Defines callbacks for handling dependencies (associations).

Such callbacks are typically declared via the has_many/3 macro in your model’s schema block. For example:

has_many :comments, MyApp.Comment, on_delete: :fetch_and_delete

`:on_delete` options

There are four different behaviors you can set for your associations when the parent is deleted:

• :nothing - Does nothing to the association;

• :delete_all - Deletes all associations without triggering lifecycle callbacks;

• :nilify_all - Sets model reference to nil for each association without triggering any lifecycle callback;

• :fetch_and_delete - Explicitly fetch all associations and delete them one by one, triggering any before_delete and after_delete callbacks;

Keep in mind these options are only available for has_many/3 macros.

Alternatives

Ecto also provides an :on_delete option when using references/2 in migrations. This allows you to set what to perform when an entry is deleted in your schema effectively at the database level. Relying on the database is often the safest way to perform this operation and should be preferred.

However using the :on_delete option may be more flexible specially if you have logic that needs to be expressed on the application side or if your database does not support references.