serializers.py

from rest-framework import serializers
from .models import Review

class ReviewSerializer(serializers.Serializer):
  class Meta : 
    model = Review
    fields = ['username','content']
    
  # to validate any field override the function validate_<field>
  def validate_username(self,value):
    if len(value) > 20 : 
      raise serializers.ValidationError("the length of your username should be less than 20 character")
    return value
  
  # to validate on the serializer level
  def validate(self, data):
    if data['username'] == data['content'] : 
      raise serializers.ValidationError("the content value should not be the same as the username value ")
    return value