Add support for nested eager loading on namespaced relation classes

[lucasmartins]

I was getting the following error when doing nested eager loading, when relations are not the default belongs_to :other_thing:

NameError: uninitialized constant OtherClass
from /Users/lucasmartins/.gem/ruby/2.3.1/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:261:in `const_get'

This is a relation like I'm using:

class NS::ModuleName::MyClass
  belongs_to :other_class, class_name: NS::ModuleName::OtherClass.to_s, index: true, touch: true
end

class NS::ModuleName::OtherClass
  has_many :my_class, class_name: NS::ModuleName::MyClass.to_s
end

While debugging the code, I've noticed that get_inclusion_metadata was trying to constantize "OtherClass" instead of "NS::ModuleName::OtherClass".

# lib/mongoid/criteria/includable.rb:134
def get_inclusion_metadata(_klass, association)
  if _klass.is_a?(Class)
    _klass.reflect_on_association(association)
  else
    # here, _klass.to_s.classify was "OtherClass", while it should be "NS::ModuleName::OtherClass"
    _klass.to_s.classify.constantize.reflect_on_association(association)
  end
end

I'm not sure about performance impact though.

I need somebody who knows this codebase better to check it out.

Any thoughts?