Always define dependent on AR relations

[jandudulski]

It is often a case that definition of dependent wasn't added because someone just forget about that. We should always do:

has_many :objects, dependent: :destroy # when the object is destroyed, destroy will be called on its associated objects
has_many :object, dependent: :delete_all # when the object is destroyed, all its associated objects will be deleted directly from the database without calling their destroy method (for has_one use :delete)
has_many :objects, dependent: :nullify # causes the foreign key to be set to NULL. Callbacks are not executed
has_many :objects, dependent: :restrict_with_exception # causes an exception to be raised if there is an associated record
has_many :objects, dependent: :restrict_with_error # causes an error to be added to the owner if there is an associated object

You shouldn't need to use dependent on belongs_to

[jandudulski]

And actually I don't like nullify but it is better to have it explicitly than expect something which doesn't match implicit option.