Allow database specific locking clauses to be used

lib/arel/nodes/lock.rb

 module Arel
   module Nodes
     class Lock < Arel::Nodes::Node
+      attr_reader :locking
+      def initialize locking = true
+        @locking = locking
+      end
     end
   end
 end

lib/arel/select_manager.rb

@@ -40,7 +40,7 @@ def where_clauses
     def lock locking = true
       # FIXME: do we even need to store this?  If locking is +false+ shouldn't
       # we just remove the node from the AST?
-      @ast.lock = Nodes::Lock.new
+      @ast.lock = Nodes::Lock.new(locking)
       self
     end
 

lib/arel/visitors/mysql.rb

@@ -3,7 +3,11 @@ module Visitors
     class MySQL < Arel::Visitors::ToSql
       private
       def visit_Arel_Nodes_Lock o
-        "FOR UPDATE"
+        if o.locking.is_a?(String)
+          o.locking
+        else
+          "FOR UPDATE"
+        end
       end
 
       ###

lib/arel/visitors/postgresql.rb

@@ -3,7 +3,11 @@ module Visitors
     class PostgreSQL < Arel::Visitors::ToSql
       private
       def visit_Arel_Nodes_Lock o
-        "FOR UPDATE"
+        if o.locking.is_a?(String)
+          o.locking
+        else
+          "FOR UPDATE"
+        end
       end
 
       def visit_Arel_Nodes_SelectStatement o

test/visitors/test_mysql.rb

@@ -29,11 +29,16 @@ module Visitors
         sql.must_be_like "SELECT FROM DUAL"
       end
 
-      it 'uses FOR UPDATE when locking' do
-        stmt = Nodes::SelectStatement.new
-        stmt.lock = Nodes::Lock.new
-        sql = @visitor.accept(stmt)
-        sql.must_be_like "SELECT FROM DUAL FOR UPDATE"
+      describe 'locking' do
+        it 'defaults to FOR UPDATE when locking' do
+          node = Nodes::Lock.new
+          @visitor.accept(node).must_be_like "FOR UPDATE"
+        end
+
+        it 'allows a custom string to be used as a lock' do
+          node = Nodes::Lock.new('LOCK IN SHARE MODE')
+          @visitor.accept(node).must_be_like "LOCK IN SHARE MODE"
+        end
       end
     end
   end

test/visitors/test_postgres.rb

@@ -7,10 +7,19 @@ module Visitors
         @visitor = PostgreSQL.new Table.engine
       end
 
-      it 'should produce a lock value' do
-        @visitor.accept(Nodes::Lock.new).must_be_like %{
-          FOR UPDATE
-        }
+      describe 'locking' do
+        it 'defaults to FOR UPDATE' do
+          @visitor.accept(Nodes::Lock.new).must_be_like %{
+            FOR UPDATE
+          }
+        end
+
+        it 'allows a custom string to be used as a lock' do
+          node = Nodes::Lock.new('FOR SHARE')
+          @visitor.accept(node).must_be_like %{
+            FOR SHARE
+          }
+        end
       end
 
       it "should escape LIMIT" do