Currently I have a lot of python objects in my code similar to the following:
class MyClass(): def __init__(self, name, friends): self.myName = name self.myFriends = [str(x) for x in friends]
Now I want to turn this into a Django model, where self.myName is a string field, and self.myFriends is a list of strings.
from django.db import models class myDjangoModelClass(): myName = models.CharField(max_length=64) myFriends = ??? # what goes here?
Since the list is such a common data structure in python, I sort of expected there to be a Django model field for it. I know I can use a ManyToMany or OneToMany relationship, but I was hoping to avoid that extra indirection in the code.
I added this related question, which people may find useful.
Would this relationship not be better expressed as a one-to-many foreign key relationship to a
Friends table? I understand that
myFriends are just strings but I would think that a better design would be to create a
Friend model and have
MyClass contain a foreign key realtionship to the resulting table.
“Premature optimization is the root of all evil.”
With that firmly in mind, let’s do this! Once your apps hit a certain point, denormalizing data is very common. Done correctly, it can save numerous expensive database lookups at the cost of a little more housekeeping.
To return a
list of friend names we’ll need to create a custom Django Field class that will return a list when accessed.
David Cramer posted a guide to creating a SeperatedValueField on his blog. Here is the code:
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)
The logic of this code deals with serializing and deserializing values from the database to Python and vice versa. Now you can easily import and use our custom field in the model class:
from django.db import models from custom.fields import SeparatedValuesField class Person(models.Model): name = models.CharField(max_length=64) friends = SeparatedValuesField()
A simple way to store a list in Django is to just convert it into a JSON string, and then save that as Text in the model. You can then retrieve the list by converting the (JSON) string back into a python list. Here’s how:
The “list” would be stored in your Django model like so:
class MyModel(models.Model): myList = models.TextField(null=True) # JSON-serialized (text) version of your list
In your view/controller code:
Storing the list in the database:
import simplejson as json # this would be just 'import json' in Python 2.7 and later ... ... myModel = MyModel() listIWantToStore = [1,2,3,4,5,'hello'] myModel.myList = json.dumps(listIWantToStore) myModel.save()
Retrieving the list from the database:
jsonDec = json.decoder.JSONDecoder() myPythonList = jsonDec.decode(myModel.myList)
Conceptually, here’s what’s going on:
1,2,3,4,5,'hello'] import simplejson as json myJsonList = json.dumps(myList) myJsonList '[1, 2, 3, 4, 5, "hello"]' myJsonList.__class__ <type 'str'> jsonDec = json.decoder.JSONDecoder() myPythonList = jsonDec.decode(myJsonList) myPythonList [1, 2, 3, 4, 5, u'hello'] myPythonList.__class__ <type 'list'>myList = [
If you are using Django >= 1.9 with Postgres you can make use of ArrayField advantages
A field for storing lists of data. Most field types can be used, you
simply pass another field instance as the base_field. You may also
specify a size. ArrayField can be nested to store multi-dimensional
It is also possible to nest array fields:
from django.contrib.postgres.fields import ArrayField from django.db import models class ChessBoard(models.Model): board = ArrayField( ArrayField( models.CharField(max_length=10, blank=True), size=8, ), size=8, )
As @thane-brimhall mentioned it is also possible to query elements directly. Documentation reference
As this is an old question, and Django techniques must have changed significantly since, this answer reflects Django version 1.4, and is most likely applicable for v 1.5.
Django by default uses relational databases; you should make use of ’em. Map friendships to database relations (foreign key constraints) with the use of ManyToManyField. Doing so allows you to use RelatedManagers for friendlists, which use smart querysets. You can use all available methods such as
ManyToManyField relations and properties:
class MyDjangoClass(models.Model): name = models.CharField(...) friends = models.ManyToManyField("self") def friendlist(self): # Watch for large querysets: it loads everything in memory return list(self.friends.all())
You can access a user’s friend list this way:
joseph = MyDjangoClass.objects.get(name="Joseph") friends_of_joseph = joseph.friendlist
Note however that these relations are symmetrical: if Joseph is a friend of Bob, then Bob is a friend of Joseph.
class Course(models.Model): name = models.CharField(max_length=256) students = models.ManyToManyField(Student) class Student(models.Model): first_name = models.CharField(max_length=256) student_number = models.CharField(max_length=128) # other fields, etc... friends = models.ManyToManyField('self')
Remember that this eventually has to end up in a relational database. So using relations really is the common way to solve this problem. If you absolutely insist on storing a list in the object itself, you could make it for example comma-separated, and store it in a string, and then provide accessor functions that split the string into a list. With that, you will be limited to a maximum number of strings, and you will lose efficient queries.
In case you’re using postgres, you can use something like this:
class ChessBoard(models.Model): board = ArrayField( ArrayField( models.CharField(max_length=10, blank=True), size=8, ), size=8, )
if you need more details you can read in the link below: