I have a polymorphic `lookups` table with a child `ContractType` model. I have a `before_save` callback in the `ContractType` model that sets a `category`, but it doesn't seem to be working.

```
class Lookup < ActiveRecord::Base
  validates :value, presence: true
  validates :category, presence: true
end

class ContractType < Lookup
  before_save { self.category = "contract_type" }
end

```

Then I open the `rails c`:

```
> c = ContractType.create(value: "test")
> c.errors.messages
=> { :category => ["can't be blank"] }

```

I don't get any errors, just a failed validation. As far as I know, the syntax looks correct, it's just that the `before_save` callback doesn't seem to be working...

Am I missing something obvious here?