[ruby-on-rails] Rails 2.3.4 Persisting Model on Validation Failure

I have a standard html form post that is being persisted and validated with a rails model on the server side.

For the sake of discussion, lets call this a "car" model.

When

car.save 

is invoked the validators fire and set a list of fields in error within the model.

How best can I get an object back that has the fields whose validation failed removed, i.e. such that I don't have to delete the fields on the form the user entered correctly?

The validation patterns they use are great but how do I get a hook to when the validation fails such that I can delete the bogus entries from the model?

I could see manually iterating over the errors list and writing a big case statement but that seems like a hack.

In your controller, render the new action from your create action if validation fails, with an instance variable, @car populated from the user input (i.e., the params hash). Then, in your view, add a logic check (either an if block around the form or a ternary on the helpers, your choice) that automatically sets the value of the form fields to the params values passed in to @car if car exists. That way, the form will be blank on first visit and in theory only be populated on re-render in the case of error. In any case, they will not be populated unless @car is set.