Django: List field in model?

Question

In my model, I want a field that has a list of triplets. e.g. [[1, 3, 4], [4, 2, 6], [8, 12, 3], [3, 3, 9]]. Is there a field that can store this data in the da

Answer 1

If you're on Django 1.10 or newer AND Postgres as your database, you can use ArrayField. It's better to use than django-taggit or other alternatives, as it's native to the Django framework. https://docs.djangoproject.com/en/3.1/ref/contrib/postgres/fields/#arrayfield

from django.db import models
from django.contrib.postgres.fields import ArrayField

class ChessBoard(models.Model):
    board = ArrayField(
        ArrayField(
            models.CharField(max_length=10, blank=True),
            size=8,
        ),
        size=8,
    )

If you're on Django 3.1 or newer they've added support for JSONField with most database backends (MariaDB 10.2.7+, MySQL 5.7.8+, Oracle, PostgreSQL, and SQLite 3.9.0+). You can use this to store your Array!

https://docs.djangoproject.com/en/3.1/ref/models/fields/#jsonfield

from django.db import models

class ChessBoard(models.Model):
    list_of_pieces = models.JSONField()

Answer 2

If you are using Google App Engine or MongoDB as your backend, and you are using the djangoappengine library, there is a built in ListField that does exactly what you want. Further, it's easy to query the Listfield to find all objects that contain an element in the list.

Answer 3

With my current reputation I have no ability to comment, so I choose answer referencing comments for sample code in reply by Prashant Gaur (thanks, Gaur - this was helpful!) - his sample is for python2, since python3 has no

unicode

method.

The replacement below for function

get_prep_value(self, value):

should work with Django running with python3 (I'll use this code soon - yet not tested). Note, though, that I'm passing

encoding='utf-8', errors='ignore'

parameters to

decode()

and

unicode() methods

. Encoding should match your Django settings.py configuration and passing

errors='ignore'

is optional (and may result in silent data loss instead of exception whith misconfigured django in rare cases).

import sys

...

    def get_prep_value(self, value):
        if value is None:
            return value
        if sys.version_info[0] >= 3:
            if isinstance(out_data, type(b'')):
                return value.decode(encoding='utf-8', errors='ignore')
        else:
            if isinstance(out_data, type(b'')):
                return unicode(value, encoding='utf-8', errors='ignore')
        return str(value)
...

---

Answer 4

It's quite an old topic, but since it is returned when searching for "django list field" I'll share the custom django list field code I modified to work with Python 3 and Django 2. It supports the admin interface now and not uses eval (which is a huge security breach in Prashant Gaur's code).

from django.db import models
from typing import Iterable

class ListField(models.TextField):
    """
    A custom Django field to represent lists as comma separated strings
    """

    def __init__(self, *args, **kwargs):
        self.token = kwargs.pop('token', ',')
        super().__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        kwargs['token'] = self.token
        return name, path, args, kwargs

    def to_python(self, value):

        class SubList(list):
            def __init__(self, token, *args):
                self.token = token
                super().__init__(*args)

            def __str__(self):
                return self.token.join(self)

        if isinstance(value, list):
            return value
        if value is None:
            return SubList(self.token)
        return SubList(self.token, value.split(self.token))

    def from_db_value(self, value, expression, connection):
        return self.to_python(value)

    def get_prep_value(self, value):
        if not value:
            return
        assert(isinstance(value, Iterable))
        return self.token.join(value)

    def value_to_string(self, obj):
        value = self.value_from_object(obj)
        return self.get_prep_value(value)

Answer 5

If you are using PostgreSQL, you can use ArrayField with a nested ArrayField: https://docs.djangoproject.com/en/2.2/ref/contrib/postgres/fields/

This way, the data structure will be known to the underlying database. Also, the ORM brings special functionality for it.

Note that you will have to create a GIN index by yourself, though (see the above link, further down: https://docs.djangoproject.com/en/2.2/ref/contrib/postgres/fields/#indexing-arrayfield).

(Edit: updated links to newest Django LTS, this feature exists at least since 1.8.)

Answer 6

You can convert it into string by using JSON and store it as string.

For example,

In [3]: json.dumps([[1, 3, 4], [4, 2, 6], [8, 12, 3], [3, 3, 9]])

Out[3]: '[[1, 3, 4], [4, 2, 6], [8, 12, 3], [3, 3, 9]]'

You can add a method into your class to convert it automatically for you.

import json


class Foobar(models.Model):
    foo = models.CharField(max_length=200)

    def set_foo(self, x):
        self.foo = json.dumps(x)

    def get_foo(self):
        return json.loads(self.foo)

If you're using Django 1.9 and postgresql, there is a new class called JSONField, you should use it instead. Here is a link to it

There is a good talk about PostgreSQL JSONs and Arrays on youtube. Watch it, it has very good information.