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.
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.
You can store virtually any object using a Django Pickle Field, ala this snippet:
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')
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()
Using one-to-many relation (FK from Friend to parent class) will make your app more scalable (as you can trivially extend the Friend object with additional attributes beyond the simple name). And thus this is the best way
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:
>>> myList = [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'>
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") @property 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.
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
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:
Storing a list of strings in Django model:
class Bar(models.Model): foo = models.TextField(blank=True) def set_list(self, element): self.foo += "," + element if self.foo else element def get_list(self): return self.foo.split(",") if self.foo else None
and you can call it like this:
bars = Bar() bars.set_list("str1") bars.set_list("str2") bar_list = bars.get_list() for bar in bar_list: print bar
Because in 2021 this post is first in google results.
This days exist nice elegant solution
from django.db.models import CharField, Model from django_mysql.models import ListCharField class Person(Model): name = CharField() post_nominals = ListCharField( base_field=CharField(max_length=10), size=6, max_length=(6 * 11) # 6 * 10 character nominals, plus commas )
from django.db.models import IntegerField, Model from django_mysql.models import ListTextField class Widget(Model): widget_group_ids = ListTextField( base_field=IntegerField(), size=100, # Maximum of 100 ids in list )
>>> Person.objects.create(name='Horatio', post_nominals=['PhD', 'Esq.', 'III']) >>> Person.objects.create(name='Severus', post_nominals=['PhD', 'DPhil']) >>> Person.objects.create(name='Paulus', post_nominals=) >>> Person.objects.filter(post_nominals__contains='PhD') [<Person: Horatio>, <Person: Severus>] >>> Person.objects.filter(post_nominals__contains='Esq.') [<Person: Horatio>] >>> Person.objects.filter(post_nominals__contains='DPhil') [<Person: Severus>] >>> Person.objects.filter(Q(post_nominals__contains='PhD') & Q(post_nominals__contains='III')) [<Person: Horatio>]
As ListCharField is a subclass of CharField, any CharField options can be set too. Most importantly you’ll need to set max_length to determine how many characters to reserve in the database.
from django.db.models import CharField, Model from django_mysql.models import ListCharField class Person(Model): name = CharField() post_nominals = ListCharField( base_field=CharField(max_length=10), size=6, max_length=(6 * 11), # 6 * 10 character nominals, plus commas )
There are various answers addressing potential paths, such as
ArrayField if one is using PostgreSQL,
TextField by converting the list into a JSON string, creating a custom Django model field or just creating a new model and then using a ForeignKey as noted by Daniel Roseman here.
Still, one of my favourites (together with the ForeignKey option) for this use case was left out. More specifically, I’d like to reference the
JSONField as a better option. As per the documentation,
A field for storing JSON encoded data. In Python the data is represented in its Python native format: dictionaries, lists, strings, numbers, booleans and None.
JSONField is supported on MariaDB, MySQL 5.7.8+, Oracle, PostgreSQL, and SQLite (with the JSON1 extension enabled).
It has advantage over
ArrayField in that it is supported in more databases than PostgreSQL (since Django 3.1).
Also, it’s better than
TextField since this one was purposefully built to store JSON and requires less steps for operations like reading.
Also, it’s better than a custom field in that it comes already with Django (no need for extra work / maintenance).