Support ActiveModel::Error translation lookup on indexed attributes.

Instances of ActiveModel::Error where `attribute` is a nested attribute
can use translation keys that don't include the index in their lookup key.

activemodel/lib/active_model/error.rb

@@ -69,9 +69,14 @@ module ActiveModel
 
       if base.class.respond_to?(:i18n_scope)
         i18n_scope = base.class.i18n_scope.to_s
+        unindexed_attribute = attribute.to_s.remove(/\[\d\]/)
+
         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}" ]
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{unindexed_attribute}.#{type}",
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}"
+          ]
         end
         defaults << :"#{i18n_scope}.errors.messages.#{type}"
 

activemodel/test/cases/error_test.rb

@@ -26,6 +26,16 @@ class ErrorTest < ActiveModel::TestCase
     end
   end
 
+  class Manager < Person
+    def read_attribute_for_validation(attr)
+      try(attr)
+    end
+
+    def self.i18n_scope
+      :activemodel
+    end
+  end
+
   def test_initialize
     base = Person.new
     error = ActiveModel::Error.new(base, :name, :too_long, foo: :bar)
@@ -144,6 +154,15 @@ class ErrorTest < ActiveModel::TestCase
     }
   end
 
+  test "message with type as a symbol and indexed attribute can lookup without index in attribute key" do
+    I18n.backend.store_translations(:en, activemodel: { errors: { models: { 'error_test/manager': {
+      attributes: { reports: { name: { presence: "must be present" } } } } } } })
+
+    error = ActiveModel::Error.new(Manager.new, :'reports[0].name', :presence)
+
+    assert_equal "must be present", error.message
+  end
+
   test "message uses current locale" do
     I18n.backend.store_translations(:en, errors: { messages: { inadequate: "Inadequate %{attribute} found!" } })
     error = ActiveModel::Error.new(Person.new, :name, :inadequate)