title: Validate specific field in DRF serializer
body: <p>I have a model with a <code>JSONField</code>.</p>
  
  <p><code>model.py</code></p>
  
  <pre><code>class Categories(models.Model):
       type = models.CharField(max_length=20)
       name = models.CharField(max_length=500)
       details = JSONField(blank=True, null=True)
  </code></pre>
  
  <p>Currently I'm using <code>serializers.ModelSerializer</code> for serializing this above model:</p>
  
  <p><code>serializers.py</code></p>
  
  <pre><code>class CategoriesSerializer(serializers.ModelSerializer):
       class Meta:
           model = Categories
           fields = ('id', 'type', 'name', 'details')
  </code></pre>
  
  <p>Due to this, the details field is only checked to contain <code>valid json</code>. What i really need to do is to perform some custom validation based on the <strong>Json Schema</strong> defined for details field. However since I don't want any other custom validations for rest of the fields, I would like to keep using the validations provided by <code>serializers.ModelSerializer</code>. Is there any way I can override a validation for just one field, probably by writing a custom serializer <em>for just the details field</em>?</p>
  
  <blockquote>
    <p>Note question is not about how to write a custom Validator, but about how to use that custom validator on a field in the serializer inheriting <code>ModelSerializer</code></p>
  </blockquote>