dealing with limits and offsets

lib/arel/visitors/oracle.rb

 module Arel
   module Visitors
     class Oracle < Arel::Visitors::ToSql
+      private
+
       def visit_Arel_Nodes_SelectStatement o
-        if o.limit && o.orders.empty?
+        if o.limit && o.orders.empty? && !o.offset
           o.cores.last.wheres.push Nodes::LessThanOrEqual.new(
             Nodes::SqlLiteral.new('ROWNUM'), o.limit
           )
@@ -10,6 +12,22 @@ def visit_Arel_Nodes_SelectStatement o
           return super
         end
 
+        if o.limit && o.offset
+          limit = o.limit.to_i
+          offset = o.offset
+          o.limit = nil
+          o.offset = nil
+          sql = super
+          return <<-eosql
+              SELECT * FROM (
+                SELECT raw_sql_.*, rownum raw_rnum_
+                FROM (#{sql}) raw_sql_
+                WHERE rownum <= #{offset.value.to_i + limit}
+              )
+              WHERE #{visit offset}
+          eosql
+        end
+
         if o.limit && !o.orders.empty?
           limit = o.limit
           o.limit = nil
@@ -18,6 +36,10 @@ def visit_Arel_Nodes_SelectStatement o
 
         super
       end
+
+      def visit_Arel_Nodes_Offset o
+        "raw_rnum_ > #{visit o.value}"
+      end
     end
   end
 end

spec/arel/visitors/oracle_spec.rb

@@ -25,6 +25,21 @@ module Visitors
               SELECT * FROM (SELECT ORDER BY foo) WHERE ROWNUM <= 10
             }
           end
+
+          it 'creates a different subquery when there is an offset' do
+            stmt = Nodes::SelectStatement.new
+            stmt.limit = 10
+            stmt.offset = Nodes::Offset.new(10)
+            sql = @visitor.accept stmt
+            sql.should be_like %{
+              SELECT * FROM (
+                SELECT raw_sql_.*, rownum raw_rnum_
+                FROM (SELECT ) raw_sql_
+                WHERE rownum <= 20
+              )
+              WHERE raw_rnum_ > 10
+            }
+          end
         end
       end
     end