Has many through - It is not necessary to manually merge in the conditions hash for the through record, because the creation is done directly on the through association, which will already handle setting the conditions.

activerecord/lib/active_record/associations/through_association.rb

@@ -88,10 +88,6 @@ module ActiveRecord
             join_attributes.merge!(@reflection.source_reflection.foreign_type => associate.class.base_class.name)
           end
 
-          if @reflection.through_reflection.options[:conditions].is_a?(Hash)
-            join_attributes.merge!(@reflection.through_reflection.options[:conditions])
-          end
-
           join_attributes
         end
 

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -20,10 +20,13 @@ require 'models/subscription'
 require 'models/categorization'
 require 'models/category'
 require 'models/essay'
+require 'models/member'
+require 'models/membership'
+require 'models/club'
 
 class HasManyThroughAssociationsTest < ActiveRecord::TestCase
   fixtures :posts, :readers, :people, :comments, :authors, :categories,
-           :owners, :pets, :toys, :jobs, :references, :companies,
+           :owners, :pets, :toys, :jobs, :references, :companies, :members,
            :subscribers, :books, :subscriptions, :developers, :categorizations
 
   # Dummies to force column loads so query counts are clean.
@@ -557,4 +560,11 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert proxy.stale_target?
     assert_equal authors(:david).categorizations.sort_by(&:id), post.author_categorizations.sort_by(&:id)
   end
+
+  def test_create_with_conditions_hash_on_through_association
+    member = members(:groucho)
+    club   = member.clubs.create!
+
+    assert_equal true, club.reload.membership.favourite
+  end
 end

activerecord/test/models/club.rb

@@ -1,4 +1,5 @@
 class Club < ActiveRecord::Base
+  has_one :membership
   has_many :memberships
   has_many :members, :through => :memberships
   has_many :current_memberships

activerecord/test/models/member.rb

@@ -13,4 +13,7 @@ class Member < ActiveRecord::Base
 
   has_many :current_memberships
   has_one :club_through_many, :through => :current_memberships, :source => :club
+
+  has_many :current_memberships, :conditions => { :favourite => true }
+  has_many :clubs, :through => :current_memberships
 end