Make Relation#includes behave exactly like the existing :include option

activerecord/lib/active_record/base.rb

@@ -1579,17 +1579,8 @@ module ActiveRecord #:nodoc:
             order(construct_order(options[:order], scope)).
             limit(construct_limit(options[:limit], scope)).
             offset(construct_offset(options[:offset], scope)).
-            from(options[:from])
-
-          include_associations = merge_includes(scope && scope[:include], options[:include])
-
-          if include_associations.any?
-            if references_eager_loaded_tables?(options)
-              relation = relation.eager_load(include_associations)
-            else
-              relation = relation.preload(include_associations)
-            end
-          end
+            from(options[:from]).
+            includes( merge_includes(scope && scope[:include], options[:include]))
 
           lock = (scope && scope[:lock]) || options[:lock]
           relation = relation.lock if lock.present?

activerecord/lib/active_record/relation.rb

@@ -76,7 +76,7 @@ module ActiveRecord
     def to_a
       return @records if loaded?
 
-      find_with_associations = @eager_load_associations.any?
+      find_with_associations = @eager_load_associations.any? || references_eager_loaded_tables?
 
       @records = if find_with_associations
         begin
@@ -90,7 +90,7 @@ module ActiveRecord
             :offset => @relation.skipped,
             :from => (@relation.send(:from_clauses) if @relation.send(:sources).present?)
             },
-            ActiveRecord::Associations::ClassMethods::JoinDependency.new(@klass, @eager_load_associations, nil))
+            ActiveRecord::Associations::ClassMethods::JoinDependency.new(@klass, @eager_load_associations + @include_associations, nil))
         rescue ThrowResult
           []
         end
@@ -157,7 +157,7 @@ module ActiveRecord
     end
 
     def reset
-      @first = @last = @create_scope = nil
+      @first = @last = @create_scope = @joined_tables = nil
       @records = []
       self
     end
@@ -216,5 +216,32 @@ module ActiveRecord
       @relation.send(:where_clauses).join(join_string)
     end
 
+    def references_eager_loaded_tables?
+      include_eager_order? || include_eager_conditions? || include_eager_select?
+    end
+
+    def include_eager_order?
+      order_clause = @relation.send(:order_clauses).join(', ')
+      (tables_in_string(order_clause) - joined_tables).any?
+    end
+
+    def include_eager_conditions?
+      (tables_in_string(where_clause) - joined_tables).any?
+    end
+
+    def include_eager_select?
+      select_clause = @relation.send(:select_clauses).join(', ')
+      (tables_in_string(select_clause) - joined_tables).any?
+    end
+
+    def joined_tables
+      @joined_tables ||= (tables_in_string(@relation.joins(relation)) + [table.name, table.table_alias]).compact.uniq
+    end
+
+    def tables_in_string(string)
+      return [] if string.blank?
+      string.scan(/([a-zA-Z_][\.\w]+).?\./).flatten.uniq
+    end
+
   end
 end