Question

Using Rails 3 validations and/or callbacks, what would be the cleanest way to ensure that only one record of a model has a boolean value ticked as true? I’d like to mark one record as the currently active model.

(I know another option is to use a has-one association, but I’m curious to know how to store this more directly in the model records.)

Answer 1

If all you want to do is validate it you can use

validates_uniqueness_of :boolean_attribute, if: :boolean_attribute

Just drop that in your model class. That will validate that the model has only one boolean_attribute set to true.

Note that you will have to work around the atomicity of swapping the boolean_attribute from one instance to another.

Depending on what database you are using you might be able to resolve it using a transaction. If your database doesn’t support transactions you might have to figure out a better way to guarantee data consistency (such us having a dedicated model that points to the “active” model and removing boolean_attribute altogether, or replacing boolean_attribute with an integer that can be atomically incremented (highest number representing the active one).