[Django]-Django 1.8 ArrayField append & extend

Note: OP code will absolutely work. We just need to save the model (because these is just a model field, not relation). Let’s see:

>>> p = Post.objects.create(tags=[str(i) for i in range(10000)])
>>> p.tags.append("working!")
>>> p.save()
>>> working_post = Post.objects.get(tags__contains=["working!"])
<Post: Post object>
>>> working_post.tags[-2:]
[u'9999', u'working!']

Going deeper

Django gets ArrayField as python list

Code reference

Everything you could do with list, you can do with ArrayField. Even sorting

Django saves ArrayField as python list

Code reference

These means that it saves structure and elements of python list.

This works:

from django.db.models import F
from django.db.models.expressions import CombinedExpression, Value

post = Post.objects.get(id=1000)
post.tags = CombinedExpression(F('tags'), '||', Value(['hello']))
post.save()

or in an update clause:

Post.objects.filter(created_on__lt=now() - timespan(days=30))\
    .update(tags=CombinedExpression(F('tags'), '||', Value(['old'])))

Another solution is using a custom expression. I tested the following code with Django 1.11 and Python 3.6 (f-strings).

from django.db.models.expressions import Func

class ArrayAppend(Func):

    function = 'array_append'
    template = "%(function)s(%(expressions)s, %(element)s)"
    arity = 1

    def __init__(self, expression: str, element, **extra):
        if not isinstance(element, (str, int)):
            raise TypeError(
                f'Type of "{element}" must be int or str, '
                f'not "{type(element).__name__}".'
            )

        super().__init__(
            expression,
            element=isinstance(element, int) and element or f"'{element}'",
            **extra,
        )

The expression can be used in update():

Post.objects \
    .filter(pk=1) \
    .update(tags=ArrayAppend('tags', 'new tag'))

You could use django_postgres_extensions. It supports a lot of functions like append, prepend, remove, concatenate.

But if you are using Django 1.8 like me, you should use only the required classes from this package. That way, you won’t have to change database backend too. I’ve pasted the required classes here. Use them as described in first link.

This 100% works

from django.db.models import F
from django.db.models.expressions import CombinedExpression, Value

Post.objects.filter(created_on__lt=now() - timespan(days=30)).update(tags=CombinedExpression(F('tags'), '||', Value('{hello}')))

Note the difference between my variant and Yotam Ofek’s:

Value(["hello"]) should be changed to Value("{hello}")

Also if you need to exclude entries with some specific tag use this:

.exclude(tags__contains="{hello}")

One more moment, field "tags" should have default=list

tags = ArrayField(models.CharField(max_length=200), default=list)