Supply run_with_isolation_level method to allow running queries at va…

…rious isolation levels (e.g. to go around other processes locking tables).

lib/active_record/connection_adapters/sqlserver_adapter.rb

@@ -325,6 +325,37 @@ def finish_statement_handle(handle)
 
       # DATABASE STATEMENTS ======================================#
 
+      # Returns the SET options active for the current connection.
+      def user_options
+        values = {}
+        select_rows("dbcc useroptions").each {|field| values.merge!(field[0].to_sym => field[1])}
+        values
+      end
+
+      VALID_ISOLATION_LEVELS = ["READ UNCOMMITTED", "READ COMMITTED", "REPEATABLE READ", "SNAPSHOT", "SERIALIZABLE"]
+
+      # Runs a block with a given isolation level.
+      # Supported isolation levels include 
+      # * <tt>"READ UNCOMMITTED"</tt>
+      # * <tt>"READ COMMITTED"</tt>
+      # * <tt>"REPEATABLE READ"</tt>
+      # * <tt>"SERIALIZABLE"</tt>      
+      # * <tt>"SNAPSHOT"</tt>
+      def run_with_isolation_level(isolation_level, &block)
+        if !VALID_ISOLATION_LEVELS.include?(isolation_level.upcase)        
+          raise ArgumentError, "#{isolation_level} not a supported isolation level.  Supported isolation levels are #{VALID_ISOLATION_LEVELS.to_sentence}"
+        end
+
+        initial_isolation_level = user_options[:"isolation level"] || "READ COMMITTED"
+        execute "SET TRANSACTION ISOLATION LEVEL #{isolation_level}"
+        begin
+          result = yield
+          return result
+        ensure
+          execute "SET TRANSACTION ISOLATION LEVEL #{initial_isolation_level}"
+        end
+      end
+
       def select_rows(sql, name = nil)
         raw_select(sql,name).last
       end

test/cases/adapter_test_sqlserver.rb

@@ -376,6 +376,68 @@ def setup
 
   context 'For DatabaseStatements' do
 
+    context "finding out what user_options are available" do
+      should "run the database consistency checker useroptions command" do
+        @connection.expects(:select_rows).with(regexp_matches(/^dbcc\s+useroptions$/i)).returns []
+        @connection.user_options
+      end
+
+      should "return a symbolized hash of the results" do
+        @connection.expects(:select_rows).with(regexp_matches(/^dbcc\s+useroptions$/i)).returns [['some', 'thing'], ['an', 'other thing']]
+        res = @connection.user_options
+        assert_equal 'thing', res[:some]
+        assert_equal 'other thing', res[:an]
+        assert_equal 2, res.keys.size
+      end
+    end
+
+    context "altering isolation levels" do
+      should "barf if the requested isolation level is not valid" do
+        @connection.class::VALID_ISOLATION_LEVELS.expects(:include?).returns false
+        assert_raise(ArgumentError) do
+          @connection.run_with_isolation_level 'something' do; end
+        end
+      end
+
+      context "with a valid isolation level" do
+        setup do
+          @connection.class::VALID_ISOLATION_LEVELS.expects(:include?).returns true
+          @connection.stubs(:user_options).returns({:"isolation level" => "something"})
+          @yieldy = states('yield').starts_as(:not_yielded)
+        end
+
+        should "set the isolation level to that supplied before calling the supplied block" do
+          @connection.expects(:execute).with(regexp_matches(/set transaction isolation level new_isolation_level/i)).when(@yieldy.is(:not_yielded))
+          @connection.stubs(:execute).when(@yieldy.is(:yielded))
+
+          @connection.run_with_isolation_level 'new_isolation_level' do
+            @yieldy.become(:yielded)
+          end
+        end
+
+        should "set the isolation level back to the original after calling the supplied block" do
+          @connection.expects(:execute).with(regexp_matches(/set transaction isolation level something/i)).when(@yieldy.is(:yielded))
+          @connection.stubs(:execute).when(@yieldy.is(:not_yielded))
+
+          @connection.run_with_isolation_level 'new_isolation_level' do
+            @yieldy.become(:yielded)
+          end
+        end
+
+        should "set the isolation level back to the original after calling the supplied block even when the block raises an exception" do
+          @connection.expects(:execute).with(regexp_matches(/set transaction isolation level something/i)).when(@yieldy.is(:yielded))
+          @connection.stubs(:execute).when(@yieldy.is(:not_yielded))
+
+          assert_raise(RuntimeError) do
+            @connection.run_with_isolation_level 'new_isolation_level' do
+              @yieldy.become(:yielded)
+              raise "a problem"
+            end
+          end
+        end
+      end
+    end
+
   end
 
   context 'For SchemaStatements' do