Move join scopes on to the reflection object

Scopes can only ever be *not* reflection objects when they are passed in
to the Reflection constructor.  Given this fact, we can eliminate is_a
checks and an intermediate array object by just asking the reflection
object for join scopes.

activerecord/lib/active_record/associations/join_dependency/join_association.rb

@@ -41,14 +41,7 @@ def join_constraints(foreign_table, foreign_klass, join_type, tables, chain)
             constraint = build_constraint(klass, table, key, foreign_table, foreign_key)
 
             predicate_builder = PredicateBuilder.new(TableMetadata.new(klass, table))
-            scope_chain_items = reflection.scopes.map do |item|
-              if item.is_a?(Relation)
-                item
-              else
-                ActiveRecord::Relation.create(klass, table, predicate_builder)
-                  .instance_exec(&item)
-              end
-            end
+            scope_chain_items = reflection.join_scopes(table, predicate_builder)
 
             klass_scope =
               if klass.current_scope

activerecord/lib/active_record/reflection.rb

@@ -187,6 +187,15 @@ def scope_chain
       end
       deprecate :scope_chain
 
+      def join_scopes(table, predicate_builder) # :nodoc:
+        if scope
+          [ActiveRecord::Relation.create(klass, table, predicate_builder)
+            .instance_exec(&scope)]
+        else
+          []
+        end
+      end
+
       def constraints
         chain.map(&:scopes).flatten
       end
@@ -806,6 +815,10 @@ def scopes
         source_reflection.scopes + super
       end
 
+      def join_scopes(table, predicate_builder) # :nodoc:
+        source_reflection.join_scopes(table, predicate_builder) + super
+      end
+
       def source_type_scope
         through_reflection.klass.where(foreign_type => options[:source_type])
       end
@@ -990,6 +1003,15 @@ def scopes
         end
       end
 
+      def join_scopes(table, predicate_builder) # :nodoc:
+        scopes = @previous_reflection.join_scopes(table, predicate_builder) + super
+        if @previous_reflection.options[:source_type]
+          scopes + [@previous_reflection.source_type_scope]
+        else
+          scopes
+        end
+      end
+
       def klass
         @reflection.klass
       end