Where does input validation happen in MVC?

Question

Ok, this has probably been asked before but i cant find a definative answer. Where in the MVC pattern should validation of input happen?

I would like to say that things like empty fields and basic general validation should happen in the controller and that rules as lengths and valid characters of for example usernames / passwords etc should happen at the model layer.

However, this means spreading this burdon around the application which surely cant be good either?

Sorry if this question is naieve but I am relatively new to this type of programming and want to get things correct from the start.

Answer 1

Validation is the job of the model.

As models have various attributes (fields), only the models can know what combination of inputs make that model valid. It's not just about whether a field is blank, or the input of that field matches some pattern, but sometimes this is a combination of field inputs, or the model's relationship to other models that determine the valid state.

Your model should encapsulate this logic so you can interrogate it ("are you valid?") and not have it spread across other parts of your code.