Fall back to parent locale before it falls back to the :errors namespace

activemodel/lib/active_model/errors.rb

@@ -479,6 +479,14 @@ module ActiveModel
     # * <tt>errors.messages.blank</tt>
     def generate_message(attribute, type = :invalid, options = {})
       type = options.delete(:message) if options[:message].is_a?(Symbol)
+      value = (attribute != :base ? @base.send(:read_attribute_for_validation, attribute) : nil)
+
+      options = {
+        model: @base.model_name.human,
+        attribute: @base.class.human_attribute_name(attribute),
+        value: value,
+        object: @base
+      }.merge!(options)
 
       if @base.class.respond_to?(:i18n_scope)
         i18n_scope = @base.class.i18n_scope.to_s
@@ -487,6 +495,11 @@ module ActiveModel
             :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
         end
         defaults << :"#{i18n_scope}.errors.messages.#{type}"
+
+        catch(:exception) do
+          translation = I18n.translate(defaults.first, options.merge(default: defaults.drop(1), throw: true))
+          return translation unless translation.nil?
+        end unless options[:message]
       else
         defaults = []
       end
@@ -496,15 +509,7 @@ module ActiveModel
 
       key = defaults.shift
       defaults = options.delete(:message) if options[:message]
-      value = (attribute != :base ? @base.send(:read_attribute_for_validation, attribute) : nil)
-
-      options = {
-        default: defaults,
-        model: @base.model_name.human,
-        attribute: @base.class.human_attribute_name(attribute),
-        value: value,
-        object: @base
-      }.merge!(options)
+      options[:default] = defaults
 
       I18n.translate(key, options)
     end

activerecord/test/cases/validations/i18n_generate_message_validation_test.rb

@@ -4,16 +4,20 @@ require "cases/helper"
 require "models/topic"
 
 class I18nGenerateMessageValidationTest < ActiveRecord::TestCase
+  class Backend < I18n::Backend::Simple
+    include I18n::Backend::Fallbacks
+  end
+
   def setup
     Topic.clear_validators!
     @topic = Topic.new
-    I18n.backend = I18n::Backend::Simple.new
+    I18n.backend = Backend.new
   end
 
   def reset_i18n_load_path
     @old_load_path, @old_backend = I18n.load_path.dup, I18n.backend
     I18n.load_path.clear
-    I18n.backend = I18n::Backend::Simple.new
+    I18n.backend = Backend.new
     yield
   ensure
     I18n.load_path.replace @old_load_path
@@ -83,4 +87,16 @@ class I18nGenerateMessageValidationTest < ActiveRecord::TestCase
       assert_equal "Custom taken message", @topic.errors.generate_message(:title, :taken, value: "title")
     end
   end
+
+  test "activerecord attributes scope falls back to parent locale before it falls back to the :errors namespace" do
+    reset_i18n_load_path do
+      I18n.backend.store_translations "en", activerecord: { errors: { models: { topic: { attributes: { title: { taken: "custom en message" } } } } } }
+      I18n.backend.store_translations "en-US", errors: { messages: { taken: "generic en-US fallback" } }
+
+      I18n.with_locale "en-US" do
+        assert_equal "custom en message", @topic.errors.generate_message(:title, :taken, value: "title")
+        assert_equal "generic en-US fallback", @topic.errors.generate_message(:heading, :taken, value: "heading")
+      end
+    end
+  end
 end