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Django querysets optimization - preventing selection of annotated fields

Let's say I have following models:
class Invoice(models.Model):
...
class Note(models.Model):
invoice = models.ForeignKey(Invoice, related_name='notes', on_delete=models.CASCADE)
text = models.TextField()
and I want to select Invoices that have some notes. I would write it using annotate/Exists like this:
Invoice.objects.annotate(
has_notes=Exists(Note.objects.filter(invoice_id=OuterRef('pk')))
).filter(has_notes=True)
This works well enough, filters only Invoices with notes. However, this method results in the field being present in the query result, which I don't need and means worse performance (SQL has to execute the subquery 2 times).
I realize I could write this using extra(where=) like this:
Invoice.objects.extra(where=['EXISTS(SELECT 1 FROM note WHERE invoice_id=invoice.id)'])
which would result in the ideal SQL, but in general it is discouraged to use extra / raw SQL.
Is there a better way to do this?

You can remove annotations from the SELECT clause using .values() query set method. The trouble with .values() is that you have to enumerate all names you want to keep instead of names you want to skip, and .values() returns dictionaries instead of model instances.
Django internaly keeps the track of removed annotations in
QuerySet.query.annotation_select_mask. So you can use it to tell Django, which annotations to skip even wihout .values():
class YourQuerySet(QuerySet):
def mask_annotations(self, *names):
if self.query.annotation_select_mask is None:
self.query.set_annotation_mask(set(self.query.annotations.keys()) - set(names))
else:
self.query.set_annotation_mask(self.query.annotation_select_mask - set(names))
return self
Then you can write:
invoices = (Invoice.objects
.annotate(has_notes=Exists(Note.objects.filter(invoice_id=OuterRef('pk'))))
.filter(has_notes=True)
.mask_annotations('has_notes')
)
to skip has_notes from the SELECT clause and still geting filtered invoice instances. The resulting SQL query will be something like:
SELECT invoice.id, invoice.foo FROM invoice
WHERE EXISTS(SELECT note.id, note.bar FROM notes WHERE note.invoice_id = invoice.id) = True
Just note that annotation_select_mask is internal Django API that can change in future versions without a warning.

Ok, I've just noticed in Django 3.0 docs, that they've updated how Exists works and can be used directly in filter:
Invoice.objects.filter(Exists(Note.objects.filter(invoice_id=OuterRef('pk'))))
This will ensure that the subquery will not be added to the SELECT columns, which may result in a better performance.
Changed in Django 3.0:
In previous versions of Django, it was necessary to first annotate and then filter against the annotation. This resulted in the annotated value always being present in the query result, and often resulted in a query that took more time to execute.
Still, if someone knows a better way for Django 1.11, I would appreciate it. We really need to upgrade :(

We can filter for Invoices that have, when we perform a LEFT OUTER JOIN, no NULL as Note, and make the query distinct (to avoid returning the same Invoice twice).
Invoice.objects.filter(notes__isnull=False).distinct()

This is best optimize code if you want to get data from another table which primary key reference stored in another table
Invoice.objects.filter(note__invoice_id=OuterRef('pk'),)

We should be able to clear the annotated field using the below method.
Invoice.objects.annotate(
has_notes=Exists(Note.objects.filter(invoice_id=OuterRef('pk')))
).filter(has_notes=True).query.annotations.clear()

Extend queryset in django python

I am looking for way how to add new objects to existing queryset, or how to implement what I want by other way.
contact = watson.filter(contacts, searchline)
This line returns queryset, which I later use to iterate.
Then I want to do this to add more objects, which watson couldn't find
contact_in_iteration = Contact.objects.get(id = fild.f_for)
contact.append(contact_in_iteration)
And sorry for my poor english
Did this
contacts = Contact.objects.filter(crm_id=request.session['crm_id'])
query = Q(contacts,searchline)
contact = watson.filter(query)
and get 'filter() missing 1 required positional argument: 'search_text'' error

You can use | and Q lookups. See the docs.
I'm not sure I've fully understood your initial query, but I think that in your case you would want to do:
query = Q(contacts='Foo', searchline='Bar')
contact = watson.filter(query)
Then later:
contact = watson.filter(query | Q(id=field.f_for))
Strictly speaking it won't append to the queryset, but will return a new queryset. But that's okay, because that's what .filter() does anyway.

You should look at a queryset as a sql query that will be executed later. When constructing a queryset and save the result in a variable, you can later filter it even more, but you can not expand it. If you need a query that has more particular rules (like, you need an OR operation) you should state that when you are constructing the query. One way of doing that is indeed using the Q object.
But it looks like you are confused about what querysets really are and how they are used. First of all:
Contact.objects.get(id = fild.f_for)
will never return a queryset, but an instance, because you use get and thus ask for a single particular record. You need to use filter() if you want to get a quersyet. So if you had an existing queryset say active_contacts and you wanted to filter it down so you only get the contacts that have a first_name of 'John' you would do:
active_contacts = Contact.objects.filter(active=True)
active_contacts_named_John = active_contacts.filter(first_name='John')
Of course you could do this in one line too, but I'm assuming you do the first queryset construction somewhere else in your code.
Second remark:
If in your example watson is a queryset, your user of filter() is unclear. This doesn't really make sense:
contact = watson.filter(contacts, searchline)
As stated earlier, filtering a queryset returns another queryset. So you should use a plurar as your variable name e.g. contacts. Then the correct use of filter would be:
contacts = watson.filter(first_name=searchline)
I'm assuming searchline here is a variable that contains a user inputted search term. So maybe here you should name your variable searchterm or similar. This will return all contacts that are filtered by whatever watson is filtering out already and whose first_name matches searchline exactly. You could also use a more liberate method and filter out results that 'contains' the searching term, like so:
contacts = watson.filter(first_name__contains=searchline)
Hope this helps you get on the right path.

Django complex ordering

I have a Django model Document, which can have Vote objects pointing on it. There's a integer field on Vote called score.
I want to order a queryset of documents according to the number of Vote objects with score=1 that are pointing at the document. i.e., the document that has the most positive votes should be the first one in the queryset.
Is it possible with Django? How?

This is a job for annotations.
from django.db.models import Count
Document.objects.filter(score=1).annotate(
positive_votes=Count('vote__count')).order_by('positive_votes')
Edit
There isn't really a way to do this without filtering, because that's the way the underlying database operations work. But one not-so-nice way would be to do a separate query for all the documents not included in the original, and chain the two querysets together:
positive_docs = <query from above>
other_docs = Document.objects.exclude(id__in=positive_docs)
all_docs = itertools.chain(positive_docs, other_docs)
This would work as long as you don't have millions of docs, but would break things like pagination.

I did this (on the QuerySet model):
def order_by_score(self):
q = django.db.models.Q(ratings__score=1)
documents_with_one_positive_rating = self.filter(q) # Annotation sees only
# the positive ratings
documents_without_one_positive_rating = self.filter(~q)
return (documents_with_one_positive_rating |
documents_without_one_positive_rating).annotate(
db_score=django.db.models.Count('ratings')
).order_by('-db_score')
Advantage is it still shows the documents without a positive rating.

Variable interpolation in python/django, django query filters [duplicate]

Given a class:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=20)
Is it possible, and if so how, to have a QuerySet that filters based on dynamic arguments? For example:
# Instead of:
Person.objects.filter(name__startswith='B')
# ... and:
Person.objects.filter(name__endswith='B')
# ... is there some way, given:
filter_by = '{0}__{1}'.format('name', 'startswith')
filter_value = 'B'
# ... that you can run the equivalent of this?
Person.objects.filter(filter_by=filter_value)
# ... which will throw an exception, since `filter_by` is not
# an attribute of `Person`.

Python's argument expansion may be used to solve this problem:
kwargs = {
'{0}__{1}'.format('name', 'startswith'): 'A',
'{0}__{1}'.format('name', 'endswith'): 'Z'
}
Person.objects.filter(**kwargs)
This is a very common and useful Python idiom.

A simplified example:
In a Django survey app, I wanted an HTML select list showing registered users. But because we have 5000 registered users, I needed a way to filter that list based on query criteria (such as just people who completed a certain workshop). In order for the survey element to be re-usable, I needed for the person creating the survey question to be able to attach those criteria to that question (don't want to hard-code the query into the app).
The solution I came up with isn't 100% user friendly (requires help from a tech person to create the query) but it does solve the problem. When creating the question, the editor can enter a dictionary into a custom field, e.g.:
{'is_staff':True,'last_name__startswith':'A',}
That string is stored in the database. In the view code, it comes back in as self.question.custom_query . The value of that is a string that looks like a dictionary. We turn it back into a real dictionary with eval() and then stuff it into the queryset with **kwargs:
kwargs = eval(self.question.custom_query)
user_list = User.objects.filter(**kwargs).order_by("last_name")

Additionally to extend on previous answer that made some requests for further code elements I am adding some working code that I am using
in my code with Q. Let's say that I in my request it is possible to have or not filter on fields like:
publisher_id
date_from
date_until
Those fields can appear in query but they may also be missed.
This is how I am building filters based on those fields on an aggregated query that cannot be further filtered after the initial queryset execution:
# prepare filters to apply to queryset
filters = {}
if publisher_id:
filters['publisher_id'] = publisher_id
if date_from:
filters['metric_date__gte'] = date_from
if date_until:
filters['metric_date__lte'] = date_until
filter_q = Q(**filters)
queryset = Something.objects.filter(filter_q)...
Hope this helps since I've spent quite some time to dig this up.
Edit:
As an additional benefit, you can use lists too. For previous example, if instead of publisher_id you have a list called publisher_ids, than you could use this piece of code:
if publisher_ids:
filters['publisher_id__in'] = publisher_ids

Django.db.models.Q is exactly what you want in a Django way.

This looks much more understandable to me:
kwargs = {
'name__startswith': 'A',
'name__endswith': 'Z',
***(Add more filters here)***
}
Person.objects.filter(**kwargs)

A really complex search forms usually indicates that a simpler model is trying to dig it's way out.
How, exactly, do you expect to get the values for the column name and operation?
Where do you get the values of 'name' an 'startswith'?
filter_by = '%s__%s' % ('name', 'startswith')
A "search" form? You're going to -- what? -- pick the name from a list of names? Pick the operation from a list of operations? While open-ended, most people find this confusing and hard-to-use.
How many columns have such filters? 6? 12? 18?
A few? A complex pick-list doesn't make sense. A few fields and a few if-statements make sense.
A large number? Your model doesn't sound right. It sounds like the "field" is actually a key to a row in another table, not a column.
Specific filter buttons. Wait... That's the way the Django admin works. Specific filters are turned into buttons. And the same analysis as above applies. A few filters make sense. A large number of filters usually means a kind of first normal form violation.
A lot of similar fields often means there should have been more rows and fewer fields.

Running into issues with Django's ModelChoiceField "queryset" attribute

In django by default the form for a model foreign key is a ModelChoiceField (where you can select out of a list of all possible models). And it's possible to change these with the query set attribute, like
// in forms.py
self.fields['possible_cars'].queryset = somequeryset
But I'm in a situation where I have a list of stuff, not a queryset, and since there is no way to convert a list into a queryset, i'm not sure how to make the options for my ModelChoiceField similar to the list of models I want. (Since they take a queryset by default, i'm assuming they get a list from that query anyways, so this kinda thing should be possible).
I tried self.fields['possible_cars']._choices = mylist , but it won't work.
Any ideas guys?

Assuming your field take a Car queryset, you can construct one like the following:
mylist = ['BMW', 'Lamborghini', 'Porsche']
cars = Car.objects.filter(name__in=mylist)
self.fields['possible_cars'].queryset = cars