Added ActiveRecord::QueryMethods#rewhere which will overwrite an existing, named where condition.

activerecord/CHANGELOG.md

@@ -2,6 +2,16 @@
 
     *Jon Leighton*
 
+*   Added ActiveRecord::QueryMethods#rewhere which will overwrite an existing, named where condition.
+
+	Examples:
+		
+    	Post.where(trashed: true).where(trashed: false)                       #=> WHERE `trashed` = 1 AND `trashed` = 0
+    	Post.where(trashed: true).rewhere(trashed: false)                     #=> WHERE `trashed` = 0
+    	Post.where(active: true).where(trashed: true).rewhere(trashed: false) #=> WHERE `active` = 1 AND `trashed` = 0
+
+	*DHH*
+
 *   Extend ActiveRecord::Base#cache_key to take an optional list of timestamp attributes of which the highest will be used.
 
     Example:

activerecord/lib/active_record/relation/query_methods.rb

@@ -550,6 +550,18 @@ module ActiveRecord
       end
     end
 
+    # Allows you to change a previously set where condition for a given attribute, instead of appending to that condition.
+    #
+    #   Post.where(trashed: true).where(trashed: false)                       #=> WHERE `trashed` = 1 AND `trashed` = 0
+    #   Post.where(trashed: true).rewhere(trashed: false)                     #=> WHERE `trashed` = 0
+    #   Post.where(active: true).where(trashed: true).rewhere(trashed: false) #=> WHERE `active` = 1 AND `trashed` = 0
+    #
+    # This is short-hand for unscope(where: conditions.keys).where(conditions). Note that unlike reorder, we're only unscoping
+    # the named conditions -- not the entire where statement.
+    def rewhere(conditions)
+      unscope(where: conditions.keys).where(conditions)
+    end
+
     # Allows to specify a HAVING clause. Note that you can't use HAVING
     # without also specifying a GROUP clause.
     #

activerecord/test/cases/relation/where_chain_test.rb

@@ -76,5 +76,35 @@ module ActiveRecord
       expected = Arel::Nodes::NotEqual.new(Post.arel_table[@name], 'ruby on rails')
       assert_equal(expected, relation.where_values[1])
     end
+    
+    def test_rewhere_with_one_condition
+      relation = Post.where(title: 'hello').where(title: 'world').rewhere(title: 'alone')
+
+      expected = Arel::Nodes::Equality.new(Post.arel_table[@name], 'alone')
+      assert_equal 1, relation.where_values.size
+      assert_equal expected, relation.where_values.first
+    end
+
+    def test_rewhere_with_multiple_overwriting_conditions
+      relation = Post.where(title: 'hello').where(body: 'world').rewhere(title: 'alone', body: 'again')
+
+      title_expected = Arel::Nodes::Equality.new(Post.arel_table['title'], 'alone')
+      body_expected  = Arel::Nodes::Equality.new(Post.arel_table['body'], 'again')
+
+      assert_equal 2, relation.where_values.size
+      assert_equal title_expected, relation.where_values.first
+      assert_equal body_expected, relation.where_values.second
+    end
+
+    def test_rewhere_with_one_overwriting_condition_and_one_unrelated
+      relation = Post.where(title: 'hello').where(body: 'world').rewhere(title: 'alone')
+
+      title_expected = Arel::Nodes::Equality.new(Post.arel_table['title'], 'alone')
+      body_expected  = Arel::Nodes::Equality.new(Post.arel_table['body'], 'world')
+
+      assert_equal 2, relation.where_values.size
+      assert_equal body_expected, relation.where_values.first
+      assert_equal title_expected, relation.where_values.second
+    end
   end
 end