From b339caca2f3c7306c3944c5fc5d8dde17ae2deb8 Mon Sep 17 00:00:00 2001
From: Emilio Tagua <miloops@gmail.com>
Date: Sun, 27 Dec 2009 18:04:35 -0300
Subject: Added lock to Arel, allowing a locking read if required.

---
 lib/arel/algebra/relations.rb                    |  1 +
 lib/arel/algebra/relations/operations/lock.rb    | 12 ++++++++++++
 lib/arel/algebra/relations/relation.rb           |  5 +++++
 lib/arel/algebra/relations/utilities/compound.rb |  2 +-
 4 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 lib/arel/algebra/relations/operations/lock.rb

(limited to 'lib/arel/algebra')

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|
-- 
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