title: How to write a Custom field Validation for ModelSerializers in Django Rest Framework(DRF) similar to Form Validation in Django?
body: <p>I am currently creating an api based on DRF.I have a model which is like:</p>
  
  <pre><code>class Task(models.Model):
      name = models.CharField(max_length = 255)
      completed = models.BooleanField(default = False)
      description = models.TextField()
      text = models.TextField(blank = False, default = "this is my text" )
  
      def __unicode__(self):
          return self.name
  </code></pre>
  
  <p>and the corresponding Serializer for this model is as :</p>
  
  <pre><code>class TaskSerializer(serializers.ModelSerializer):
      class Meta:
          model = Task
          fields = ('name','description','completed','text')
  </code></pre>
  
  <p>Now my question is that I want to validate the 'name' field of my model while taking up data. 
  For instance I may end checking the first name or second name of the user by a Python code similar to a Django Form:</p>
  
  <pre><code>def clean_name(self):
      name = form.cleaned_data.get('name')
      first,second = name.split(' ')
      if second is None:
          raise forms.ValidationError("Please enter full name")
  </code></pre>
  
  <p>I know of something called 'validate_(fieldname)' in Serializers.serializer class. But I want this to be used in Serializers.ModelSerializer instead.(Just similar to the custom forms validation in Django)</p>