I use only django framework with his ORM.
And there is some code:
User.objects.filter(username='test').exists()
that return True or False.
this orm query generate SQL:
SELECT (1) AS "a" FROM "auth_user" WHERE "auth_user"."username" = E'test' LIMIT 1
How I can perform equal query in SQLAlchemy?
I try understand this page
http://www.sqlalchemy.org/docs/05/ormtutorial.html#using-exists
but can't understand how this works.
Help please.
Thanks!
(ret,), = Session.query(exists().where(User.username=='test'))
Related
I have the following code -
testcase = Table1.objects.filter(ID=id,serialno ='1234', condition='False').exists()
if not testcase:
c = Table1.objects.filter(ID=id,serialno ='1234').select_for_update().update(condition='True')
Here, I am querying the same DB twice. Is there any way where I can do the filter exists and update in a single query?
Filtering doesn't send a query.
I agree with the answer above, but if you want a correction for you code, then this would be correct:
testcase = Table1.objects.filter(ID=id,serialno ='1234')
if not testcase.filter(condition='False'):
c = testcase.select_for_update().update(condition='True')
You can just use the update method with your filter query.
Table1.objects.filter(ID=id,serialno ='1234', condition='False').update(condition=True)
I'm trying to count the dates users register from a DateTime field. In the database this is stored as '2016-10-31 20:49:38' but I'm only interested in the date '2016-10-31'.
The raw SQL query is:
select DATE(registered_at) registered_date,count(registered_at) from User
where course='Course 1' group by registered_date;
It is possible using 'extra' but I've read this is deprecated and should not be done. It works like this though:
User.objects.all()
.filter(course='Course 1')
.extra(select={'registered_date': "DATE(registered_at)"})
.values('registered_date')
.annotate(**{'total': Count('registered_at')})
Is it possible to do without using extra?
I read that TruncDate can be used and I think this is the correct queryset however it does not work:
User.objects.all()
.filter(course='Course 1')
.annotate(registered_date=TruncDate('registered_at'))
.values('registered_date')
.annotate(**{'total': Count('registered_at')})
I get <QuerySet [{'total': 508346, 'registered_date': None}]> so there is something going wrong with TruncDate.
If anyone understands this better than me and can point me in the right direction that would be much appreciated.
Thanks for your help.
I was trying to do something very similar and was having the same problems as you. I managed to get my problem working by adding in an order_by clause after applying the TruncDate annotation. So I imagine that this should work for you too:
User.objects.all()
.filter(course='Course 1')
.annotate(registered_date=TruncDate('registered_at'))
.order_by('registered_date')
.values('registered_date')
.annotate(**{'total': Count('registered_at')})
Hope this helps?!
This is an alternative to using TruncDate by using `registered_at__date' and Django does the truncate for you.
from django.db.models import Count
from django.contrib.auth import get_user_model
metrics = {
'total': Count('registered_at__date')
}
get_user_model().objects.all()
.filter(course='Course 1')
.values('registered_at__date')
.annotate(**metrics)
.order_by('registered_at__date')
For Postgresql this transforms to the DB query:
SELECT
("auth_user"."registered_at" AT TIME ZONE 'Asia/Kolkata')::date,
COUNT("auth_user"."registered_at") AS "total"
FROM
"auth_user"
GROUP BY
("auth_user"."registered_at" AT TIME ZONE 'Asia/Kolkata')::date
ORDER BY
("auth_user"."registered_at" AT TIME ZONE 'Asia/Kolkata')::date ASC;
From the above example you can see that Django ORM reverses SELECT and GROUP_BY arguments. In Django ORM .values() roughly controls the GROUP_BY argument while .annotate() controls the SELECT columns and what aggregations needs to be done. This feels a little odd but is simple when you get the hang of it.
my models are designed like so
class Warehouse:
name = ...
sublocation = FK(Sublocation)
class Sublocation:
name = ...
city = FK(City)
class City:
name = ..
state = Fk(State)
Now if i throw a query.
wh = Warehouse.objects.value_list(['name', 'sublocation__name',
'sublocation__city__name']).first()
it returns correct result but internally how many query is it throwing? is django fetching the data in one request?
Django makes only one query to the database for getting the data you described.
When you do:
wh = Warehouse.objects.values_list(
'name', 'sublocation__name', 'sublocation__city__name').first()
It translates in to this query:
SELECT "myapp_warehouse"."name", "myapp_sublocation"."name", "myapp_city"."name"
FROM "myapp_warehouse" INNER JOIN "myapp_sublocation"
ON ("myapp_warehouse"."sublocation_id" = "myapp_sublocation"."id")
INNER JOIN "myapp_city" ON ("myapp_sublocation"."city_id" = "myapp_city"."id")'
It gets the result in a single query. You can count number of queries in your shell like this:
from django.db import connection as c, reset_queries as rq
In [42]: rq()
In [43]: len(c.queries)
Out[43]: 0
In [44]: wh = Warehouse.objects.values_list('name', 'sublocation__name', 'sublocation__city__name').first()
In [45]: len(c.queries)
Out[45]: 1
My suggestion would be to write a test for this using assertNumQueries (docs here).
from django.test import TestCase
from yourproject.models import Warehouse
class TestQueries(TestCase):
def test_query_num(self):
"""
Assert values_list query executes 1 database query
"""
values = ['name', 'sublocation__name', 'sublocation__city__name']
with self.assertNumQueries(1):
Warehouse.objects.value_list(values).first()
FYI I'm not sure how many queries are indeed sent to the database, 1 is my current best guess. Adjust the number of queries expected to get this to pass in your project and pin the requirement.
There is extensive documentation on how and when querysets are evaluated in Django docs: QuerySet API Reference.
The pretty much standard way to have a good insight of how many and which queries are taken place during a page render is to use the Django Debug Toolbar. This could tell you precisely how many times this recordset is evaluated.
You can use django-debug-toolbar to see real queries to db
I have two models:
class Note(model):
<attribs>
class Permalink(model):
note = foreign key to Note
I want to execute a query: get all notes which don't have a permalink.
In SQL, I would do it as something like:
SELECT * FROM Note WHERE id NOT IN (SELECT note FROM Permalink);
Wondering how to do this in ORM.
Edit: I don't want to get all the permalinks out into my application. Would instead prefer it to run as a query inside the DB.
You should be able to use this query:
Note.objects.filter(permalink_set__isnull=True)
you can use:
Note.objects.exclude(id__in=Permalink.objects.all().values_list('id', flat=True))
Is it possible to use AS sql statement with Django ORM:
SELECT my_field AS something_shiny WHERE my_condition = 1
If it is possible then how?
By now the Django documentation says that one should use extra as a last resort.
So here is a better way to do this query:
from django.db.models import F
Foo.objects.filter(cond=1).annotate(sth_shiny=F('my_field'))
use extra()
Foo.objects.filter(cond=1).extra(select={'sth_shiny':'my_field'})
Then you could access sth_shiny attr of resulted Foo instances