Always validate record if validating a virtual attribute

Fixes #23645

When you're using an `attr_accessor` for a record instead of an
attribute in the database there's no way for the record to know if it
has `changed?` unless you tell it `attribute_will_change!("attribute")`.

The change made in 27aa4dd updated validations to check if a record was
`changed?` or `marked_for_destruction?` or not `persisted?`. It did not
take into account virtual attributes that do not affect the model's
dirty status.

The only way to fix this is to always validate the record if the
attribute does not belong to the set of attributes the record expects
(in `record.attributes`) because virtual attributes will not be in that
hash.

I think we should consider deprecating this particular behavior in the
future and requiring that the user mark the record dirty by noting that
the virtual attribute will change. Unfortunately this isn't easy because
we have no way of knowing that you did the "right thing" in your
application by marking it dirty and will get the deprecation warning
even if you are doing the correct thing.

For now this restores expected behavior when using a virtual attribute
by always validating the record, as well as adds tests for this case.

I was going to add the `!record.attributes.include?(attribute)` to the
`should_validate?` method but `uniqueness` cannot validate a virtual
attribute with nothing to hold on to the attribute. Because of this
`should_validate?` was about to become a very messy method so I decided
to split them up so we can handle it specifically for each case.

1 files changed, 7 insertions, 0 deletions

diff --git a/activemodel/lib/active_model/validator.rb b/activemodel/lib/active_model/validator.rb
index 1d2888a818..21339b628e 100644
--- a/activemodel/lib/active_model/validator.rb
+++ b/activemodel/lib/active_model/validator.rb
@@ -167,6 +167,13 @@ module ActiveModel
     def should_validate?(record) # :nodoc:
       !record.persisted? || record.changed? || record.marked_for_destruction?
     end
+
+    # Always validate the record if the attribute is a virtual attribute.
+    # We have no way of knowing that the record was changed if the attribute
+    # does not exist in the database.
+    def unknown_attribute?(record, attribute) # :nodoc:
+      !record.attributes.include?(attribute.to_s)
+    end
   end
 
   # +BlockValidator+ is a special +EachValidator+ which receives a block on initialization