Add support for table aliasing, with a test that needs aliasing in order to work correctly. This test incidentally provides a more complicated test case (4 inner joins, 2 using polymorphism).

activerecord/lib/active_record/associations/through_association_scope.rb

@@ -22,9 +22,8 @@ module ActiveRecord
       # Build SQL conditions from attributes, qualified by table name.
       def construct_conditions
         reflection = @reflection.through_reflection_chain.last
-        table_name = reflection.quoted_table_name
         conditions = construct_quoted_owner_attributes(reflection).map do |attr, value|
-          "#{table_name}.#{attr} = #{value}"
+          "#{table_aliases[reflection]}.#{attr} = #{value}"
         end
         conditions << sql_conditions if sql_conditions
         "(" + conditions.join(') AND (') + ")"
@@ -67,21 +66,23 @@ module ActiveRecord
           polymorphic_join  = nil
           
           case
-            when left.options[:as]
+            when left.source_reflection.nil?
               left_primary_key  = left.primary_key_name
               right_primary_key = right.klass.primary_key
               
-              polymorphic_join = "AND %s.%s = %s" % [
-                left.quoted_table_name, "#{left.options[:as]}_type",
-                @owner.class.quote_value(right.klass.name)
-              ]
+              if left.options[:as]
+                polymorphic_join = "AND %s.%s = %s" % [
+                  table_aliases[left], "#{left.options[:as]}_type",
+                  @owner.class.quote_value(right.klass.name)
+                ]
+              end
             when left.source_reflection.macro == :belongs_to
               left_primary_key  = left.klass.primary_key
               right_primary_key = left.source_reflection.primary_key_name
               
               if left.options[:source_type]
                 polymorphic_join = "AND %s.%s = %s" % [
-                  right.quoted_table_name,
+                  table_aliases[right],
                   left.source_reflection.options[:foreign_type].to_s,
                   @owner.class.quote_value(left.options[:source_type])
                 ]
@@ -92,22 +93,45 @@ module ActiveRecord
               
               if left.source_reflection.options[:as]
                 polymorphic_join = "AND %s.%s = %s" % [
-                  left.quoted_table_name,
+                  table_aliases[left],
                   "#{left.source_reflection.options[:as]}_type",
                   @owner.class.quote_value(right.klass.name)
                 ]
               end
           end
           
+          if right.quoted_table_name == table_aliases[right]
+            table = right.quoted_table_name
+          else
+            table = "#{right.quoted_table_name} #{table_aliases[right]}"
+          end
+          
           joins << "INNER JOIN %s ON %s.%s = %s.%s %s" % [
-            right.quoted_table_name,
-            left.quoted_table_name,  left_primary_key,
-            right.quoted_table_name, right_primary_key,
+            table,
+            table_aliases[left],  left_primary_key,
+            table_aliases[right], right_primary_key,
             polymorphic_join
           ]
         end
         
-        joins
+        joins.join(" ")
+      end
+
+      def table_aliases
+        @table_aliases ||= begin
+          tally = {}
+          @reflection.through_reflection_chain.inject({}) do |aliases, reflection|
+            if tally[reflection.table_name].nil?
+              tally[reflection.table_name] = 1
+              aliases[reflection] = reflection.quoted_table_name
+            else
+              tally[reflection.table_name] += 1
+              aliased_table_name = reflection.table_name + "_#{tally[reflection.table_name]}"
+              aliases[reflection] = reflection.klass.connection.quote_table_name(aliased_table_name)
+            end
+            aliases
+          end
+        end
       end
 
       # Construct attributes for associate pointing to owner.

activerecord/test/cases/associations/nested_has_many_through_associations_test.rb

@@ -40,4 +40,9 @@ class NestedHasManyThroughAssociationsTest < ActiveRecord::TestCase
     author = authors(:david)
     assert_equal [subscribers(:first), subscribers(:second)], author.distinct_subscribers
   end
+  
+  def test_nested_has_many_through_with_a_table_referenced_multiple_times
+    author = authors(:bob)
+    assert_equal [posts(:misc_by_bob), posts(:misc_by_mary)], author.similar_posts.sort_by(&:id)
+  end
 end

activerecord/test/fixtures/authors.yml

@@ -7,3 +7,7 @@ david:
 mary:
   id: 2
   name: Mary
+
+bob:
+  id: 3
+  name: Bob

activerecord/test/fixtures/posts.yml

@@ -50,3 +50,17 @@ eager_other:
   title: eager loading with OR'd conditions
   body: hello
   type: Post
+
+misc_by_bob:
+  id: 8
+  author_id: 3
+  title: misc post by bob
+  body: hello
+  type: Post
+
+misc_by_mary:
+  id: 9
+  author_id: 2
+  title: misc post by mary
+  body: hello
+  type: Post

activerecord/test/fixtures/taggings.yml

@@ -26,3 +26,15 @@ godfather:
 orphaned:
   id: 5
   tag_id: 1
+
+misc_post_by_bob:
+  id: 6
+  tag_id: 2
+  taggable_id: 8
+  taggable_type: Post
+
+misc_post_by_mary:
+  id: 7
+  tag_id: 2
+  taggable_id: 9
+  taggable_type: Post

activerecord/test/fixtures/tags.yml

@@ -4,4 +4,4 @@ general:
 
 misc:
   id: 2
-  name: Misc
+  name: Misc

activerecord/test/models/author.rb

@@ -84,8 +84,9 @@ class Author < ActiveRecord::Base
   has_many :favorite_authors, :through => :author_favorites, :order => 'name'
 
   has_many :tagging,         :through => :posts # through polymorphic has_one
-  has_many :taggings,        :through => :posts, :source => :taggings # through polymorphic has_many TODO: Why is the :source needed?
+  has_many :taggings,        :through => :posts # through polymorphic has_many
   has_many :tags,            :through => :posts # through has_many :through (on source reflection + polymorphic)
+  has_many :similar_posts,   :through => :tags,  :source => :tagged_posts
   has_many :distinct_tags,   :through => :posts, :source => :tags, :select => "DISTINCT tags.*", :order => "tags.name"
   has_many :post_categories, :through => :posts, :source => :categories