scope with same attributes on different models misbehaves

I have two AR models

class Project < ActiveRecord::Base
has_many :teimerecords end

and

class Timerecord < ActiveRecord::Base
belongs_to :project

scope :billable, where(:billable => true, :projects => { :billable => true}). joins(:project) end

when use the billable scope, the sql is:

ruby-1.9.2-p0 > Timerecord.billable.to_sql
=> "SELECT "timerecords".* FROM "timerecords" INNER JOIN "projects" ON "projects"."id" = "timerecords"."project_id" WHERE ("projects"."billable" = 't')"

therefore not including the where clause on the timerecord

when I use it without the scope:

Timerecord. where(:billable => true, :projects => { :billable => true}). joins(:project).to_sql => "SELECT "timerecords".* FROM "timerecords" INNER JOIN "projects" ON "projects"."id" = "timerecords"."project_id" WHERE ("timerecords"."billable" = 't') AND ("projects"."billable" = 't')"

The build SQL statement is correct.

for ease, find the attached rails project

Bert