1 files changed, 19 insertions, 7 deletions
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 942b4fa9bb..971bdd08b1 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support/core_ext/array/conversions"
 require "active_support/core_ext/string/inflections"
 require "active_support/core_ext/object/deep_dup"
@@ -93,6 +95,18 @@ module ActiveModel
       @details  = other.details.dup
     end
 
+    # Merges the errors from <tt>other</tt>.
+    #
+    # other - The ActiveModel::Errors instance.
+    #
+    # Examples
+    #
+    #   person.errors.merge!(other)
+    def merge!(other)
+      @messages.merge!(other.messages) { |_, ary1, ary2| ary1 + ary2 }
+      @details.merge!(other.details) { |_, ary1, ary2| ary1 + ary2 }
+    end
+
     # Clear the error messages.
     #
     #   person.errors.full_messages # => ["name cannot be nil"]
@@ -384,21 +398,19 @@ module ActiveModel
       type = options.delete(:message) if options[:message].is_a?(Symbol)
 
       if @base.class.respond_to?(:i18n_scope)
-        defaults = @base.class.lookup_ancestors.map do |klass|
-          [ :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
-            :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
+        i18n_scope = @base.class.i18n_scope.to_s
+        defaults = @base.class.lookup_ancestors.flat_map do |klass|
+          [ :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
         end
+        defaults << :"#{i18n_scope}.errors.messages.#{type}"
       else
         defaults = []
       end
 
-      defaults << :"#{@base.class.i18n_scope}.errors.messages.#{type}" if @base.class.respond_to?(:i18n_scope)
       defaults << :"errors.attributes.#{attribute}.#{type}"
       defaults << :"errors.messages.#{type}"
 
-      defaults.compact!
-      defaults.flatten!
-
       key = defaults.shift
       defaults = options.delete(:message) if options[:message]
       value = (attribute != :base ? @base.send(:read_attribute_for_validation, attribute) : nil)