Rails: :inverse_of and Association extensions

Question

I have the following set-up

class Player < ActiveRecord::Base
  has_many :cards, :inverse_of => :player do
    def in_hand
      find_all_by_location('hand')
    end
  end
end

class Card < ActiveRecord::Base
  belongs_to :player, :inverse_of => :cards
end

This means the following works:

p = Player.find(:first)
c = p.cards[0]
p.score # => 2
c.player.score # => 2
p.score += 1
c.player.score # => 3
c.player.score += 2
p.score # => 5

But the following doesn't behave the same way:

p = Player.find(:first)
c = p.cards.in_hand[0]
p.score # => 2
c.player.score # => 2
p.score += 1
c.player.score # => 2
c.player.score += 2
p.score # => 3

d = p.cards.in_hand[1]
d.player.score # => 2

How can I make the :inverse_of relationship extend to the extension methods? (Is this just a bug?)

Answer 1

I have found a workaround if (as I am) you're willing to give up the SQL optimisation granted by Arel and just do it all in Ruby.

class Player < ActiveRecord::Base
  has_many :cards, :inverse_of => :player do
    def in_hand
      select {|c| c.location == 'hand'}
    end
  end
end

class Card < ActiveRecord::Base
  belongs_to :player, :inverse_of => :cards
end

By writing the extension to filter in Ruby the full results of the association, rather than narrowing down the SQL query, results returned by the extension behave correctly with :inverse_of:

p = Player.find(:first)
c = p.cards[0]
p.score # => 2
c.player.score # => 2
p.score += 1
c.player.score # => 3
c.player.score += 2
p.score # => 5

d = p.cards.in_hand[0]
d.player.score # => 5
d.player.score += 3
c.player.score # => 8

Answer 2

It does not work because the "in_hand" method has a query that goes back to the database.

Because of the inverse_of option, the working code knows how to use the objects that are already in memory.

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html