I will show two different validations, one with the validation of one attribute of the serializer and the second with the validation of different attributes.
validation of different attributes in validate():
# serializers.py class ProjectCreateSerializer(serializers.ModelSerializer): class Meta: model = Project fields = [ "id", "name", "description", "project_type", ] def validate(self, attrs): if ( self.context["view"] .project.filter(name=attrs["name"], project_type=attrs["project_type"]) .exists() ): raise serializers.ValidationError("Attention! This project exists already.") return attrs self.context["view"].project this project property/attribute is created in view (ProjectViewSet). With this syntax we can get the project property and filter for existing projects. If this combination of name and project_type is found, the ValidationError will be raised. Otherwise the project will be validated.
validation with one attributes in validate_user():
validate_user the user part is the name of the attribute from the serializer. If you have an attribute project the function name would be validate_project().
# serializers.py class ContributorSerializer(serializers.ModelSerializer): # create attribute 'user', which is write_only because we just need to give a value user = serializers.IntegerField(write_only=True) class Meta: model = UserModel fields = ["id", "user"] def validate_user(self, value): user = UserModel.objects.filter(pk=value).first() if user is None: raise serializers.ValidationError("User does not exists!") if user.is_superuser: raise serializers.ValidationError( "Superusers cannot be added as contributors." ) if self.context["view"].project.contributors.filter(pk=value).exists(): raise serializers.ValidationError( "This user is already a contributor of this project." ) return user 
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