How do you declare an optional condition for an ActiveRecord belongs_to association?

Question

Rails ActiveRecord provides an optional option for belongs_to. Consider the use case of allowing null for the foreign key and allowing the association to be null during object creation but requiring its presence during subsequent saves. For example, a new Member may have no initial Group, but any further updates of Member require a Group association.

Can the optional option value itself be conditional? For example,

class Member < ApplicationRecord
  belongs_to :group, optional: -> { new_record? }
end

behaves the same as optional: true, and we can infer that the optional option parsing only checks for a truthy value.

Is a custom validator the pragmatic way to meet this use case?

Answer 1

It looks like providing a lambda to the optional option won't work (although I haven't tried it). I looked at the source code and this is how optional is used.

required = !reflection.options[:optional]

If required, Rails just adds a presence validation like this:

model.validates_presence_of reflection.name, message: :required

I believe you could go the custom route with something like this:

class Member < ApplicationRecord
  belongs_to :group, optional: true
  validates :group, presence: true, on: :update
end