Add SelectManager#distinct to set/unset the Arel::Nodes::Distinct.new quantifier

lib/arel/select_manager.rb

@@ -139,6 +139,14 @@ module Arel
       @ctx.projections = projections
     end
 
+    def distinct(value = true)
+      if value
+        @ctx.set_quantifier = Arel::Nodes::Distinct.new
+      else
+        @ctx.set_quantifier = nil
+      end
+    end
+
     def order *expr
       # FIXME: We SHOULD NOT be converting these to SqlLiteral automatically
       @ast.orders.concat expr.map { |x|

test/test_select_manager.rb

@@ -981,5 +981,18 @@ module Arel
         manager.source.must_equal manager.ast.cores.last.source
       end
     end
+
+    describe 'distinct' do
+      it 'sets the quantifier' do
+        table   = Table.new :users
+        manager = Arel::SelectManager.new Table.engine
+
+        manager.distinct
+        manager.ast.cores.last.set_quantifier.class.must_equal Arel::Nodes::Distinct
+
+        manager.distinct(false)
+        manager.ast.cores.last.set_quantifier.must_equal nil
+      end
+    end
   end
 end