64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
|
# Foreign Keys & Associations
|
||
|
|
|
||
|
|
When adding an association to a model you must also add a foreign key. For
|
||
|
|
example, say you have the following model:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
class User < ActiveRecord::Base
|
||
|
|
has_many :posts
|
||
|
|
end
|
||
|
|
```
|
||
|
|
|
||
|
|
Here you will need to add a foreign key on column `posts.user_id`. This ensures
|
||
|
|
that data consistency is enforced on database level. Foreign keys also mean that
|
||
|
|
the database can very quickly remove associated data (e.g. when removing a
|
||
|
|
user), instead of Rails having to do this.
|
||
|
|
|
||
|
|
## Adding Foreign Keys In Migrations
|
||
|
|
|
||
|
|
Foreign keys can be added concurrently using `add_concurrent_foreign_key` as
|
||
|
|
defined in `Gitlab::Database::MigrationHelpers`. See the [Migration Style
|
||
|
|
Guide](migration_style_guide.md) for more information.
|
||
|
|
|
||
|
|
Keep in mind that you can only safely add foreign keys to existing tables after
|
||
|
|
you have removed any orphaned rows. The method `add_concurrent_foreign_key`
|
||
|
|
does not take care of this so you'll need to do so manually.
|
||
|
|
|
||
|
|
## Cascading Deletes
|
||
|
|
|
||
|
|
Every foreign key must define an `ON DELETE` clause, and in 99% of the cases
|
||
|
|
this should be set to `CASCADE`.
|
||
|
|
|
||
|
|
## Indexes
|
||
|
|
|
||
|
|
When adding a foreign key in PostgreSQL the column is not indexed automatically,
|
||
|
|
thus you must also add a concurrent index. Not doing so will result in cascading
|
||
|
|
deletes being very slow.
|
||
|
|
|
||
|
|
## Dependent Removals
|
||
|
|
|
||
|
|
Don't define options such as `dependent: :destroy` or `dependent: :delete` when
|
||
|
|
defining an association. Defining these options means Rails will handle the
|
||
|
|
removal of data, instead of letting the database handle this in the most
|
||
|
|
efficient way possible.
|
||
|
|
|
||
|
|
In other words, this is bad and should be avoided at all costs:
|
||
|
|
|
||
|
|
```ruby
|
||
|
|
class User < ActiveRecord::Base
|
||
|
|
has_many :posts, dependent: :destroy
|
||
|
|
end
|
||
|
|
```
|
||
|
|
|
||
|
|
Should you truly have a need for this it should be approved by a database
|
||
|
|
specialist first.
|
||
|
|
|
||
|
|
You should also not define any `before_destroy` or `after_destroy` callbacks on
|
||
|
|
your models _unless_ absolutely required and only when approved by database
|
||
|
|
specialists. For example, if each row in a table has a corresponding file on a
|
||
|
|
file system it may be tempting to add a `after_destroy` hook. This however
|
||
|
|
introduces non database logic to a model, and means we can no longer rely on
|
||
|
|
foreign keys to remove the data as this would result in the filesystem data
|
||
|
|
being left behind. In such a case you should use a service class instead that
|
||
|
|
takes care of removing non database data.
|