I think the unspoken implication is "for you.". Many newbie Rails devs assume that a line like "has_many :friends" is magic and makes user.friends execute quickly. They won't understand this explanation, but that can plausibly cause a full table scan, or worse, N+2 full table scans if you have something like:

photos_i_am_in = user.friends.map {|f| f.photos.select {|p| p.include? user}}.flatten

(Code somewhat minimalist - don't think too hard about it.)