How to add an model instance to a django queryset?
PythonDjangoPython Problem Overview
It seems like a django queryset behaves somehow like a python list.
But it doesn't support list's .append() method as I know.
What I want to do is like:
from my_django_app.models import MyModel
queryset = MyModel.objects.none()
queryset.append(MyModel.objects.first()) ## no list's .append() method!
Is there any way to add an model instance to an existing queryset?
Python Solutions
Solution 1 - Python
You can also use the | operator to create a union:
queryset = MyModel.objects.none()
instance = MyModel.objects.first()
queryset |= MyModel.objects.filter(pk=instance.pk)
But be warned that this will generate different queries depending on the number of items you append this way, making caching of compiled queries inefficient.
Solution 2 - Python
No. A queryset is a representation of a query - hence the name - not an arbitrary collection of instances.
If you really need an actual queryset rather than a list, you could try accumulating the IDs of the objects you need and then getting the objects via an __in query:
list_of_ids = []
list_of_ids.append(my_id)
...
queryset = MyModel.objects.filter(id__in=list_of_ids)
This isn't very efficient, though.
Solution 3 - Python
This can be done using union. After doing this, the type of the result can be seen as <class 'django.db.models.query.QuerySet'>. So two querysets can be combined. Lets see an example.
query1 = User.objects.filter(is_active=True)
query2 = User.objects.filter(is_active=False)
combined_query = query1.union(query2)
print (type(combined_query))
The above program will print result as below, confirming it is a queryset
<class 'django.db.models.query.QuerySet'>
So basically Django executes the below query for union.
(SELECT "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."is_active" = True)
UNION
(SELECT "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."is_active" = False)
This also means that there will be error(django.db.utils.ProgrammingError: each UNION query must have the same number of columns) if union is tried with two different tables.
Solution 4 - Python
Queryset is not a list
So
to_list = queryset.values()
To combine queryset
from itertools import chain
result_queryset = list(chain(queryset1, queryset2))
or
querysets = [queryset1, queryset2]
result_queryset = list(chain(*querysets))