According to the Rails Guides for i18n regarding Error Message Scopes (5.1.1) for Active Record validation error messages, what you're attempting to do should work:
Consider a User model with a validation for the name attribute like this:
class User < ActiveRecord::Base
validates :name, :presence => true
end
<...snip...>
When your models are additionally using inheritance then the messages are looked up in the inheritance chain.
For example, you might have an Admin model inheriting from User:
class Admin < User
validates :name, :presence => true
end
Then Active Record will look for messages in this order:
activerecord.errors.models.admin.attributes.name.blank
activerecord.errors.models.admin.blank
activerecord.errors.models.user.attributes.name.blank
activerecord.errors.models.user.blank
activerecord.errors.messages.blank
errors.attributes.name.blank
errors.messages.blank
This way you can provide special translations for various error messages at different points in your models inheritance chain and in the attributes, models, or default scopes.
So, in your case, assuming your classes look something like this:
app/models/user.rb
User < ActiveRecord::Base
validates :username, :format => {/regex/}, :message => :name_format
end
app/models/user_subclass.rb
UserSubclass < User
validates :username, :format => {/regex/}, :message => :name_format
end
and your config/locales/en.yml looks something like:
en:
activerecord:
errors:
models:
user:
attributes:
username:
name_format: 'has the way-wrong format, bro!'
then the message searching for a validation on UserSubClass
should go:
activerecord.errors.models.user_sublcass.attributes.username.name_format # fail
activerecord.errors.models.user_sublcass.name_format # fail
activerecord.errors.models.user.attributes.username.name_format # success
activerecord.errors.models.user.name_format
# ...
Assuming that your model files and yaml files look similar to what's above, then the potential complication you mentioned may be the issue, but obviously I can't be certain.