Way back at the beginning of this course, I created Serializers for the course and

review models those Serializers were fairly simple,

thanks to Rest framework's model Serializer.

If you looked over the models in the course's app, you might have noticed

that there is a unique together in the review models metaclass.

The unique together means that a reviewer with a specific email address

can only review a specific course once.

If you were to try to create two reviews for a course and use the same email

you'd get back an error saying that course an email must be unique.

REST rust framework model Serializer automatically sets a validate or

on the Serializer to match what we have on the model.

How then, would you specify your own custom validation on a serializer?

It's very similar to how you handle custom validation on a Django model form.

Currently the rating field on the review model accepts any integer.

I think I'd like the ratings to be an integer between one and five.

Let me jump into Workspaces and build it.

since I'm only trying to validate a single field,

I'm gonna solve this with field level validation.

Okay, so what I'm gonna do is I'm going to add a new method to my ReviewSerializer.

And I'm gonna make sure and come outside of class.

Meta.

That has bit me a couple of times.

And I'm gonna name this one, validate rating.

So just like with field level validation for forms,

you do a set of clean field name, you do validate field name, and

then this is going to get an argument named, value and

that's going to be the value that's passed then to the API For that field.

All right, so if value in range one through six,

then I'm gonna return the value.

If that doesn't happen, then I'm going to raise serializers.ValidationError.

And the message I'm gonna send back is, Rating must be an integer between 1 and 5.

All right cool that sends the information to them.

Now this isn't a political to this current thing but

if you have a field that is not required like for

instance yeah this comment field here because we have a link it was true so

it's not required Then any validator applied to that field

won't fire unless that field is in the provided data.

So somebody comes and they post a review that doesn't include comment

then validate comment would never run.

So all right just a thing to be aware of if you're like why

isn't my validation working.

All right so come back over here to postman and

I want to post a new a new review.

So the course I want to put this on is course number two which is Python testing

my name is Kenneth my email

not emaile is kenneth@teamtreehouse.com.

And I'm just gonna give a rating here.

I really like this course, I think it's an amazing course.

I want to put in a rating of 20 because it's just it's the best course.

I'm gonna send, and looks like my rating has to be an integer between 1 and 5.

I don't have any other choices.

All right, well, I mean,

if the best I can do is 5 then I'll give it a 5 because it's what it is.

And send, all right, cool.

So that's awesome that that works, that enforces my 5.

Now, there is one of the thing to know about and

this applies more to something like this where you've Explicitly named a field and

you can add in a rule here that's like validators equals and then a list.

I'm gonna take that out because I don't need it and

that will list all the functions or custom classes or

whatever that you want it to apply those validations to that field.

So you don't have to do validate whatever.

Maybe you need to do two or

three different validations On the field a couple of or a combination of things.

And if that's the case then you can just provide the validaters key argument

to the field when you create the field.

Field level validation is up and running as usual or provide a link to

the Serializer validation documentation and the teachers notes.

Something I didn't cover though, Is validating the serializer as a whole.

Field level validation only validates a single field, like the rating field.

Sometimes you need to validate multiple fields together, like maybe for user

registration, where you'd have a password field, and a password confirmation field,

and you'd want to make sure that the two fields were equal to each other.

This is covered very well in the Rest framework documentation.

So check the teacher's notes when you need to implement this yourself.