Includes (eager loads) of associations that includes joins fails

[Gunnar Blom]

When an association has joins (either to filter the association or calculate something), it cannot be eager loaded with includes. The joins command seems to be omitted and I get an error saying the table doesn't exist cause it didn't the join when eager loading.

Example:

class User < ActiveRecord::Base
  has_many :purchases

  # Perform joins and attach some calculations to the User object
  scope :add_stats, -> { group("users.id").joins(:purchases).select("users.*, SUM(purchases.price) AS total_purchases") }
end

class Purchase < ActiveRecord::Base
  belongs_to :user
end

The add_stats scope represents heavy calculations attached to the User objects. So if I want to get all User objects with stats, I just write User.all.add_stats.

So far so good. Now I want to fetch some Purchase objects and eager load the Users with stats as well. I've tried this:

belongs_to :user, -> { add_stats }

But then when Rails eager load the users, it seems to remove.group("user.id").joins(:purchases) and complain on purchases.price - "purchases table unknown". So the .select() is the only thing preserved from the scope.

How do I apply a scope (with working .group().joins()) to the eager load query of all included belongs_to :user objects?