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

"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()

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.

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')

Because in 2021 this post is first in google results. This days exist nice elegant solution

MySql Docs

PostgreSQL

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
    )

Form

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
    )

Query

>>> 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>]

You can store virtually any object using a Django Pickle Field, ala this snippet:

http://www.djangosnippets.org/snippets/513/

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

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.

Example instantiation:

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).