Rails - Feb 2011 - Arel: misunderstanding with intersections

Hi,

I''ve been playing with Arel for a few days, but I feel really confused
about predicates / arrays intersection.

Let''s say I have 3 models: users, (bank) accounts and transfers. An
account belongs to a user, and a transfer is made of a source and
target accounts (see the snippets below).

Given a user, I want to query his transfers. I can go this way in the
User class:

def transfers
    Transfer.joins(:source) & Account.where(:user_id => id)
end

Here, I intersect the sources accounts of all the transfers with the
user''s accounts. Having said that (as always wanting more and more
conciseness), I wonder why the following is not working:

def transfers
    Transfer.joins(:source) & accounts
end

where ''accounts'' is the user''s accounts dependency.

To me, "Account.where(:user_id => id)" and "accounts"
represent
exactly the same concept. Is it because "Account.where(:user_id =>
id)" is a predicate and "accounts" is an Active Record
association, so
they aren''t homogeneous?

Actually, "Transfer.joins(:source) & accounts" doesn''t
return anything
(there is no error though), while "Transfer.joins(:source) &
Account.where(:user_id => id)" works like a charm.

Can someone clarify this for me?

Thanks in advance for your help :)

Here are the snippets:

[models/user.rb]
class User < ActiveRecord::Base
    has_many :accounts, :dependent => :destroy

    def transfers
      # Useless to join the targets, since source and target
necessarily belong to this user
      # Why this doesn''t work ? Transfer.joins(:source) & accounts
      Transfer.joins(:source) & Account.where(:user_id => id)
    end

end

[models/account.rb]
class Account < ActiveRecord::Base
    belongs_to :user
end

[models/transfer.rb]
class Transfer < ActiveRecord::Base
    belongs_to :source, :class_name => "Account"
    belongs_to :target, :class_name => "Account"
end

Environment: Rails 3.0.4 / Ruby 1.9.2

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