[Answered ]-Django 1054, "Unknown column 'emp_id' in 'field list"

here how I solved it using SerializerMethodField,

emp_id = serializers.SerializerMethodField('emp_id_from_nickname') 

here, emp_id_from_nickname is the method, which helps to add value into my field.

def emp_id_from_nickname(self, model_obj):
   get_id = Employee.objects.get(usename=model_obj.username).emp_id
   return get_id

class Meta:
    model = User
    fields = ('id', 'username', 'first_name', 'last_name', 'emp_id')

Change your serializer field to,

emp_id = serializers.PrimaryKeyRelatedField(queryset=Employee.objects.all())