Question

Our Rails 3 application has models Person and Message. Messages can be specific to a Person (when the message person_id column is set) or they can be “global” (when the person_id column is NULL).

We would like to have a simple has_many relationship using the :conditions option as such:

class Person < ActiveRecord::Base
  has_many :messages,
      :conditions => proc { ['(messages.person_id IS NULL) OR ' +
                             '(messages.person_id = ?)'], self.id }
  # ...
end

But it appears that the has_many class method encodes the “conditions” option as a logical “AND” clause after enforcing the foreign key constraint of equality to the Person object’s id (e.g. “FROM messages WHERE person_id=123 AND (person_id IS NULL OR person_id=123)“). It appears that there is no way to allow associated objects with null foreign keys to belong to such associations.

Does Rails 3 / ActiveRecord provide a way to do this or must I hack my own association-like methods?

Answer 1

You can’t have an OR clause like you want using conditions on the ActiveRecord assocation. You could have the association without conditions and then add a method to include the global messages you want. That way you could still take advantage of the association when building associated records.

# person.rb
has_many :messages

def all_messages
  Message.where('messages.person_id=? OR messages.person_id IS NULL', id)
end

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Here’s my standard plug for the Squeel gem, which is handy for more advanced queries like this if you don’t want to have bits of SQL in your code. Check it out.