I have an association user.lessons. Sometimes i just want the lessons,
other times i want the lessons to eager-load one of their own
associations, ‘assets’. Theres a couple of ways i can see to deal with
this and i’m not super happy with any of them.
approach 1) Make two associations:
has_many :lessons
has_many :lessons_with_assets, :class_name => “Lesson”, :include =>
[:assets]
#then, if i want the assets, call @user.lessons_with_assets
This is actually my favorite but it still feels a bit clumsy
approach 2) Don’t use the association if i want to eager load #just do a regular find
Lesson.find_all_by_user_id(@user.id, :include => [:assets])
Is there another way? This example is simple but i have other
associations that go through join tables and are polymorphic (eg using
the has_favorite plugin) that are a bit more arsey to use.
aha…so, if i have lots of different associations (in user and other
classes) that return a collection of lessons i can just add
“.with_assets” to the end? eg
After a bit of experimentation i confirmed for myself that that is
indeed the case
thanks a lot Andrew, this is great. I’d seen named scopes before but
never really used them much and never realised their potential for
managing includes.
One thing i noticed which is interesting…lessons are joined to assets
through a join table, ‘elements’.
if i use the with_assets scope for an association that goes through a
join table, then rails does a further join to the assets table, like
you’d expect with an include, so we have this situation:
user JOIN user_usb_lessons JOIN lessons JOIN elements JOIN assets
If, however, it’s a simple association (such as user.lessons, which just
uses Lesson’s user_id field) then rails splits it off into three sql
calls: the first to load the lessons, the second to load the elements
(join table records) and the third to load the assets.
I guess that three seperate simple calls were seen as preferable to a
single JOIN?
