Set default array to details

maintaining behavior errors.details[:foo].any?

activemodel/lib/active_model/errors.rb

@@ -318,7 +318,7 @@ def details
       group_by_attribute.each do |attribute, errors|
         hash[attribute] = errors.map(&:detail)
       end
-      hash
+      DeprecationHandlingDetailsHash.new(hash)
     end
 
     def group_by_attribute
@@ -672,6 +672,14 @@ def <<(message)
     end
   end
 
+  class DeprecationHandlingDetailsHash < SimpleDelegator
+    def initialize(details)
+      details.default = []
+      details.freeze
+      super(details)
+    end
+  end
+
   # Raised when a validation cannot be corrected by end users and are considered
   # exceptional.
   #

activemodel/test/cases/errors_test.rb

@@ -576,6 +576,12 @@ def test_no_key
     assert_equal [:name], person.errors.details.keys
   end
 
+  test "details returns empty array when accessed with non-existent attribute" do
+    errors = ActiveModel::Errors.new(Person.new)
+
+    assert_equal [], errors.details[:foo]
+  end
+
   test "copy errors" do
     errors = ActiveModel::Errors.new(Person.new)
     errors.add(:name, :invalid)