diff --git a/lib/composite_primary_keys/associations.rb b/lib/composite_primary_keys/associations.rb
index a95d767..3bcbb30 100644
--- a/lib/composite_primary_keys/associations.rb
+++ b/lib/composite_primary_keys/associations.rb
@@ -376,31 +376,37 @@ module ActiveRecord::Associations
      return conditions
    end

-    def construct_joins(custom_joins = nil)
+    def construct_joins(custom_joins = nil)
      polymorphic_join = nil
      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
+        if @reflection.options[:source_type]
+          polymorphic_join = "AND %s.%s = %s" % [
+            @reflection.through_reflection.table_name, "#{@reflection.source_reflection.options[:foreign_type]}",
+            @owner.class.quote_value(@reflection.options[:source_type])
+          ]
+        end
      else
        reflection_primary_key = @reflection.source_reflection.primary_key_name
        source_primary_key     = @reflection.klass.primary_key
        if @reflection.source_reflection.options[:as]
          polymorphic_join = "AND %s.%s = %s" % [
            @reflection.table_name, "#{@reflection.source_reflection.options[:as]}_type",
-                @owner.class.quote_value(@reflection.through_reflection.klass.name)
-              ]
-            end
-          end
-
-          "INNER JOIN %s ON %s %s #{@reflection.options[:joins]} #{custom_joins}" % [
-            @reflection.through_reflection.table_name,
-            composite_join_clause(
-              full_keys(@reflection.table_name, reflection_primary_key),
-              full_keys(@reflection.through_reflection.table_name, source_primary_key)),
-            polymorphic_join
+            @owner.class.quote_value(@reflection.through_reflection.klass.name)
          ]
+        end
+      end
+
+      "INNER JOIN %s ON %s %s #{@reflection.options[:joins]} #{custom_joins}" % [
+        @reflection.through_reflection.table_name,
+        composite_join_clause(
+          full_keys(@reflection.table_name, reflection_primary_key),
+          full_keys(@reflection.through_reflection.table_name, source_primary_key)),
+        polymorphic_join
+      ]
    end
-
+
    def construct_sql
      case
        when @reflection.options[:finder_sql]