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Merge pull request #8375 from steveklabnik/migrations_guide

Updates to the Migration Guide
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2 parents ea53526 + 4a93836 commit c9c1e6b9e640a34b4f2d4caaae052ff61ee2ee7e @fxn fxn committed Dec 3, 2012
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582 guides/source/migrations.md
@@ -15,139 +15,75 @@ After reading this guide, you will know:
--------------------------------------------------------------------------------
-What are Migrations?
---------------------
+Migration Overview
+------------------
-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.
+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.
-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
+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.
-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.
-
-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:
+Here's an example of a migration:
```ruby
class CreateProducts < ActiveRecord::Migration
- def up
+ def change
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.
+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.
-Migrations are not limited to changing the schema. You can also use them to fix
-bad data in the database or populate new fields:
+If you wish for a migration to do something that Active Record doesn't know how
+to reverse, you can use `up` and `down` instead of `change`:
```ruby
-class AddReceiveNewsletterToUsers < ActiveRecord::Migration
+class ChangeProductsPrice < ActiveRecord::Migration
def up
- change_table :users do |t|
- t.boolean :receive_newsletter, default: false
+ change_table :products do |t|
+ t.string :price, null: 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
+ change_table :products do |t|
+ t.integer :price, null: false
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.
+Creating a Migration
+--------------------
-### What's in a Name
+### 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
@@ -157,119 +93,10 @@ 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
+`AddDetailsToProducts`.
-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:
+Of course, calculating timestamps is no fun, so Active Record provides a
+generator to handle making it for you:
```bash
$ rails generate migration AddPartNumberToProducts
@@ -344,10 +171,11 @@ 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.
+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
+Also, the generator accepts column type as `references`(also available as
+`belongs_to`). For instance
```bash
$ rails generate migration AddUserRefToProducts user:references
@@ -363,12 +191,40 @@ class AddUserRefToProducts < ActiveRecord::Migration
end
```
-This migration will create a user_id column and appropriate index.
+This migration will create a `user_id` column and appropriate index.
+
+### 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
+$ 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:
+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
@@ -400,8 +256,9 @@ get to work!
### Creating a Table
-Migration method `create_table` will be one of your workhorses. A typical use
-would be
+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|
@@ -412,31 +269,11 @@ 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,
+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|
@@ -456,31 +293,34 @@ 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.
+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,
+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,
+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`.
+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
+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|
@@ -494,67 +334,15 @@ 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:
+### When Helpers Aren't Enough
-```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:
+If the helpers provided by Active Record aren't enough you can use the `execute`
+method to execute arbitrary SQL:
```ruby
-create_table :products do |t|
- t.references :category, index: true
-end
+Products.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1')
```
-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)
@@ -567,9 +355,10 @@ and
### 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:
+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`
@@ -585,37 +374,42 @@ If you're going to need to use any other methods, you'll have to write the
### 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,
+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. 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
@@ -630,10 +424,9 @@ 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.
+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
+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
@@ -693,9 +486,10 @@ version to migrate to.
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. 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.rb](#schema-dumping-and-you).
+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).'
### Running Specific Migrations
@@ -708,13 +502,16 @@ example,
$ 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.
+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:
+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
@@ -752,9 +549,12 @@ class CreateProducts < ActiveRecord::Migration
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
@@ -778,11 +578,29 @@ generates the following output
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.
+
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
+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)
@@ -795,7 +613,7 @@ 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
+initializes it. She also adds a validation to the `Product` model for the new
column.
```ruby
@@ -844,8 +662,8 @@ 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.
+* 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.
@@ -860,10 +678,10 @@ 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.
+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
+When using a local 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.
@@ -902,20 +720,20 @@ 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
+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.
+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
+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.
+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
+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.
@@ -948,12 +766,13 @@ 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`.
+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:
+at this file you'll find that it looks an awful lot like one very big
+migration:
```ruby
ActiveRecord::Schema.define(version: 20080906171750) do
@@ -976,20 +795,20 @@ 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.
+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.
+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
@@ -1010,14 +829,47 @@ 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`).
+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`).
+
+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.

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