Support in memory ordering better.

3 files changed, 21 insertions, 11 deletions

diff --git a/lib/arel/algebra/relations/operations/order.rb b/lib/arel/algebra/relations/operations/order.rb
index a589b56997..df54735630 100644
--- a/lib/arel/algebra/relations/operations/order.rb
+++ b/lib/arel/algebra/relations/operations/order.rb
@@ -1,7 +1,8 @@
 module Arel
   class Order < Compound
     attributes :relation, :orderings
-    deriving :==
+    deriving   :==
+    requires   :ordering
 
     def initialize(relation, *orderings, &block)
       @relation = relation
diff --git a/lib/arel/algebra/relations/operations/where.rb b/lib/arel/algebra/relations/operations/where.rb
index 2fc51c7f24..6c9c5ed755 100644
--- a/lib/arel/algebra/relations/operations/where.rb
+++ b/lib/arel/algebra/relations/operations/where.rb
@@ -1,7 +1,8 @@
 module Arel
   class Where < Compound
     attributes :relation, :predicate
-    deriving :==
+    deriving   :==
+    requires   :restricting
 
     def initialize(relation, *predicates, &block)
       predicate = block_given?? yield(relation) : predicates.shift
@@ -9,15 +10,6 @@ module Arel
       @predicate = predicate.bind(@relation)
     end
 
-    def engine
-      # Temporary check of whether or not the engine supports where.
-      if relation.engine.respond_to?(:supports) && !relation.engine.supports(:where)
-        Memory::Engine.new
-      else
-        relation.engine
-      end
-    end
-
     def wheres
       @wheres ||= (relation.wheres + [predicate]).collect { |p| p.bind(self) }
     end
diff --git a/lib/arel/algebra/relations/utilities/compound.rb b/lib/arel/algebra/relations/utilities/compound.rb
index 1a43c76a94..882cf8a76b 100644
--- a/lib/arel/algebra/relations/utilities/compound.rb
+++ b/lib/arel/algebra/relations/utilities/compound.rb
@@ -5,6 +5,11 @@ module Arel
              :column_for, :engine, :sources, :locked, :table_alias,
              :to => :relation
 
+    def self.requires(feature = nil)
+      @requires = feature if feature
+      @requires
+    end
+
     [:attributes, :wheres, :groupings, :orders, :havings, :projections].each do |operation_name|
       class_eval <<-OPERATION, __FILE__, __LINE__
         def #{operation_name}
@@ -21,6 +26,18 @@ module Arel
       self == other
     end
 
+    def engine
+      requires = self.class.requires
+      engine   = relation.engine
+
+      # Temporary check of whether or not the engine supports where.
+      if requires && engine.respond_to?(:supports) && !engine.supports(requires)
+        Memory::Engine.new
+      else
+        engine
+      end
+    end
+
   private
 
     def arguments_from_block(relation, &block)