Make sure nested base errors are translatable

If the user defined a translation to a nested error on base we should
look it up in the same way we do for the other attributes.

If no translation is set, we fallback to the name of the association.

Fixes #48884.

activemodel/lib/active_model/error.rb

@@ -49,9 +49,7 @@ def self.full_message(attribute, message, base) # :nodoc:
       defaults << :"errors.format"
       defaults << "%{attribute} %{message}"
 
-      attribute = attribute.remove(/\.base\z/)
-
-      attr_name = attribute.tr(".", "_").humanize
+      attr_name = attribute.remove(/\.base\z/).tr(".", "_").humanize
       attr_name = base_class.human_attribute_name(attribute, {
         default: attr_name,
         base: base,

activemodel/test/cases/error_test.rb

@@ -219,7 +219,7 @@ def test_initialize
 
   test "full_message returns the given message when the attribute contains base" do
     error = ActiveModel::Error.new(Person.new, :"foo.base", "press the button")
-    assert_equal "foo press the button", error.full_message
+    assert_equal "foo.base press the button", error.full_message
   end
 
   # details

activerecord/test/cases/autosave_association_test.rb

@@ -599,6 +599,7 @@ def self.name; "Person"; end
     assert_not_predicate p, :valid?
     assert_equal [{ error: "should be favorite" }], p.errors.details[:"references[1].base"]
     assert_equal "should be favorite", p.errors[:"references[1].base"].first
+    assert_equal ["References[1] should be favorite"], p.errors.full_messages
   end
 
   def test_indexed_errors_should_be_properly_translated
@@ -649,6 +650,55 @@ def self.name; "Person"; end
     I18n.backend = I18n::Backend::Simple.new
   end
 
+  def test_indexed_errors_on_base_attribute_should_be_properly_translated
+    I18n.backend.store_translations(
+      :en,
+      activerecord: {
+        attributes: {
+          person: {
+            reference: "Super reference",
+          },
+          reference: {
+            base: ""
+          }
+        }
+      }
+    )
+    reference = Class.new(ActiveRecord::Base) do
+      self.table_name = "references"
+      def self.name; "Reference"; end
+
+      validate :should_be_favorite
+      validates_presence_of :job_id
+
+      private
+        def should_be_favorite
+          errors.add(:base, "should be favorite") unless favorite?
+        end
+    end
+
+    person = Class.new(ActiveRecord::Base) do
+      self.table_name = "people"
+      def self.name; "Person"; end
+
+      has_one :reference, autosave: true, anonymous_class: reference
+      validates :reference, presence: true
+    end
+
+    p = person.new
+    assert_not_predicate p, :valid?
+    assert_equal ["Super reference can’t be blank"], p.errors.full_messages
+
+    reference_invalid = reference.new(favorite: false)
+    p.reference = reference_invalid
+
+    assert_not_predicate reference_invalid, :valid?
+    assert_not_predicate p, :valid?
+    assert_equal [" should be favorite", "Reference job can’t be blank"], p.errors.full_messages
+  ensure
+    I18n.backend = I18n::Backend::Simple.new
+  end
+
   def test_errors_details_should_be_indexed_when_global_flag_is_set
     old_attribute_config = ActiveRecord.index_nested_attribute_errors
     ActiveRecord.index_nested_attribute_errors = true