Added lock to Arel, allowing a locking read if required.

5 files changed, 23 insertions, 3 deletions

diff --git a/lib/arel/algebra/relations.rb b/lib/arel/algebra/relations.rb
index cee308299b..951b69d970 100644
--- a/lib/arel/algebra/relations.rb
+++ b/lib/arel/algebra/relations.rb
@@ -13,3 +13,4 @@ require 'arel/algebra/relations/operations/project'
 require 'arel/algebra/relations/operations/where'
 require 'arel/algebra/relations/operations/skip'
 require 'arel/algebra/relations/operations/take'
+require 'arel/algebra/relations/operations/lock'
diff --git a/lib/arel/algebra/relations/operations/lock.rb b/lib/arel/algebra/relations/operations/lock.rb
new file mode 100644
index 0000000000..0d6c12e65b
--- /dev/null
+++ b/lib/arel/algebra/relations/operations/lock.rb
@@ -0,0 +1,12 @@
+module Arel
+  class Lock < Compound
+    attributes :relation, :locked
+    deriving :initialize, :==
+
+    def initialize(relation, locked, &block)
+      @relation = relation
+      @locked   = locked.blank? ? " FOR UPDATE" : locked
+    end
+  end
+end
+
diff --git a/lib/arel/algebra/relations/relation.rb b/lib/arel/algebra/relations/relation.rb
index 2ce3fdcce8..b24e7c24d0 100644
--- a/lib/arel/algebra/relations/relation.rb
+++ b/lib/arel/algebra/relations/relation.rb
@@ -51,6 +51,10 @@ module Arel
         OPERATION
       end
 
+      def lock(locking = nil)
+        Lock.new(self, locking)
+      end
+
       def alias
         Alias.new(self)
       end
@@ -131,6 +135,7 @@ module Arel
       def taken;                  nil end
       def skipped;                nil end
       def sources;                []  end
+      def locked;                 []  end
     end
     include DefaultOperations
   end
diff --git a/lib/arel/algebra/relations/utilities/compound.rb b/lib/arel/algebra/relations/utilities/compound.rb
index 06bfce4ac0..9967472d88 100644
--- a/lib/arel/algebra/relations/utilities/compound.rb
+++ b/lib/arel/algebra/relations/utilities/compound.rb
@@ -2,7 +2,7 @@ module Arel
   class Compound < Relation
     attr_reader :relation
     delegate :joins, :join?, :inserts, :taken, :skipped, :name, :externalizable?,
-             :column_for, :engine, :sources,
+             :column_for, :engine, :sources, :locked,
              :to => :relation
 
     [:attributes, :wheres, :groupings, :orders].each do |operation_name|
diff --git a/lib/arel/engines/sql/relations/relation.rb b/lib/arel/engines/sql/relations/relation.rb
index 448cf7951d..8b71aa8c04 100644
--- a/lib/arel/engines/sql/relations/relation.rb
+++ b/lib/arel/engines/sql/relations/relation.rb
@@ -16,7 +16,8 @@ module Arel
           "FROM       #{from_clauses}",
           (joins(self)                                   unless joins(self).blank? ),
           ("WHERE     #{where_clauses.join("\n\tAND ")}" unless wheres.blank?      ),
-          ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   )
+          ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   ),
+          ("#{locked}"                                   unless locked.blank?     )
 
         build_query \
           "SELECT * FROM (#{query}) AS id_list",
@@ -33,7 +34,8 @@ module Arel
           ("GROUP BY  #{group_clauses.join(', ')}"       unless groupings.blank?   ),
           ("ORDER BY  #{order_clauses.join(', ')}"       unless orders.blank?      ),
           ("LIMIT     #{taken}"                          unless taken.blank?       ),
-          ("OFFSET    #{skipped}"                        unless skipped.blank?     )
+          ("OFFSET    #{skipped}"                        unless skipped.blank?     ),
+          ("#{locked}"                                   unless locked.blank?     )
       end
     end