diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 29b721a38ace4cf86ba2068266c4ac5c551e5e1a..8eb9ac1720b7ee1ee267cb919d50ff5df209fe35 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,21 @@
 *SVN*
 
+* Allow has_many :through to work on has_many associations (closes #3864) [sco@scottraymond.net]  Example:
+
+    class Firm < ActiveRecord::Base
+      has_many :clients
+      has_many :invoices, :through => :clients
+    end
+  
+    class Client < ActiveRecord::Base
+      belongs_to :firm
+      has_many   :invoices
+    end
+  
+    class Invoice < ActiveRecord::Base
+      belongs_to :client
+    end
+
 * Raise error when trying to select many polymorphic objects with has_many :through or :include (closes #4226) [josh@hasmanythrough.com]
 
 * Fixed has_many :through to include :conditions set on the :through association. closes #4020 [jonathan@bluewire.net.nz]
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index 562d0e93fb9bbe829043bd9062f489dd6263756d..1baac52cdd8fabed314fe641a23a7d302ae5d6e6 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -26,6 +26,7 @@ def find(*args)
         
         options[:select] = construct_select
         options[:from]   = construct_from
+        options[:joins]  = construct_joins
         
         merge_options_from_reflection!(options)
 
@@ -53,44 +54,57 @@ def find_target
             :select     => construct_select,
             :conditions => construct_conditions,
             :from       => construct_from,
+            :joins      => construct_joins,
             :order      => @reflection.options[:order], 
             :limit      => @reflection.options[:limit],
-            :joins      => @reflection.options[:joins],
             :group      => @reflection.options[:group]
           )
         end
 
-        def construct_conditions
-          # Get the actual primary key of the belongs_to association that the reflection is going through
-          source_primary_key = @reflection.source_reflection.primary_key_name
-          
-          if @reflection.through_reflection.options[:as]
-            conditions = 
-              "#{@reflection.table_name}.#{@reflection.klass.primary_key} = #{@reflection.through_reflection.table_name}.#{source_primary_key} " +
-              "AND #{@reflection.through_reflection.table_name}.#{@reflection.through_reflection.options[:as]}_id = #{@owner.quoted_id} " + 
+        def construct_conditions          
+          conditions = if @reflection.through_reflection.options[:as]
+              "#{@reflection.through_reflection.table_name}.#{@reflection.through_reflection.options[:as]}_id = #{@owner.quoted_id} " + 
               "AND #{@reflection.through_reflection.table_name}.#{@reflection.through_reflection.options[:as]}_type = #{@owner.class.quote @owner.class.base_class.name.to_s}"
           else
-            conditions = 
-              "#{@reflection.klass.table_name}.#{@reflection.klass.primary_key} = #{@reflection.through_reflection.table_name}.#{source_primary_key} " +
-              "AND #{@reflection.through_reflection.table_name}.#{@reflection.through_reflection.primary_key_name} = #{@owner.quoted_id}"
+            case @reflection.source_reflection.macro
+              when :belongs_to, :has_many
+                "#{@reflection.through_reflection.table_name}.#{@reflection.through_reflection.primary_key_name} = #{@owner.quoted_id}"
+              else
+                raise ActiveRecordError, "Invalid source reflection macro :#{@reflection.source_reflection.macro} for has_many #{@reflection.name}, :through => #{@reflection.through_reflection.name}"
+            end
           end
-          
           conditions << " AND (#{sql_conditions})" if sql_conditions
           
           return conditions
         end
 
         def construct_from
-          "#{@owner.class.reflections[@reflection.options[:through]].table_name}, #{@reflection.table_name}"
+          @reflection.table_name
         end
         
         def construct_select
           selected = @reflection.options[:select] || "#{@reflection.table_name}.*"          
         end
         
+        def construct_joins
+          if @reflection.through_reflection.options[:as] || @reflection.source_reflection.macro == :belongs_to
+            reflection_primary_key = @reflection.klass.primary_key
+            source_primary_key     = @reflection.source_reflection.primary_key_name
+          else
+            reflection_primary_key = @reflection.source_reflection.primary_key_name
+            source_primary_key     = @reflection.klass.primary_key
+          end
+          
+          "INNER JOIN %s ON %s.%s = %s.%s #{@reflection.options[:joins]}" % [
+            @owner.class.reflections[@reflection.options[:through]].table_name,
+            @reflection.table_name, reflection_primary_key,
+            @reflection.through_reflection.table_name, source_primary_key
+          ]
+        end
+        
         def construct_scope
           {
-            :find   => { :from => construct_from, :conditions => construct_conditions },
+            :find   => { :from => construct_from, :conditions => construct_conditions, :joins => construct_joins },
             :create => { @reflection.primary_key_name => @owner.id }
           }
         end
@@ -115,9 +129,14 @@ def construct_sql
           end
         end
         
-        def sql_conditions
-          @conditions ||= interpolate_sql(@reflection.active_record.send(:sanitize_sql, @reflection.through_reflection.options[:conditions])) if @reflection.through_reflection.options[:conditions]
+        def conditions
+          @conditions ||= [
+            (interpolate_sql(@reflection.active_record.send(:sanitize_sql, @reflection.options[:conditions])) if @reflection.options[:conditions]),
+            (interpolate_sql(@reflection.active_record.send(:sanitize_sql, @reflection.through_reflection.options[:conditions])) if @reflection.through_reflection.options[:conditions])
+          ].compact.collect { |condition| "(#{condition})" }.join(' AND ') unless (!@reflection.options[:conditions] && !@reflection.through_reflection.options[:conditions])
         end
+        
+        alias_method :sql_conditions, :conditions
     end
   end
 end
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 3011ba66257c935aa493d59fc568f97d143a747a..0d3333e0e14fee1675cc4726eef975692931c2e2 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -156,7 +156,9 @@ def source_reflection_name
       #
       def source_reflection
         return nil unless through_reflection
-        @source_reflection ||= through_reflection.klass.reflect_on_association(source_reflection_name)
+        @source_reflection ||= \
+          through_reflection.klass.reflect_on_association(source_reflection_name) || # has_many :through a :belongs_to
+          through_reflection.klass.reflect_on_association(name)                      # has_many :through a :has_many
       end
 
       def check_validity!
@@ -183,7 +185,7 @@ def name_to_class_name(name)
             if options[:class_name]
               options[:class_name]
             elsif through_reflection # get the class_name of the belongs_to association of the through reflection
-              through_reflection.klass.reflect_on_association(name.to_s.singularize.to_sym).class_name
+              source_reflection.class_name
             else
               class_name = name.to_s.camelize
               class_name = class_name.singularize if [ :has_many, :has_and_belongs_to_many ].include?(macro)
diff --git a/activerecord/test/associations_join_model_test.rb b/activerecord/test/associations_join_model_test.rb
index f53a7669f4d50c6dc093d8f93255d44f06eaf9de..4b0f4e4c334841754ec780f7ed23eadbb04ba493 100644
--- a/activerecord/test/associations_join_model_test.rb
+++ b/activerecord/test/associations_join_model_test.rb
@@ -230,6 +230,22 @@ def test_has_many_polymorphic
     end
   end
 
+  def test_has_many_through_has_many_find_all
+    assert_equal comments(:greetings), authors(:david).comments.find(:all).first
+  end
+
+  def test_has_many_through_has_many_find_first
+    assert_equal comments(:greetings), authors(:david).comments.find(:first)
+  end
+
+  def test_has_many_through_has_many_find_conditions
+    assert_equal comments(:does_it_hurt), authors(:david).comments.find(:first, :conditions => "comments.type='SpecialComment'", :order => 'comments.id')
+  end
+
+  def test_has_many_through_has_many_find_by_id
+    assert_equal comments(:more_greetings), authors(:david).comments.find(2)
+  end
+
   private
     # create dynamic Post models to allow different dependency options
     def find_post_with_dependency(post_id, association, association_name, dependency)
diff --git a/activerecord/test/fixtures/author.rb b/activerecord/test/fixtures/author.rb
index 06b0aaa1aeb630f9317efc4043673ab1e70bc455..1a7aa6e3889a0afd5ddde1aee049c8c58ed4602d 100644
--- a/activerecord/test/fixtures/author.rb
+++ b/activerecord/test/fixtures/author.rb
@@ -3,6 +3,7 @@ class Author < ActiveRecord::Base
   has_many :posts_with_comments, :include => :comments, :class_name => "Post"
   has_many :posts_with_categories, :include => :categories, :class_name => "Post"
   has_many :posts_with_comments_and_categories, :include => [ :comments, :categories ], :order => "posts.id", :class_name => "Post"
+  has_many :comments, :through => :posts
 
   has_many :special_posts,        :class_name => "Post"
   has_many :hello_posts,          :class_name => "Post", :conditions=>"\#{aliased_table_name}.body = 'hello'"