Rails - Mar 2014 - Four way JOIN (many-to-many + lookup); preventing duplicates when using scopes

I have two "top-level" entities joined with a third table
(many-to-many
relationship) which uses yet another table (lookup/reference table) to 
describe the nature of the relationship of the join.  ActiveRecord is 
adding an unnecessary JOIN which results in duplicate rows being returned.

[See model class definitions below.]

# users INNER JOIN request_user_roles INNER JOIN request_role_types
@request.users.count # 1
# users INNER JOIN request_user_roles INNER JOIN request_role_types INNER 
JOIN request_user_roles
@request.users.is_request_initiator.count # 898
@request.users.is_request_initiator.uniq.count # 1
# users INNER JOIN request_user_roles INNER JOIN request_role_types INNER 
JOIN request_user_roles
@request.users.joins(:request_role_types).merge(RequestRoleType.is_initiator).count
# 898

I want to select users for a particular request based on their role with 
respect to it.

I can write this in SQL, and I can write this in a single AR query, but I 
would like to simplify my code
with scopes, if possible.  What I can't do is structure a scope such that I 
don't return duplicates.  Or,
to put it another way, I can't structure the scope to not automatically 
include the extra JOIN (on request_user_roles).
Is there a way to express this with AR scopes or in another modular, 
re-usable way?  The uniq() feels dirty.

# relevant classes

# User
# |
# RequestUserRole - RequestRoleType
# |
# Request

class User < ActiveRecord::Base
  has_many :request_user_roles
  has_many :requests, :through => :request_user_roles
  has_many :request_role_types, :through => :request_user_roles

  scope :is_request_initiator, 
joins(:request_role_types).where(:request_role_types => {:request_role_type 
=> 'Initiator'})
end

class Request < ActiveRecord::Base
  has_many :request_user_roles
  has_many :users, :through => :request_user_roles
  has_many :request_role_types, :through => :request_user_roles
end

class RequestUserRole < ActiveRecord::Base
  belongs_to :user
  belongs_to :request
  belongs_to :request_role_type
end

class RequestRoleType < ActiveRecord::Base
  has_many :request_user_roles

  scope :is_initiator, where(:request_role_type => 'Initiator')
end



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