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Delete entries from ManyToMany table using _raw_delete

I have a huge amount of data in my db.
I cannot use .delete() method cause performance of Django ORM is insufficient in my case.
_raw_delete() method suits me cause I can do it python instead using raw SQL.
But I have problem I have no idead how can I delete relation tables using _raw_delete. They need to be deleted before models cause I have restrict in DB. Any ideas how can I achieve this?

I have found a solution.
You can operate on link model with this:
link_model = MyModel._meta.get_field('my_m2m_field').remote_field.through
qs = link_model.objects.filter(mymodel_id__in=mymodel_ids)
qs._raw_delete(qs.db)

Insert statment created by django ORM at bulk_create

I am kind of new to python and django.
I am using bulk_create to insert a lot of rows and as a former DBA I would very much like to see what insert statments are being executed. I know that for querys you can use .query but for insert statments I can't find a command.
Is there something I'm missing or is there no easy way to see it? (A regular print is fine by me.)

The easiest way is to set DEBUG = True and check connection.queries after executing the query. This stores the raw queries and the time each query takes.
from django.db import connection
MyModel.objects.bulk_create(...)
print(connection.queries[-1]['sql'])
There's more information in the docs.
A great tool to make this information easily accessible is the django-debug-toolbar.

get_or_create in Peewee

The paragraph titled Get or create on the peewee documentation says:
While peewee has a get_or_create() method, this should really not be
used outside of tests as it is vulnerable to a race condition. The
proper way to perform a get or create with peewee is to rely on the
database to enforce a constraint.
And then it goes on with an example that only shows the create part, not the get part.
What is the best way to perform a get or create with peewee?

Everything you are doing inside a transaction is atomic.
So as long as you are calling get_or_create() inside a transaction, that paragraph is wrong.

Django conditionally excluding fields

I am new to Django and am having some trouble with filtering. Please excuse the contrived example. I have some model for events, with a one to many relationship with attendees. From my set of events I want to exclude events occurring in Boston, but only if there's only 1 attendee. The code below is what I've tried that definitely does not work:
queryset = queryset.annotate(attendees_count=Count('attendees')).exclude(attendees_count=1, event_location__city="Boston")
I don't get an error. Rather it just filters out "Boston" regardless of "attendees_count".
In addition to a query that actually does what I want it to do, I'd like to understand why what I did doesn't work. Thanks in advance.

Are you sure that your original queryset includes Boston with >1 attendee?
https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.exclude
Multiple parameters are joined via AND in the underlying SQL
statement, and the whole thing is enclosed in a NOT()

Django models - assign id instead of object

I apologize if my question turns out to be silly, but I'm rather new to Django, and I could not find an answer anywhere.
I have the following model:
class BlackListEntry(models.Model):
user_banned = models.ForeignKey(auth.models.User,related_name="user_banned")
user_banning = models.ForeignKey(auth.models.User,related_name="user_banning")
Now, when i try to create an object like this:
BlackListEntry.objects.create(user_banned=int(user_id),user_banning=int(banning_id))
I get a following error:
Cannot assign "1": "BlackListEntry.user_banned" must be a "User" instance.
Of course, if i replace it with something like this:
user_banned = User.objects.get(pk=user_id)
user_banning = User.objects.get(pk=banning_id)
BlackListEntry.objects.create(user_banned=user_banned,user_banning=user_banning)
everything works fine. The question is:
Does my solution hit the database to retrieve both users, and if yes, is it possible to avoid it, just passing ids?

The answer to your question is: YES.
Django will hit the database (at least) 3 times, 2 to retrieve the two User objects and a third one to commit your desired information. This will cause an absolutelly unnecessary overhead.
Just try:
BlackListEntry.objects.create(user_banned_id=int(user_id),user_banning_id=int(banning_id))
These is the default name pattern for the FK fields generated by Django ORM. This way you can set the information directly and avoid the queries.
If you wanted to query for the already saved BlackListEntry objects, you can navigate the attributes with a double underscore, like this:
BlackListEntry.objects.filter(user_banned__id=int(user_id),user_banning__id=int(banning_id))
This is how you access properties in Django querysets. with a double underscore. Then you can compare to the value of the attribute.
Though very similar, they work completely different. The first one sets an atribute directly while the second one is parsed by django, that splits it at the '__', and query the database the right way, being the second part the name of an attribute.
You can always compare user_banned and user_banning with the actual User objects, instead of their ids. But there is no use for this if you don't already have those objects with you.
Hope it helps.

I do believe that when you fetch the users, it is going to hit the db...
To avoid it, you would have to write the raw sql to do the update using method described here:
https://docs.djangoproject.com/en/dev/topics/db/sql/
If you decide to go that route keep in mind you are responsible for protecting yourself from sql injection attacks.
Another alternative would be to cache the user_banned and user_banning objects.
But in all likelihood, simply grabbing the users and creating the BlackListEntry won't cause you any noticeable performance problems. Caching or executing raw sql will only provide a small benefit. You're probably going to run into other issues before this becomes a problem.