manually construct associations to get around the eager loading limitations with mongoid?

Question

suppose i have 3 models, A, B and C:

class A
    include Mongoid::Document
    has_many :bs
    ...

class B
    include Mongoid::Document
    belongs_to :a
    has_many :cs
    ...

class C
    include Mongoid::Document
    belongs_to :B
    ...

It appears that mongoid does not support complex eager loading as of version 6. Is it possible to instead just run my 3 queries to grab all As, Bs and Cs, and manually override the associations. Something like this:

a = A.(#some criteria).first
bs = B.where(a_id: a.id).group_by{|x| x.id}
cs = C.where(:b_id.in => bs.values.collect{|x| x.id}).group_by{|x| x.b_id}

a.bs = bs
bs.values.each do |k,v|
    v.a = a
    v.cs = cs[k]
end

cs.each do |k,v|
    v.b = bs[v.b_id]
end

This seems to work just fine, but it fires tons of queries, when you go to access a.bs and b.cs, etc., so it's obviously not working at the mongoid level. Is there a way to manually override associations so that I can just run the queries and assemble them by hand?

Basically, I want to do something like what this post describes but with mongoid (and with Rails 5):

https://mrbrdo.wordpress.com/2013/09/25/manually-preloading-associations-in-rails-using-custom-scopessql/

Thanks for any help, kevin

Answer 1

The answer I was looking for was the set_relation method. From the code above, the solution is:

bs.values.each do |k,v|
    v.a = a
    v.set_relation(:cs, cs[k])
end

set_relation takes two arguments, the symbol for the relation and the array or singleton of objects.