Rails - Oct 2013 - Includes (eager loads) of associations that includes joins fails

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 :userend

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?*

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