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diff --git a/guides/source/migrations.md b/guides/source/migrations.md index 035f9499de..6742c05946 100644 --- a/guides/source/migrations.md +++ b/guides/source/migrations.md @@ -18,9 +18,10 @@ After reading this guide, you will know: Migration Overview ------------------ -Migrations are a convenient way to alter your database schema over time in a -consistent and easy way. They use a Ruby DSL so that you don't have to write -SQL by hand, allowing your schema and changes to be database independent. +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 @@ -120,7 +121,7 @@ Of course, calculating timestamps is no fun, so Active Record provides a generator to handle making it for you: ```bash -$ rails generate migration AddPartNumberToProducts +$ bin/rails generate migration AddPartNumberToProducts ``` This will create an empty but appropriately named migration: @@ -137,7 +138,7 @@ 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 +$ bin/rails generate migration AddPartNumberToProducts part_number:string ``` will generate @@ -153,7 +154,7 @@ end If you'd like to add an index on the new column, you can do that as well: ```bash -$ rails generate migration AddPartNumberToProducts part_number:string:index +$ bin/rails generate migration AddPartNumberToProducts part_number:string:index ``` will generate @@ -171,7 +172,7 @@ end Similarly, you can generate a migration to remove a column from the command line: ```bash -$ rails generate migration RemovePartNumberFromProducts part_number:string +$ bin/rails generate migration RemovePartNumberFromProducts part_number:string ``` generates @@ -184,10 +185,10 @@ class RemovePartNumberFromProducts < ActiveRecord::Migration end ``` -You are not limited to one magically generated column. For example +You are not limited to one magically generated column. For example: ```bash -$ rails generate migration AddDetailsToProducts part_number:string price:decimal +$ bin/rails generate migration AddDetailsToProducts part_number:string price:decimal ``` generates @@ -206,7 +207,7 @@ 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 -$ rails generate migration CreateProducts name:string part_number:string +$ bin/rails generate migration CreateProducts name:string part_number:string ``` generates @@ -227,10 +228,10 @@ 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 +`belongs_to`). For instance: ```bash -$ rails generate migration AddUserRefToProducts user:references +$ bin/rails generate migration AddUserRefToProducts user:references ``` generates @@ -248,7 +249,7 @@ 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 -rails g migration CreateJoinTableCustomerProduct customer product +$ bin/rails g migration CreateJoinTableCustomerProduct customer product ``` will produce the following migration: @@ -269,10 +270,10 @@ end 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 +adding these columns will also be created. For example, running: ```bash -$ rails generate model Product name:string description:text +$ bin/rails generate model Product name:string description:text ``` will create a migration that looks like this @@ -297,15 +298,16 @@ You can append as many column name/type pairs as you want. 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 +* `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. -For instance, running +For instance, running: ```bash -$ rails generate migration AddDetailsToProducts price:decimal{5,2} supplier:references{polymorphic} +$ bin/rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic} ``` will produce a migration that looks like this @@ -313,7 +315,7 @@ will produce a migration that looks like this ```ruby class AddDetailsToProducts < ActiveRecord::Migration def change - add_column :products, :price, precision: 5, scale: 2 + add_column :products, :price, :decimal, precision: 5, scale: 2 add_reference :products, :supplier, polymorphic: true, index: true end end @@ -344,7 +346,7 @@ 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, +you can place an SQL fragment in the `:options` option. For example: ```ruby create_table :products, options: "ENGINE=BLACKHOLE" do |t| @@ -358,7 +360,7 @@ will append `ENGINE=BLACKHOLE` to the SQL statement used to create the table ### Creating a Join Table Migration method `create_join_table` creates a HABTM join table. A typical use -would be +would be: ```ruby create_join_table :products, :categories @@ -376,8 +378,8 @@ 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` with you want to customize the table -name. For example, +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 @@ -399,7 +401,7 @@ end 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 +yielded to the block knows more tricks. For example: ```ruby change_table :products do |t| @@ -419,7 +421,7 @@ If the helpers provided by Active Record aren't enough you can use the `execute` method to execute arbitrary SQL: ```ruby -Products.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1') +Product.connection.execute('UPDATE `products` SET `price`=`free` WHERE 1') ``` For more details and examples of individual methods, check the API documentation. @@ -463,7 +465,7 @@ or write the `up` and `down` methods instead of using the `change` method. 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, +migration what else to do when reverting it. For example: ```ruby class ExampleMigration < ActiveRecord::Migration @@ -493,6 +495,7 @@ class ExampleMigration < ActiveRecord::Migration 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 @@ -641,16 +644,16 @@ 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 +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 +to version 20080906120000 run: ```bash -$ rake db:migrate VERSION=20080906120000 +$ bin/rake db:migrate VERSION=20080906120000 ``` If version 20080906120000 is greater than the current version (i.e., it is @@ -664,10 +667,10 @@ down to, but not including, 20080906120000. 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 +number associated with the previous migration you can run: ```bash -$ rake db:rollback +$ bin/rake db:rollback ``` This will rollback the latest migration, either by reverting the `change` @@ -675,42 +678,47 @@ method or by running the `down` method. If you need to undo several migrations you can provide a `STEP` parameter: ```bash -$ rake db:rollback STEP=3 +$ 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 +if you need to go more than one version back, for example: ```bash -$ rake db:migrate:redo STEP=3 +$ 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, recreate it and load the -current schema into it. +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).' +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, +invoked, for example: ```bash -$ rake db:migrate:up VERSION=20080906120000 +$ bin/rake db:migrate:up VERSION=20080906120000 ``` will run the 20080906120000 migration by running the `change` method (or the @@ -726,7 +734,7 @@ To run migrations against another environment you can specify it using the migrations against the `test` environment you could run: ```bash -$ rake db:migrate RAILS_ENV=test +$ bin/rake db:migrate RAILS_ENV=test ``` ### Changing the Output of Running Migrations @@ -749,7 +757,7 @@ Several methods are provided in migrations that allow you to control all this: | 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 +For example, this migration: ```ruby class CreateProducts < ActiveRecord::Migration @@ -812,158 +820,6 @@ 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). -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 - reversible do |dir| - dir.up { Product.update_all flag: false } - end - Product.update_all flag: false - end -end -``` - -```ruby -# app/models/product.rb - -class Product < ActiveRecord::Base - validates :flag, inclusion: { in: [true, false] } -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 - reversible do |dir| - dir.up { Product.update_all fuzz: 'fuzzy' } - end - end -end -``` - -```ruby -# app/models/product.rb - -class Product < ActiveRecord::Base - validates :flag, inclusion: { in: [true, false] } - validates :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 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. - -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 - reversible do |dir| - dir.up { Product.update_all flag: false } - end - 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 - reversible do |dir| - dir.up { Product.update_all fuzz: 'fuzzy' } - end - end -end -``` - -There are other ways in which the above example could have gone badly. - -For example, imagine that Alice creates a migration that selectively -updates the `description` field on certain products. She runs the -migration, commits the code, and then begins working on the next feature, -which is to add a new column `fuzz` to the products table. - -She creates two migrations for this new feature, one which adds the new -column, and a second which selectively updates the `fuzz` column based on -other product attributes. - -These migrations run just fine, but when Bob comes back from his vacation -and calls `rake db:migrate` to run all the outstanding migrations, he gets a -subtle bug: The descriptions have defaults, and the `fuzz` column is present, -but `fuzz` is nil on all products. - -The solution is again to use `Product.reset_column_information` before -referencing the Product model in a migration, ensuring the Active Record's -knowledge of the table structure is current before manipulating data in those -records. - Schema Dumping and You ---------------------- @@ -1033,8 +889,8 @@ 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. +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 @@ -1046,6 +902,11 @@ schema into a RDBMS other than the one used to create it. 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 --------------------------------------- |