counting related records

I have 2 related models, model_A and model_B There's a 1:n relationship among them Meaning one record in model_A can have several related records in model_B I'd like model_A to have a generated (and stored) column Such generated column should be boolean It should be set to true if there are 1 or more related records in model_B It should be false if there are 0 related records in model_B How do I write the expression to generate such boolean values ?