title: unable to change validate field error message in Model serializer
body: <p>I'm working on <code>ModelSerializer</code> I'm facing these below issues.</p>
  
  <p>1) Unable to validate <code>.validate(self, data)</code> method as I want to return a custom validator message which is not working.</p>
  
  <p>model.py</p>
  
  <pre><code>class BlogModel(models.Model):
  
      BLOG_STATUS = (
          ('PUBLISH', 'Publish'),
          ('DRAFT', 'Draft'),
      )
  
      blog_id = models.AutoField(primary_key=True)
      user = models.ForeignKey(
          User, on_delete=models.CASCADE, related_name='blogs')
      title = models.CharField(max_length=255)
      content = models.TextField(blank=True, null=True)
      status = models.CharField(max_length=7, choices=BLOG_STATUS)
      created_at = models.DateTimeField(auto_now_add=True)
      updated_at = models.DateTimeField(auto_now=True)
  
      class Meta():
          db_table = 'blogs'
          verbose_name = 'Blog'
          verbose_name_plural = 'Blogs'
  
      def __str__(self):
          return self.title
  
  </code></pre>
  
  <p>serializers.py</p>
  
  <pre><code>
  import datetime
  from django.contrib.auth.models import User
  from django.utils.timezone import now
  from rest_framework import serializers
  from rest_framework.serializers import ModelSerializer,Serializer
  from blogs.models import BlogModel, BlogFilesModel
  
  class UserSerializer(ModelSerializer):
      class Meta:
          model = User
          fields = '__all__'
          # exclude = ['password']
  
  
  class BlogFilesSerializer(ModelSerializer):
      count_files = serializers.IntegerField(required=False)
  
      class Meta:
          model = BlogFilesModel
          fields = ('blog_files_id', 'blog', 'path',
                    'created_at', 'updated_at', 'count_files')
  
      def get_path(self, obj):
          formatted_date = obj.created_at.strftime("%d-%m-%Y")
          return formatted_date
  
  class BlogSerializer(ModelSerializer):
      blog_files = serializers.SerializerMethodField()
  
      def get_blog_files(self, obj):
          info = BlogFilesSerializer(BlogFilesModel.objects.filter(
              blog=obj).order_by('-pk'), many=True)
  
          if info.data:
              for i in info.data:
                  user_detail = User.objects.get(pk=obj.user.id)
                  i.__setitem__('user_detail', UserSerializer(user_detail).data)
                  if i.get('user_detail'):
                      try:
                          del i['user_detail']['password']
                      except expression as identifier:
                          pass
  
          return info.data
  
      blog_created_at=serializers.SerializerMethodField()
  
      def get_blog_created_at(self, obj):
          formatted_date=obj.created_at.strftime("%d-%m-%Y")
          return formatted_date
  
      def validate(self, data):
          if data.get('title') == "":
              #this below error message not showing
              raise serializers.ValidationError("Title field must not be left blank.") 
          return data
  
      def validate_title(self, value):
          if value != "Hello":
              raise serializers.ValidationError("Title field must be 'Hello' ")
          return value
  
      def validate_content(self, value):
          if len(value) &lt; 2:
              raise serializers.ValidationError("Content must have at least 2 letters")
          return value
  
      class Meta:
          model=BlogModel
          fields=('blog_id', 'user', 'title', 'content',
                    'status', 'blog_files', 'blog_created_at')
  
  
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>
  class BlogViewSet(viewsets.ModelViewSet):
      queryset = BlogModel.objects.all()
      lookup_field = 'blog_id'
      serializer_class = BlogSerializer
  
  </code></pre>
  
  <p>below is the screen shot of my output</p>
  
  <p><a href="https://i.stack.imgur.com/RZWa1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RZWa1.png" alt="validation output while creating new object"></a></p>
  
  <p>Hope is have explained my problem well. </p>
  
  <p>Thank you.</p>