3 files changed, 21 insertions, 4 deletions
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index f0e1242554..0d9e761b1e 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -141,7 +141,9 @@ module ActiveModel
       @mutations_from_database = nil
     end
 
-    def changes_applied # :nodoc:
+    # Clears dirty data and moves +changes+ to +previously_changed+ and
+    # +mutations_from_database+ to +mutations_before_last_save+ respectively.
+    def changes_applied
       unless defined?(@attributes)
         @previously_changed = changes
       end
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index af94d52d45..969effdc20 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -112,6 +112,17 @@ module ActiveModel
       @details.merge!(other.details) { |_, ary1, ary2| ary1 + ary2 }
     end
 
+    # Removes all errors except the given keys. Returns a hash containing the removed errors.
+    #
+    #   person.errors.keys                  # => [:name, :age, :gender, :city]
+    #   person.errors.slice!(:age, :gender) # => { :name=>["cannot be nil"], :city=>["cannot be nil"] }
+    #   person.errors.keys                  # => [:age, :gender]
+    def slice!(*keys)
+      keys = keys.map(&:to_sym)
+      @details.slice!(*keys)
+      @messages.slice!(*keys)
+    end
+
     # Clear the error messages.
     #
     #   person.errors.full_messages # => ["name cannot be nil"]
@@ -327,11 +338,11 @@ module ActiveModel
     #  person.errors.added? :name, :too_long                                # => false
     #  person.errors.added? :name, "is too long"                            # => false
     def added?(attribute, message = :invalid, options = {})
+      message = message.call if message.respond_to?(:call)
+
       if message.is_a? Symbol
-        self.details[attribute.to_sym].map { |e| e[:error] }.include? message
+        details[attribute.to_sym].include? normalize_detail(message, options)
       else
-        message = message.call if message.respond_to?(:call)
-        message = normalize_message(attribute, message, options)
         self[attribute].include? message
       end
     end
diff --git a/activemodel/lib/active_model/type/decimal.rb b/activemodel/lib/active_model/type/decimal.rb
index e8ee18c00e..b37dad1c41 100644
--- a/activemodel/lib/active_model/type/decimal.rb
+++ b/activemodel/lib/active_model/type/decimal.rb
@@ -12,6 +12,10 @@ module ActiveModel
         :decimal
       end
 
+      def serialize(value)
+        cast(value)
+      end
+
       def type_cast_for_schema(value)
         value.to_s.inspect
       end