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Diffstat (limited to 'guides/source/migrations.md')
| -rw-r--r-- | guides/source/migrations.md | 97 |
1 files changed, 51 insertions, 46 deletions
diff --git a/guides/source/migrations.md b/guides/source/migrations.md index 550f8fdc3c..b7283d16cc 100644 --- a/guides/source/migrations.md +++ b/guides/source/migrations.md @@ -184,7 +184,7 @@ 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 @@ -227,7 +227,7 @@ 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 @@ -269,7 +269,7 @@ 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 @@ -301,11 +301,12 @@ braces. You can use the following modifiers: * `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 +* `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} +$ rails generate migration AddDetailsToProducts 'price:decimal{5,2}' supplier:references{polymorphic} ``` will produce a migration that looks like this @@ -313,8 +314,8 @@ 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 + add_column :products, :price, :decimal, precision: 5, scale: 2 + add_reference :products, :supplier, polymorphic: true, index: true end end ``` @@ -344,7 +345,7 @@ 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 +359,7 @@ 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 @@ -366,34 +367,32 @@ 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, +`false` by default. This can be overridden by specifying the `:column_options` +option. ```ruby -create_join_table :products, :categories, table_name: :categorization +create_join_table :products, :categories, column_options: {null: true} ``` -will create a `categorization` table. +will create the `product_id` and `category_id` with the `:null` option as +`true`. -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, +You can pass the option `:table_name` when you want to customize the table +name. For example: ```ruby -create_join_table :products, :categories, column_options: {null: true} +create_join_table :products, :categories, table_name: :categorization ``` -will create the `product_id` and `category_id` with the `:null` option as -`true`. +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 :products - t.index :categories + t.index :product_id + t.index :category_id end ``` @@ -401,7 +400,7 @@ 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| @@ -448,7 +447,7 @@ definitions: * `create_table` * `create_join_table` * `drop_table` (must supply a block) -* `drop_join_table` (must supply a block) +* `drop_join_table` (must supply a block) * `remove_timestamps` * `rename_column` * `rename_index` @@ -465,7 +464,7 @@ 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 @@ -649,7 +648,7 @@ 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 @@ -666,7 +665,7 @@ 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 @@ -684,7 +683,7 @@ 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 @@ -694,22 +693,27 @@ 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 @@ -751,7 +755,7 @@ 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 @@ -831,8 +835,7 @@ 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 -column. +initializes it: ```ruby # db/migrate/20100513121110_add_flag_to_product.rb @@ -843,11 +846,12 @@ class AddFlagToProduct < ActiveRecord::Migration reversible do |dir| dir.up { Product.update_all flag: false } end - Product.update_all flag: false end end ``` +She also adds a validation to the `Product` model for the new column: + ```ruby # app/models/product.rb @@ -856,9 +860,8 @@ class Product < ActiveRecord::Base 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. +Alice adds a second migration which adds another column to the `products` +table and initializes it: ```ruby # db/migrate/20100515121110_add_fuzz_to_product.rb @@ -873,11 +876,13 @@ class AddFuzzToProduct < ActiveRecord::Migration end ``` +She also adds a validation to the `Product` model for the new column: + ```ruby # app/models/product.rb class Product < ActiveRecord::Base - validates :flag, inclusion: { in: [true, false] } + validates :flag, inclusion: { in: [true, false] } validates :fuzz, presence: true end ``` @@ -906,7 +911,7 @@ 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.reset_column_information` to refresh the Active Record cache for the `Product` model prior to updating data in the database. If Alice had done this instead, there would have been no problem: @@ -959,7 +964,7 @@ 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. +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 @@ -1035,8 +1040,8 @@ 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 |