Rails3: Make update/create fail from model?

There's got to be an easy way to do this, but I cannot find an answer...

When some creates or updates a WorkRequest in开发者_高级运维 my app, I do other processing, including creating a Workflow object. I do some checking to make sure, for example, there isn't more than one Workflow already created for this WorkRequest. If there is, I want the update/create to fail with an error message. I just can't see how to do this. I tried returing false from my before_update callback method, but this did not work.

Do I raise an error and rescue it in my controller? What is the right way to do this in Rails 3?

Any help would be much appreciated.

---

This is why you have validations. You can implement an own validation like this:

class ...
  validate :my_validation

  def my_validation
    if workflows > 1
      errors.add(:workflow, "cannot be more than one" )
    end
  end
end