Question

(Ruby/Rails guru’s preferably :P)

I’ve got a bit of an interesting question. Hope it’s not already answered (wait, yes I do!) but I’ve looked and couldn’t find it. (Hopefully not because it’s not possible)

I’ve got two classes (and I’d like to keep it that way) Group and Event (see below).

class Group < ActiveRecord::Base
    has_and_belongs_to_many :events
end

However in my join table (group_events), I have additional columns which provide extenuating circumstances to the event… I want this information to be available on the event object. (For example, whether attendance is mandatory or not etc)

My second slightly related question is, can I not do the following:

class Event < ActiveRecord::Base
    has_and_belongs_to_many :groups
end

class GroupEvent < Event
    # Implies that a GroupEvent would be an Event, with all the other attributes of the group_events table minus the
    # two id's (group_id, event_id) that allowed for its existence (those would just be references to the group and event object)
end

Answer 1

I would first rewrite the has_and_belongs_to_many, explicitly describing the relationship between Event and the model for GroupEvent.

class Group < ActiveRecord::Base
  has_many :group_events
  has_many :events, :through => :group_events
end

class Event < ActiveRecord::Base
  has_many :group_events
  has_many :groups, :through => :group_events
end

class GroupEvent < ActiveRecord::Base
  belongs_to :group
  belongs_to :event
end

You can then methods within the Event class to refer to the GroupEvent attributes your’e after. For some boolean :attendance_mandatory attribute in GroupEvent, you could do something like

class Event < ActiveRecord::Base
  has_many :group_events
  has_many :groups, :through => :group_events

  def attendance_mandatory?(group)
    group_events.find(group.id).attendance_mandatory?
  end
end

With some Event as e and associated Group as g, you could now do

e.attentdance_mandatory?(g)

As for your 2nd question, I think you’re looking for a piece of what I’ve posted in the first code block above.

class GroupEvent < ActiveRecord::Base
  belongs_to :group
  belongs_to :event
end

Every table containing data you wish to interact with should have a representative model in your application. The above meets your stated criteria (exposes the attributes of a GroupEvent).

Note: Your syntax of class GroupEvent < Event is used for Single Table Inheritance (you’d move the attributes like attendance_mandatory into the events table, and use that events table for both a regular Event and a GroupEvent – though this is outside of the scope of this question)