Question

Let’s say that I’m looking for a specific item in a has_many association.

Is there a way to find that won’t rehit the database if what I’m requesting is already cached?

Here’s my situation (which is not working and does not do what I requested):

class User
  has_many :friendships

  def some_function(buddy)
    f = friendships.detect{|friendship| friendship.user == self && 
                                        friendship.friend == buddy}
    f.do_something
  end
end

The function is susceptible to N+1 problems, but most of the time I will already have either used :include or preloaded the user’s friendships elsewhere. However, if I didn’t do this, then this current function will fall back to requesting all the friendships, even though it only needs one. The solution would seem to be something like this:

f = Friendship.find_by_user_id_and_friend_id id, buddy.id

However, this will hit the database again even if all the friendships are cached and I have what I need right there.

Is there any way to either use the cached copy, or if none is available, search a specific record?

Answer 1

What you want is the #loaded? method of the association proxy:

class User
  has_many :friendships

  def some_function(buddy)
    if friendships.loaded? then
      f = friendships.detect{|friendship| friendship.user == self && 
                                          friendship.friend == buddy}
    else
      Friendship.find_by_user_id_and_friend_id id, buddy.id
    end
  end
end

Unfortunately, #loaded? does not appear in the public documentation. You can always guard your call to #loaded? with friendships.respond_to?(:loaded?).

Documentation for AssociationProxy#loaded? on GitHub.