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In the Django framework, custom model field validation can be implemented in the following ways:

• Using the clean() method:
  Define a clean() method in the model to validate the model’s fields. This method is automatically called when invoking the full_clean() method and is commonly used in form validation.

  from django.core.exceptions import ValidationError
  
  class MyModel(models.Model):
      my_field = models.CharField(max_length=100)
  
      def clean(self):
          if 'bad_value' in self.my_field:
              raise ValidationError('my_field cannot contain "bad_value"')

• Using the clean_<fieldname>() method:
  For validating specific fields, you can define a method prefixed with clean_ followed by the field name.

  class MyModel(models.Model):
      my_field = models.CharField(max_length=100)
  
      def clean_my_field(self):
          if 'bad_value' in self.my_field:
              raise ValidationError('my_field cannot contain "bad_value"')

• Using the validators parameter:
  When defining a field, you can add custom validators through the validators parameter.

  from django.core.validators import RegexValidator
  
  def my_custom_validator(value):
      if 'bad_value' in value:
          raise ValidationError('Value cannot contain "bad_value"')
  
  class MyModel(models.Model):
      my_field = models.CharField(max_length=100, validators=[my_custom_validator])

• Using the override method:
  If you need to perform validation when saving the model, you can override the save() method.

  class MyModel(models.Model):
      my_field = models.CharField(max_length=100)
  
      def save(self, *args, **kwargs):
          if 'bad_value' in self.my_field:
              raise ValidationError('my_field cannot contain "bad_value"')
          super().save(*args, **kwargs)

• Using signals:
  Use Django signals, such as pre_save or post_save, to perform validation before or after saving the model.

  from django.db.models.signals import pre_save
  from django.dispatch import receiver
  
  @receiver(pre_save, sender=MyModel)
  def my_model_pre_save(sender, instance, **kwargs):
      if 'bad_value' in instance.my_field:
          raise ValidationError('my_field cannot contain "bad_value"')

These are several common methods for custom field validation in Django models. Choose the appropriate method based on your specific requirements.