Question

There are Things in Places which I’m looking to find. One Thing could be in many different Places, and many Things can be in one Place.

class Thing < ActiveRecord::Base
  has_and_belongs_to_many :places
end

class Place < ActiveRecord::Base
  has_and_belongs_to_many :things
end

I want to record the Finds of my Users so that I know where they found what.

class Find < ActiveRecord::Base
  belongs_to :user
  belongs_to :places_thing # Is this depluralization correct?
end

class User < ActiveRecord::Base
  has_many :finds
  # Now, how can I link in the Things the user has found? Like this?
  has_many :found_things_in_places, :class_name => :places_things, :through => :finds
  has_many :things, :through => :thought_things_in_places
end

Does this seem right? is it efficient? Thanks.

Answer 1

I think you were on the right track, the big change I’d make is that rather than having a join table (places_things) you should make it a proper model. I decided to call this an existence.

The data only exists in one place, so it’s properly normalized. These relationships are clear and will be easy to manage. I think it’s efficient.

class Place < ActiveRecord::Base
  has_many :existences
  has_many :things, :through => :existences
end

class Thing < ActiveRecord::Base
  has_many :existences
  has_many :places, :through => :existences
end

class Existence < ActiveRecord::Base
  belongs_to :place
  belongs_to :thing
end

class Find < ActiveRecord::Base
  belongs_to :user
  belongs_to :existence
end

class User < ActiveRecord::Base
  has_many :finds
  has_many :existences, :through => :finds
  has_many :things, :through => :existences
end

You’ll need rails 3.1 to do the nested has many through’s like we did in User.

BTW the correct association declaration should be: belongs_to :places_things