Allow to override the full_message error format

activemodel/lib/active_model/errors.rb

@@ -364,12 +364,51 @@ module ActiveModel
     # Returns a full message for a given attribute.
     #
     #   person.errors.full_message(:name, 'is invalid') # => "Name is invalid"
+    #
+    #   The `"%{attribute} %{message}"` error format can be overridden with either
+    #
+    # * <tt>activemodel.errors.models.person/contacts/addresses.attributes.street.format</tt>
+    # * <tt>activemodel.errors.models.person/contacts/addresses.format</tt>
+    # * <tt>activemodel.errors.models.person.attributes.name.format</tt>
+    # * <tt>activemodel.errors.models.person.format</tt>
+    # * <tt>errors.format</tt>
     def full_message(attribute, message)
       return message if attribute == :base
+
+      if @base.class.respond_to?(:i18n_scope)
+        parts = attribute.to_s.split(".")
+        attribute_name = parts.pop
+        namespace = parts.join("/") unless parts.empty?
+        attributes_scope = "#{@base.class.i18n_scope}.errors.models"
+
+        if namespace
+          defaults = @base.class.lookup_ancestors.map do |klass|
+            [
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.attributes.#{attribute_name}.format",
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.format",
+            ]
+          end
+        else
+          defaults = @base.class.lookup_ancestors.map do |klass|
+            [
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}.attributes.#{attribute_name}.format",
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}.format",
+            ]
+          end
+        end
+
+        defaults.flatten!
+      else
+        defaults = []
+      end
+
+      defaults << :"errors.format"
+      defaults << "%{attribute} %{message}"
+
       attr_name = attribute.to_s.tr(".", "_").humanize
       attr_name = @base.class.human_attribute_name(attribute, default: attr_name)
-      I18n.t(:"errors.format",
-        default:  "%{attribute} %{message}",
+      I18n.t(defaults.shift,
+        default:  defaults,
         attribute: attr_name,
         message:   message)
     end

activemodel/test/cases/validations/i18n_validation_test.rb

@@ -42,6 +42,42 @@ class I18nValidationTest < ActiveModel::TestCase
     assert_equal ["Field Name empty"], @person.errors.full_messages
   end
 
+  def test_errors_full_messages_uses_attribute_format
+    I18n.backend.store_translations("en", activemodel: {
+      errors: { models: { person: { attributes: { name: { format: "%{message}" } } } } } })
+
+    person = Person.new
+    assert_equal "cannot be blank", person.errors.full_message(:name, "cannot be blank")
+    assert_equal "Name test cannot be blank", person.errors.full_message(:name_test, "cannot be blank")
+  end
+
+  def test_errors_full_messages_uses_model_format
+    I18n.backend.store_translations("en", activemodel: {
+      errors: { models: { person: { format: "%{message}" } } } })
+
+    person = Person.new
+    assert_equal "cannot be blank", person.errors.full_message(:name, "cannot be blank")
+    assert_equal "cannot be blank", person.errors.full_message(:name_test, "cannot be blank")
+  end
+
+  def test_errors_full_messages_uses_deeply_nested_model_attributes_format
+    I18n.backend.store_translations("en", activemodel: {
+      errors: { models: { 'person/contacts/addresses': { attributes: { street: { format: "%{message}" } } } } } })
+
+    person = Person.new
+    assert_equal "cannot be blank", person.errors.full_message(:'contacts/addresses.street', "cannot be blank")
+    assert_equal "Contacts/addresses country cannot be blank", person.errors.full_message(:'contacts/addresses.country', "cannot be blank")
+  end
+
+  def test_errors_full_messages_uses_deeply_nested_model_model_format
+    I18n.backend.store_translations("en", activemodel: {
+      errors: { models: { 'person/contacts/addresses': { format: "%{message}" } } } })
+
+    person = Person.new
+    assert_equal "cannot be blank", person.errors.full_message(:'contacts/addresses.street', "cannot be blank")
+    assert_equal "cannot be blank", person.errors.full_message(:'contacts/addresses.country', "cannot be blank")
+  end
+
   # ActiveModel::Validations
 
   # A set of common cases for ActiveModel::Validations message generation that