) constraint) then the
+`execute` method allows you to execute arbitrary SQL. A migration is just a
+regular Ruby class so you're not limited to these functions. For example, after
+adding a column you could write code to set the value of that column for
+existing records (if necessary using your models).
+
+On databases that support transactions with statements that change the schema
+(such as PostgreSQL or SQLite3), migrations are wrapped in a transaction. If the
+database does not support this (for example MySQL) 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.
+
+### What's in a Name
+
+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`. If you do feel the need to change the file name then you
+<em>have to</em> update the name of the class inside or Rails will complain
+about a missing class.
+
+Internally Rails only uses the migration's number (the timestamp) to identify
+them. Prior to Rails 2.1 the migration number started at 1 and was incremented
+each time a migration was generated. With multiple developers it was easy for
+these to clash requiring you to rollback migrations and renumber them. With
+Rails 2.1+ this is largely avoided by using the creation time of the migration
+to identify them. You can revert to the old numbering scheme by adding the
+following line to `config/application.rb`.
+
+```ruby
+config.active_record.timestamped_migrations = false
+```
+
+The combination of timestamps and recording which migrations have been run
+allows Rails to handle common situations that occur with multiple developers.
+
+For example, Alice adds migrations `20080906120000` and `20080906123000` and Bob
+adds `20080906124500` and runs it. Alice finishes her changes and checks in her
+migrations and Bob pulls down the latest changes. When Bob runs `rake db:migrate`,
+Rails knows that it has not run Alice's two migrations so it executes the `up` method for each migration.
+
+Of course this is no substitution for communication within the team. For
+example, if Alice's migration removed a table that Bob's migration assumed to
+exist, then trouble would certainly strike.
+
+### Changing 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.
+
+### Supported Types
+
+Active Record supports the following database column types:
+
+* `:binary`
+* `:boolean`
+* `:date`
+* `:datetime`
+* `:decimal`
+* `:float`
+* `:integer`
+* `:primary_key`
+* `:string`
+* `:text`
+* `:time`
+* `:timestamp`
+
+These will be mapped onto an appropriate underlying database type. For example,
+with MySQL the type `:string` is mapped to `VARCHAR(255)`. You can create
+columns of types not supported by Active Record when using the non-sexy syntax such as
+
+```ruby
+create_table :products do |t|
+  t.column :name, 'polygon', :null => false
+end
+```
+
+This may however hinder portability to other databases.
+
+Creating a Migration
+--------------------
+
+### Creating a Model
+
+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
+$ rails generate model Product name:string description:text
+```
+
+TIP: All lines starting with a dollar sign `$` are intended to be run on the command line.
+
+will create a migration that looks like this
+
+```ruby
+class CreateProducts < ActiveRecord::Migration
+  def change
+    create_table :products do |t|
+      t.string :name
+      t.text :description
+
+      t.timestamps
+    end
+  end
+end
+```
+
+You can append as many column name/type pairs as you want. By default, the
+generated migration will include `t.timestamps` (which creates the
+`updated_at` and `created_at` columns that are automatically populated
+by Active Record).
+
+### Creating a Standalone Migration
+
+If you are creating migrations for other purposes (e.g., to add a column
+to an existing table) then you can also use the migration generator:
+
+```bash
+$ 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
+$ rails generate migration AddPartNumberToProducts part_number:string
+```
+
+will generate
+
+```ruby
+class AddPartNumberToProducts < ActiveRecord::Migration
+  def change
+    add_column :products, :part_number, :string
+  end
+end
+```
+
+Similarly,
+
+```bash
+$ rails generate migration RemovePartNumberFromProducts part_number:string
+```
+
+generates
+
+```ruby
+class RemovePartNumberFromProducts < ActiveRecord::Migration
+  def up
+    remove_column :products, :part_number
+  end
+
+  def down
+    add_column :products, :part_number, :string
+  end
+end
+```
+
+You are not limited to one magically generated column. For example
+
+```bash
+$ 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
+```
+
+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.
+
+NOTE: The generated migration file for destructive migrations will still be
+old-style using the `up` and `down` methods. This is because Rails needs to know
+the original data types defined when you made the original changes.
+
+Also, the generator accepts column type as `references`(also available as `belongs_to`). For instance
+
+```bash
+$ 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.
+
+### 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
+* `scale`        Defines the scale for the `decimal` fields
+* `polymorphic`  Adds a `type` column for `belongs_to` associations
+
+For instance, running
+
+```bash
+$ 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, :precision => 5, :scale => 2
+    add_reference :products, :user, :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
+
+Migration method `create_table` will be one of your workhorses. 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).
+
+The object yielded to the block allows you to create columns on the table. There
+are two ways of doing it. The first (traditional) form looks like
+
+```ruby
+create_table :products do |t|
+  t.column :name, :string, :null => false
+end
+```
+
+The second form, the so called "sexy" migration, drops the somewhat redundant
+`column` method. Instead, the `string`, `integer`, etc. methods create a column
+of that type. Subsequent parameters are the same.
+
+```ruby
+create_table :products do |t|
+  t.string :name, :null => false
+end
+```
+
+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 (for
+example for a HABTM join table), 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.
+
+You can pass the option `:table_name` with you want to customize the table name. For example,
+
+```ruby
+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,
+
+```ruby
+create_join_table :products, :categories, :column_options => {:null => true}
+```
+
+will create the `product_id` and `category_id` with the `:null` option as `true`.
+
+### 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.
+
+### Special Helpers
+
+Active Record provides some shortcuts for common functionality. It is for
+example very common to add both the `created_at` and `updated_at` columns and so
+there is a method that does exactly that:
+
+```ruby
+create_table :products do |t|
+  t.timestamps
+end
+```
+
+will create a new products table with those two columns (plus the `id` column)
+whereas
+
+```ruby
+change_table :products do |t|
+  t.timestamps
+end
+```
+adds those columns to an existing table.
+
+Another helper is called `references` (also available as `belongs_to`). In its
+simplest form it just adds some readability.
+
+```ruby
+create_table :products do |t|
+  t.references :category
+end
+```
+
+will create a `category_id` column of the appropriate type. Note that you pass
+the model name, not the column name. Active Record adds the `_id` for you. If
+you have polymorphic `belongs_to` associations then `references` will add both
+of the columns required:
+
+```ruby
+create_table :products do |t|
+  t.references :attachment, :polymorphic => {:default => 'Photo'}
+end
+```
+
+will add an `attachment_id` column and a string `attachment_type` column with
+a default value of 'Photo'. `references` also allows you to define an
+index directly, instead of using `add_index` after the `create_table` call:
+
+```ruby
+create_table :products do |t|
+  t.references :category, :index => true
+end
+```
+
+will create an index identical to calling `add_index :products, :category_id`.
+
+NOTE: The `references` helper does not actually create foreign key constraints
+for you. You will need to use `execute` or a plugin that adds [foreign key
+support](#active-record-and-referential-integrity).
+
+If the helpers provided by Active Record aren't enough you can use the `execute`
+method to execute arbitrary SQL.
+
+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 `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 removes the need to write both `up` and `down` methods in
+those cases that Rails knows how to revert the changes automatically. Currently,
+the `change` method supports only these migration definitions:
+
+* `add_column`
+* `add_index`
+* `add_timestamps`
+* `create_table`
+* `remove_timestamps`
+* `rename_column`
+* `rename_index`
+* `rename_table`
+
+If you're going to need to use any other methods, you'll have to write the
+`up` and `down` methods instead of using the `change` method.
+
+### Using the `up`/`down` Methods
+
+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. For example,
+
+```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
+```
+
+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` from your `down` method. If someone tries
+to revert your migration, an error message will be displayed saying that it
+can't be done.
+
+Running Migrations
+------------------
+
+Rails provides a set of rake tasks to work with migrations which boil down to
+running 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 `up` or `change`
+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
+(up, down or change) 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
+$ rake db:migrate VERSION=20080906120000
+```
+
+If version 20080906120000 is greater than the current version (i.e., it is
+migrating upwards), this will run the `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
+$ rake db:rollback
+```
+
+This will run the `down` method from the latest migration. If you need to undo
+several migrations you can provide a `STEP` parameter:
+
+```bash
+$ rake db:rollback STEP=3
+```
+
+will run the `down` method from 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
+$ 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.
+
+### Resetting the Database
+
+The `rake db:reset` task will drop the database, recreate it and load the
+current schema into it.
+
+NOTE: This is not the same as running all the migrations - see the section on
+[schema.rb](#schema-dumping-and-you).
+
+### 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 `up` or `down` method invoked, for
+example,
+
+```bash
+$ rake db:migrate:up VERSION=20080906120000
+```
+
+will run the `up` method from the 20080906120000 migration. 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
+$ 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.
+
+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.  She also adds a validation to the `Product` model for the new
+column.
+
+```ruby
+# db/migrate/20100513121110_add_flag_to_product.rb
+
+class AddFlagToProduct < ActiveRecord::Migration
+  def change
+    add_column :products, :flag, :boolean
+    Product.update_all :flag => false
+  end
+end
+```
+
+```ruby
+# app/model/product.rb
+
+class Product < ActiveRecord::Base
+  validates :flag, :presence => true
+end
+```
+
+Alice adds a second migration which adds and initializes another column to the
+`products` table and also adds a validation to the `Product` model for the new
+column.
+
+```ruby
+# db/migrate/20100515121110_add_fuzz_to_product.rb
+
+class AddFuzzToProduct < ActiveRecord::Migration
+  def change
+    add_column :products, :fuzz, :string
+    Product.update_all :fuzz => 'fuzzy'
+  end
+end
+```
+
+```ruby
+# app/model/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 the `Product` 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 faux model, it's a good idea to call
+`Product.reset_column_information` to refresh the `ActiveRecord` cache for the
+`Product` model prior to updating data in the database.
+
+If Alice had done this instead, there would have been no problem:
+
+```ruby
+# 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
+    Product.update_all :flag => false
+  end
+end
+```
+
+```ruby
+# 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
+    Product.update_all :fuzz => 'fuzzy'
+  end
+end
+```
+
+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 the PostgreSQL RDBMS, 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.
+
+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 plugin like [foreigner](https://github.com/matthuhiggins/foreigner)
+which add foreign key support to Active Record (including support for dumping
+foreign keys in `db/schema.rb`).