From a5b3f372ab30e043d25b25b05e603e6ed33c0ee9 Mon Sep 17 00:00:00 2001 From: Yves Senn Date: Thu, 12 Jun 2014 08:42:00 +0200 Subject: fk: add docs --- guides/source/active_record_migrations.md | 129 ++++++++++++++++-------------- 1 file changed, 71 insertions(+), 58 deletions(-) (limited to 'guides/source/active_record_migrations.md') diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md index fd2125424b..1b0ae3ec7e 100644 --- a/guides/source/active_record_migrations.md +++ b/guides/source/active_record_migrations.md @@ -452,6 +452,40 @@ Column modifiers can be applied when creating or changing a column: Some adapters may support additional options; see the adapter specific API docs for further information. +### Foreign Keys + +While it's not required you might want to add foreign key constraints to +[guarantee referential integrity](#active-record-and-referential-integrity). + +```ruby +add_foreign_key :articles, :authors +``` + +This adds a new foreign key to the `author_id` column of the `articles` +table. The key references the `id` column of the `articles` table. If the +column names can not be derived from the table names, you can use the +`:column` and `:primary_key` options. + +Rails will generate a name for every foreign key starting with +`fk_rails_` followed by 10 random characters. +There is a `:name` option to specify a different name if needed. + +NOTE: Active Record only supports single column foreign keys. `execute` and +`structure.sql` are required to use composite foreign keys. + +Removing a foreign key is easy as well: + +```ruby +# let Active Record figure out the column name +remove_foreign_key :accounts, :branches + +# remove foreign key for a specific column +remove_foreign_key :accounts, column: :owner_id + +# remove foreign key by name +remove_foreign_key :accounts, name: :special_fk_name +``` + ### When Helpers aren't Enough If the helpers provided by Active Record aren't enough you can use the `execute` @@ -482,6 +516,7 @@ definitions: * `add_index` * `add_reference` * `add_timestamps` +* `add_foreign_key` * `create_table` * `create_join_table` * `drop_table` (must supply a block) @@ -507,24 +542,23 @@ migration what else to do when reverting it. For example: ```ruby class ExampleMigration < ActiveRecord::Migration def change - create_table :products do |t| - t.references :category + create_table :distributors do |t| + t.string :zipcode end reversible do |dir| dir.up do - #add a foreign key + # add a CHECK constraint execute <<-SQL - ALTER TABLE products - ADD CONSTRAINT fk_products_categories - FOREIGN KEY (category_id) - REFERENCES categories(id) + ALTER TABLE distributors + ADD CONSTRAINT zipchk + CHECK (char_length(zipcode) = 5) NO INHERIT; SQL end dir.down do execute <<-SQL - ALTER TABLE products - DROP FOREIGN KEY fk_products_categories + ALTER TABLE distributors + DROP CONSTRAINT zipchk SQL end end @@ -538,7 +572,7 @@ end Using `reversible` will ensure that the instructions are executed in the right order too. If the previous example migration is reverted, the `down` block will be run after the `home_page_url` column is removed and -right before the table `products` is dropped. +right before the table `distributors` is dropped. Sometimes your migration will do something which is just plain irreversible; for example, it might destroy some data. In such cases, you can raise @@ -561,16 +595,15 @@ made in the `up` method. The example in the `reversible` section is equivalent t ```ruby class ExampleMigration < ActiveRecord::Migration def up - create_table :products do |t| - t.references :category + create_table :distributors do |t| + t.string :zipcode end - # add a foreign key + # add a CHECK constraint execute <<-SQL - ALTER TABLE products - ADD CONSTRAINT fk_products_categories - FOREIGN KEY (category_id) - REFERENCES categories(id) + ALTER TABLE distributors + ADD CONSTRAINT zipchk + CHECK (char_length(zipcode) = 5); SQL add_column :users, :home_page_url, :string @@ -582,11 +615,11 @@ class ExampleMigration < ActiveRecord::Migration remove_column :users, :home_page_url execute <<-SQL - ALTER TABLE products - DROP FOREIGN KEY fk_products_categories + ALTER TABLE distributors + DROP CONSTRAINT zipchk SQL - drop_table :products + drop_table :distributors end end ``` @@ -617,43 +650,27 @@ end The `revert` method also accepts a block of instructions to reverse. This could be useful to revert selected parts of previous migrations. For example, let's imagine that `ExampleMigration` is committed and it -is later decided it would be best to serialize the product list instead. -One could write: +is later decided it would be best to use Active Record validations, +in place of the `CHECK` constraint, to verify the zipcode. ```ruby -class SerializeProductListMigration < ActiveRecord::Migration +class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration def change - add_column :categories, :product_list - - reversible do |dir| - dir.up do - # transfer data from Products to Category#product_list - end - dir.down do - # create Products from Category#product_list - end - end - revert do # copy-pasted code from ExampleMigration - create_table :products do |t| - t.references :category - end - reversible do |dir| dir.up do - #add a foreign key + # add a CHECK constraint execute <<-SQL - ALTER TABLE products - ADD CONSTRAINT fk_products_categories - FOREIGN KEY (category_id) - REFERENCES categories(id) + ALTER TABLE distributors + ADD CONSTRAINT zipchk + CHECK (char_length(zipcode) = 5); SQL end dir.down do execute <<-SQL - ALTER TABLE products - DROP FOREIGN KEY fk_products_categories + ALTER TABLE distributors + DROP CONSTRAINT zipchk SQL end end @@ -918,10 +935,10 @@ 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`. +items such as 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` @@ -948,7 +965,7 @@ 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, +the database. As such, features such as triggers or constraints, which push some of that intelligence back into the database, are not heavily used. @@ -957,14 +974,10 @@ which models can enforce data integrity. The `:dependent` option on associations allows models to automatically destroy child objects when the parent is destroyed. Like anything which operates at the application level, these cannot guarantee referential integrity and so some people augment them -with foreign key constraints in the database. - -Although Active Record does not provide any tools for working directly with -such features, the `execute` method can be used to execute arbitrary SQL. You -can also use a gem like -[foreigner](https://github.com/matthuhiggins/foreigner) which adds foreign key -support to Active Record (including support for dumping foreign keys in -`db/schema.rb`). +with [foreign key constraints](#foreign-keys) in the database. + +Although Active Record does not provide all the tools for working directly with +such features, the `execute` method can be used to execute arbitrary SQL. Migrations and Seed Data ------------------------ -- cgit v1.2.3