Pass through the `prepend` option to `AS::Callback`

I'm not sure what's the use case for this, but apparently it broke some apps.
Since it was not the intended result from #16210 I fixed it to not raise an
exception anymore. However, I didn't add documentation for it because I don't
know if this should be officially supported without knowing how it's meant to
be used.

In general, validations should be side-effect-free (other than adding to the
error message to `@errors`). Order-dependent validations seems like a bad idea.

Fixes #18002

1 files changed, 1 insertions, 1 deletions

diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index c1e344b215..6a2668b8f7 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -87,7 +87,7 @@ module ActiveModel
         validates_with BlockValidator, _merge_attributes(attr_names), &block
       end
 
-      VALID_OPTIONS_FOR_VALIDATE = [:on, :if, :unless].freeze
+      VALID_OPTIONS_FOR_VALIDATE = [:on, :if, :unless, :prepend].freeze
 
       # Adds a validation method or block to the class. This is useful when
       # overriding the +validate+ instance method becomes too unwieldy and