What is the most efficient way to store a list in the Django models?

Question

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
         


        

Answer 1

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

Answer 2

Storing a list of strings in Django model:

class Bar(models.Model):
    foo = models.TextField(blank=True)

    def set_list(self, element):
        if self.foo:
            self.foo = self.foo + "," + element
        else:
            self.foo = element

    def get_list(self):
        if self.foo:
            return self.foo.split(",")
        else:
            None

and you can call it like this:

bars = Bar()
bars.set_list("str1")
bars.set_list("str2")
list = bars.get_list()
if list is not None:
    for bar in list:
        print bar
else:
    print "List is empty."      

Answer 3

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')

Answer 4

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 arrays.

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

Answer 5

"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()

Answer 6

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 filter or values_list.

Using 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.