NEW ExistenceValidator to check if value exists in model or not. It's better to define model() static method in the models to aviod new model initation and open a new connection to database. https://github.com/wajdijurry/phalcon/blob/master/Validation/Validator/ExistenceValidator.php

Have fun

What's the difference compared to Uniqueness. I mean what are the cases when you could use this?

Did you got look at the validator ? It checks if value is exists under specific column, on contrary of uniquness validator which checks for duplication. This validator takes additional conditions to append in the query. It may become useful sometimes.