Custom Fields in Django
Django has a built-in field type for CommaSeparatedIntegerField, but most of the time I’m storing strings, as I already have the integers available elsewhere. As I began to answer the person’s question by giving him an example of usage of serialization + custom properties, until I realized that it would be much easier to just write this as a Field subclass.
So I quickly did, and replaced a few lines of repetitive code with two new field classes in our source:
Update: There were some issues with my understanding of how the metaclass was working. I’ve corrected the code and it should function properly now.
SerializedDataField
This field is typically used to store raw data, such as a dictionary, or a list of items, or could even be used for more complex objects.
from django.db import models
try:
import cPickle as pickle
except:
import pickle
import base64
class SerializedDataField(models.TextField):
"""Because Django for some reason feels its needed to repeatedly call
to_python even after it's been converted this does not support strings."""
__metaclass__ = models.SubfieldBase
def to_python(self, value):
if value is None: return
if not isinstance(value, basestring): return value
value = pickle.loads(base64.b64decode(value))
return value
def get_db_prep_save(self, value):
if value is None: return
return base64.b64encode(pickle.dumps(value))
SeparatedValuesField
An alternative to the CommaSeparatedIntegerField, it allows you to store any separated values. You can also optionally specify a
token
parameter.
from django.db import models
class SeparatedValuesField(models.TextField):
__metaclass__ = models.SubfieldBase
def __init__(self, *args, **kwargs):
self.token = kwargs.pop('token', ',')
super(SeparatedValuesField, self).__init__(*args, **kwargs)
def to_python(self, value):
if not value: return
if isinstance(value, list):
return value
return value.split(self.token)
def get_db_prep_value(self, value):
if not value: return
assert(isinstance(value, list) or isinstance(value, tuple))
return self.token.join([unicode(s) for s in value])
def value_to_string(self, obj):
value = self._get_val_from_obj(obj)
return self.get_db_prep_value(value)
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