Django Rest update many to many field of multiple objects at once - django

Question

I'm working on chat app django rest backend. Btw I have a problem to update m2m field of multiple objects at once.
Inside the Message model there is an m2m field deleted which represents a list of users who deleted this message.
class Message(models.Model):
# other fields
deleted = models.ManyToManyField(User)
So I can implement the delete functionality by adding user inside that field when user deletes a specific message.
But the problem is when user deletes a conversation(all messages in it), how can I implement to update the delete field of multiple Message objects at once. Because each object has empty delete field, another user inside delete field, or same user inside delete field(means that user already deleted a message before).
Edit
According to validname's comment I'm adding an example to make clear about my problem.
Please imagine user1 and user2 has conversation1 and there are 3 messages inside that conversation. So currently message list of that conversation is like following:
[
{
id: 304,
conversation: 1,
...
deleted: []
},
{
id: 305,
conversation: 1,
...
deleted: [2] // user2 already deleted this message before
},
{
id: 306,
conversation: 1,
...
deleted: [1] // user1 already deleted this message before
},
]
And user2 just has deleted conversation1, so after that message list should be like following:
[
{
id: 304,
conversation: 1,
...
deleted: [2]
},
{
id: 305,
conversation: 1,
...
deleted: [2] // no need to change for existing user2
},
{
id: 306,
conversation: 1,
...
deleted: [1, 2] // insert user2
},
]
I just want a django function that works like above changes.

Answer 1

All m2m relations are implemented by extra table and have their own models which are accessible by field's thorough attribute.
So if you have a set of messages and you want to add user to 'deleted' for each of message, you can do it by bulk creating records for though model:
from itertools import cycle
message_qs = ... # Query set of message.
user = ... # User model instance.
# Construct list of [(message_1, user), (message_2, user), ...] etc.
data = list(zip(message_qs, cycle(user)))
# Transform data to through model instances to pass to bulk create.
through_model = Message.deleted.through
instances = [
through_model(message=m, user=u) for m, u in data
]
through_model.objects.bulk_create(instances)
This is supposed to be very efficient way. However if you don't care about efficiency at the moment, you can just iterate over each user and call message.deleted.add(user) for each.