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Léo-Paul Géneau
gitlab-ce
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3d2a3e57
Commit
3d2a3e57
authored
Aug 01, 2018
by
Andreas Brandl
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Docs: FK constraints require an index.
Closes #49789.
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doc/development/migration_style_guide.md
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3d2a3e57
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@@ -182,6 +182,34 @@ class MyMigration < ActiveRecord::Migration
end
```
## Adding foreign-key constraints
When adding a foreign-key constraint to either an existing or new
column remember to also add a index on the column.
This is _required_ if the foreign-key constraint specifies
`ON DELETE CASCADE`
or
`ON DELETE SET NULL`
behavior. On a cascading
delete, the
[
corresponding record needs to be retrieved using an
index
](
https://www.cybertec-postgresql.com/en/postgresql-indexes-and-foreign-keys/
)
(otherwise, we'd need to scan the whole table) for subsequent update or
deletion.
Here's an example where we add a new column with a foreign key
constraint. Note it includes
`index: true`
to create an index for it.
```
ruby
class
Migration
<
ActiveRecord
::
Migration
def
change
add_reference
:model
,
:other_model
,
index:
true
,
foreign_key:
{
on_delete: :cascade
}
end
end
```
When adding a foreign-key constraint to an existing column, we
have to employ
`add_concurrent_foreign_key`
and
`add_concurrent_index`
instead of
`add_reference`
.
## Adding Columns With Default Values
When adding columns with default values you must use the method
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