If an attribute alias reaches method missing, send the original

Fixes rails#51717

activemodel/lib/active_model/attribute_methods.rb

@@ -480,6 +480,10 @@ def method_missing(method, ...)
       if respond_to_without_attributes?(method, true)
         super
       else
+        # If the missing method is an attribute alias, let's dispatch the original.
+        if self.class.attribute_alias?(method) && self.class.send(:instance_method_already_implemented?, method)
+          method = self.class.attribute_alias(method).to_sym
+        end
         match = matched_attribute_method(method.name)
         match ? attribute_missing(match, ...) : super
       end
@@ -505,7 +509,7 @@ def respond_to?(method, include_private_methods = false)
         # but found among all methods. Which means that the given method is private.
         false
       else
-        !matched_attribute_method(method.to_s).nil?
+        self.class.send(:instance_method_already_implemented?, method) || !matched_attribute_method(method.to_s).nil?
       end
     end
 

activemodel/test/cases/attribute_methods_test.rb

@@ -19,6 +19,10 @@ def attributes
   def attribute(name)
     attributes[name.to_sym]
   end
+
+  def attribute_method?(name)
+    attributes.include?(name.to_sym)
+  end
 end
 
 class ModelWithAttributes2
@@ -236,11 +240,11 @@ def foo
   end
 
   test "#undefine_attribute_methods removes attribute methods" do
-    ModelWithAttributes.define_attribute_methods(:foo)
-    ModelWithAttributes.undefine_attribute_methods
+    ModelWithoutAttributesMethod.define_attribute_methods(:foo)
+    assert_respond_to ModelWithoutAttributesMethod.new, :foo
 
-    assert_not_respond_to ModelWithAttributes.new, :foo
-    assert_raises(NoMethodError) { ModelWithAttributes.new.foo }
+    ModelWithoutAttributesMethod.undefine_attribute_methods
+    assert_not_respond_to ModelWithoutAttributesMethod.new, :foo
   end
 
   test "#undefine_attribute_methods undefines alias attribute methods" do
@@ -341,6 +345,25 @@ def m.attribute_missing(match, *args, &block)
     assert_equal "attribute_test", match.proxy_target
   end
 
+  test "should use attribute_missing to dispatch a missing attribute alias" do
+    klass = Class.new(ModelWithAttributes) do
+      define_attribute_method(:foo)
+      alias_attribute :foo2, :foo
+    end
+
+    m = klass.new
+    def m.attribute_missing(match, ...)
+      match
+    end
+    klass.undef_method :foo
+    klass.undef_method :foo2
+
+    match = m.foo2
+
+    assert_equal "foo",       match.attr_name
+    assert_equal "attribute", match.proxy_target
+  end
+
   module NameClash
     class Model1
       include ActiveModel::AttributeMethods