set_leader_serializer.py

class SetLeaderSerializer(serializers.Serializer):
    user = serializers.PrimaryKeyRelatedField(queryset=CustomUser.objects.all())
    leader = serializers.PrimaryKeyRelatedField(queryset=CustomUser.objects.all())
    
    def validate(self, data):
        """
        Check that a user can't be a leader of himself.
        Check that a circular relationship is not allowed.
        """
        data = super().validate(data)

        user = data["user"]
        leader = data["leader"]
        
        if user.id == leader.id:
            raise ValidationError("The user can't be the same as the leader.")
        
        if leader.leader_id == user.id:
            raise ValidationError("Circular relationship not allowed.")

        return data