I use postgresql database in my project and I use below example from django documentation.
from django.db.models import OuterRef, Subquery
newest = Comment.objects.filter(post=OuterRef('pk')).order_by('-created_at')
Post.objects.annotate(newest_commenter_email=Subquery(newest.values('email')[:1]))
but instead of newest commenter email, i need last two commenters emails. i changed [:1] to [:2] but this exception raised: ProgrammingError: more than one row returned by a subquery used as an expression.
You'll need to aggregate the subquery results in some way: perhaps by using an ARRAY() construct.
You can create a subclass of Subquery to do this:
class Array(Subquery):
template = 'ARRAY(%(subquery)s)`
output_field = ArrayField(base_field=models.TextField())
(You can do a more automatic method of getting the output field, but this should work for you for now: see https://schinckel.net/2019/07/30/subquery-and-subclasses/ for more details).
Then you can use:
posts = Post.objects.annotate(
newest_commenters=Array(newest.values('email')[:2]),
)
The reason this is happening is because a correlated subquery in postgres may only return one row, with one column. You can use this mechanism to deal with multiple rows, and perhaps use JSONB construction if you need multiple columns.
Related
I need to fetch the top performer for each month, here is the below MySql query which gives me the correct output.
select id,Name,totalPoints, createdDateTime
from userdetail
where app=4 and totalPoints in ( select
max(totalPoints)
FROM userdetail
where app=4
group by month(createdDateTime), year(createdDateTime))
order by totalPoints desc
I am new to Django ORM. I am not able to write an equivalent Django query which does the task. I have been struggling with this logic for 2 days. Any help would be highly appreciated.
While the GROUP BY clause in a subquery is slightly difficult to express with the ORM because aggregate() operations don't emit querysets, a similar effect can be achieved with a Window function:
UserDetail.objects.filter(total_points__in=UserDetail.objects.annotate(max_points=Window(
expression=Max('total_points'),
partition_by=[Trunc('created_datetime', 'month')]
)).values('max_points')
)
In general, this sort of pattern is implemented with Subquery expressions. In this case, I've implicitly used a subquery by passing a queryset to an __in predicate.
The Django documentation's notes on using aggregates within subqueries is are also relevant to this sort of query, since you want to use the results of an aggregate in a subquery (which I've avoided by using a window function).
However, I believe your query may not correctly capture what you want to do: as written it could return rows for users who weren't the best in a given month but did have the same score as another user who was the best in any month.
I have some table ports(switch_ip, slot_number, port_number, many, more, columns) and would like to achieve the following PostgreSQL query using Django:
SELECT switch_ip, array_agg((slot_number, port_number, many, more, columns) ORDER BY slot_number, port_number) info
FROM ports
GROUP BY switch_ip
ORDER BY switch_ip
Using django.contrib.postgres.aggregates here's what I got so far:
Port.objects \
.values('switch_ip') \
.annotate(
info=ArrayAgg('slot_number', ordering=('slot_number', 'port_number'))
) \
.order_by('switch_ip')
I am unable to include more than one column in the ArrayAgg. None of ArrayAgg(a, b, c), ArrayAgg((a, b, c)), ArrayAgg([a, b, c]) seem to work. A workaround could involve separate ArrayAggs for each column and each with the same ordering. I would despise this because I have many columns. Is there any nicer workaround, possibly more low-level?
I suspect this is no issue with ArrayAgg itself but rather with tuple expressions in general. Is there any way to have tuples at all in Django queries? For example, what would be the corresponding Django of:
SELECT switch_ip, (slot_number, port_number, many, more, columns) info
FROM ports
If this is not yet possible in Django, how feasible would it be to implement?
I have spent lot of time searching for a working solution and here is a full recipe with code example.
You need to define Array "function" with square brackets in template
from django.db.models.expressions import Func
class Array(Func):
template = '%(function)s[%(expressions)s]'
function = 'ARRAY'
You need to define output field format (it must be array of some django field). For example an array of strings
from django.contrib.postgres.fields import ArrayField
from django.db.models.fields import CharField
out_format = ArrayField(CharField(max_length=200))
Finally make an ArrayAgg expression
from django.db.models import F
annotate = {'2-fields': ArrayAgg(Array(F('field1'), F('field2'), output_field=out_format), distinct=True) }
model.objects.all().annotate(**annotate)
(Optional) If field1 or field2 are not CharFields, you may include Cast as an argument of Array
from django.db.models.functions import Cast
annotate = {'2-fields': ArrayAgg(Array(Cast(F('field1'), output_field=CharField(max_length=200)), F('field2'), output_field=out_format), distinct=True) }
Having done a bit more research I guess one could add the missing tuple functionality as follows:
Create a new model field type named TupleField. The implementation might look kind of similar to django.contrib.postgres.fields.ArrayField. TupleField would be rather awkward because I don't think any RDBMS allows for composite types to be used as column types so usage of TupleField would be limited to (possibly intermediate?) query results.
Create a new subclass of django.db.models.Expression which wraps multiple expressions on its own (like Func in general, so looking at Func's implementation might be worthwile) and evaluates to a TupleField. Name this subclass TupleExpression for example.
Then I could simply annotate with ArrayAgg(TupleExpression('slot_number', 'port_number', 'many', 'more', 'columns'), ordering=('slot_number', 'port_number')) to solve my original problem. This would annotate each switch_ip with correctly-ordered arrays of tuples where each tuple represents one switch port.
I have these two models:
class A(models.Model):
name=models.CharField(max_length=10)
class D(models.Model):
code=models.IntegerField()
the code field can have a number that exists in model A but it cant be related due to other factors. But what I want know is to list items from A whose value is the same with code
items=D.objects.values('code__name')
would work but since they are not related nor can be related, how can I handle that?
You can use Subquery() expressions in Django 1.11 or newer.
from django.db.models import OuterRef, Subquery
code_subquery = A.objects.filter(id=OuterRef('code'))
qs = D.objects.annotate(code_name=Subquery(code_subquery.values('name')))
The output of qs is a queryset of objects D with an added field code_name.
Footnotes:
It is compiled to a very similar SQL (like the Bear Brown's solution with "extra" method, but without disadvantages of his solution, see there):
SELECT app_d.id, app_d.code,
(SELECT U0.name FROM app_a U0 WHERE U0.id = (app_d.code)) AS code_name
FROM app_d
If a dictionary output is required it can be converted by .values() finally. It can work like a left join i.e. if the pseudo related field allows null (code = models.IntegerField(none=True)) then the objects D are not restricted and the output code_name value could be None. A feature of Subquery is that it returns only one field expression must be eventually repeated for another fields. (That is similar to extra(select={...: "SELECT ..."}), but thanks to object syntax it can be more readable customized than an explicit SQL.)
you can use django extra, replace YOUAPP on your real app name
D.objects.extra(select={'a_name': 'select name from YOUAPP_a where id=code'}).values('a_name')
# Replace YOUAPP^^^^^
This is a bleeding-edge feature that I'm currently skewered upon and quickly bleeding out. I want to annotate a subquery-aggregate onto an existing queryset. Doing this before 1.11 either meant custom SQL or hammering the database. Here's the documentation for this, and the example from it:
from django.db.models import OuterRef, Subquery, Sum
comments = Comment.objects.filter(post=OuterRef('pk')).values('post')
total_comments = comments.annotate(total=Sum('length')).values('total')
Post.objects.filter(length__gt=Subquery(total_comments))
They're annotating on the aggregate, which seems weird to me, but whatever.
I'm struggling with this so I'm boiling it right back to the simplest real-world example I have data for. I have Carparks which contain many Spaces. Use Book→Author if that makes you happier but —for now— I just want to annotate on a count of the related model using Subquery*.
spaces = Space.objects.filter(carpark=OuterRef('pk')).values('carpark')
count_spaces = spaces.annotate(c=Count('*')).values('c')
Carpark.objects.annotate(space_count=Subquery(count_spaces))
This gives me a lovely ProgrammingError: more than one row returned by a subquery used as an expression and in my head, this error makes perfect sense. The subquery is returning a list of spaces with the annotated-on total.
The example suggested that some sort of magic would happen and I'd end up with a number I could use. But that's not happening here? How do I annotate on aggregate Subquery data?
Hmm, something's being added to my query's SQL...
I built a new Carpark/Space model and it worked. So the next step is working out what's poisoning my SQL. On Laurent's advice, I took a look at the SQL and tried to make it more like the version they posted in their answer. And this is where I found the real problem:
SELECT "bookings_carpark".*, (SELECT COUNT(U0."id") AS "c"
FROM "bookings_space" U0
WHERE U0."carpark_id" = ("bookings_carpark"."id")
GROUP BY U0."carpark_id", U0."space"
)
AS "space_count" FROM "bookings_carpark";
I've highlighted it but it's that subquery's GROUP BY ... U0."space". It's retuning both for some reason. Investigations continue.
Edit 2: Okay, just looking at the subquery SQL I can see that second group by coming through ☹
In [12]: print(Space.objects_standard.filter().values('carpark').annotate(c=Count('*')).values('c').query)
SELECT COUNT(*) AS "c" FROM "bookings_space" GROUP BY "bookings_space"."carpark_id", "bookings_space"."space" ORDER BY "bookings_space"."carpark_id" ASC, "bookings_space"."space" ASC
Edit 3: Okay! Both these models have sort orders. These are being carried through to the subquery. It's these orders that are bloating out my query and breaking it.
I guess this might be a bug in Django but short of removing the Meta-order_by on both these models, is there any way I can unsort a query at querytime?
*I know I could just annotate a Count for this example. My real purpose for using this is a much more complex filter-count but I can't even get this working.
Shazaam! Per my edits, an additional column was being output from my subquery. This was to facilitate ordering (which just isn't required in a COUNT).
I just needed to remove the prescribed meta-order from the model. You can do this by just adding an empty .order_by() to the subquery. In my code terms that meant:
from django.db.models import Count, OuterRef, Subquery
spaces = Space.objects.filter(carpark=OuterRef('pk')).order_by().values('carpark')
count_spaces = spaces.annotate(c=Count('*')).values('c')
Carpark.objects.annotate(space_count=Subquery(count_spaces))
And that works. Superbly. So annoying.
It's also possible to create a subclass of Subquery, that changes the SQL it outputs. For instance, you can use:
class SQCount(Subquery):
template = "(SELECT count(*) FROM (%(subquery)s) _count)"
output_field = models.IntegerField()
You then use this as you would the original Subquery class:
spaces = Space.objects.filter(carpark=OuterRef('pk')).values('pk')
Carpark.objects.annotate(space_count=SQCount(spaces))
You can use this trick (at least in postgres) with a range of aggregating functions: I often use it to build up an array of values, or sum them.
I just bumped into a VERY similar case, where I had to get seat reservations for events where the reservation status is not cancelled. After trying to figure the problem out for hours, here's what I've seen as the root cause of the problem:
Preface: this is MariaDB, Django 1.11.
When you annotate a query, it gets a GROUP BY clause with the fields you select (basically what's in your values() query selection). After investigating with the MariaDB command line tool why I'm getting NULLs or Nones on the query results, I've came to the conclusion that the GROUP BY clause will cause the COUNT() to return NULLs.
Then, I started diving into the QuerySet interface to see how can I manually, forcibly remove the GROUP BY from the DB queries, and came up with the following code:
from django.db.models.fields import PositiveIntegerField
reserved_seats_qs = SeatReservation.objects.filter(
performance=OuterRef(name='pk'), status__in=TAKEN_TYPES
).values('id').annotate(
count=Count('id')).values('count')
# Query workaround: remove GROUP BY from subquery. Test this
# vigorously!
reserved_seats_qs.query.group_by = []
performances_qs = Performance.objects.annotate(
reserved_seats=Subquery(
queryset=reserved_seats_qs,
output_field=PositiveIntegerField()))
print(performances_qs[0].reserved_seats)
So basically, you have to manually remove/update the group_by field on the subquery's queryset in order for it to not have a GROUP BY appended on it on execution time. Also, you'll have to specify what output field the subquery will have, as it seems that Django fails to recognize it automatically, and raises exceptions on the first evaluation of the queryset. Interestingly, the second evaluation succeeds without it.
I believe this is a Django bug, or an inefficiency in subqueries. I'll create a bug report about it.
Edit: the bug report is here.
Problem
The problem is that Django adds GROUP BY as soon as it sees using an aggregate function.
Solution
So you can just create your own aggregate function but so that Django thinks it is not aggregate. Just like this:
total_comments = Comment.objects.filter(
post=OuterRef('pk')
).order_by().annotate(
total=Func(F('length'), function='SUM')
).values('total')
Post.objects.filter(length__gt=Subquery(total_comments))
This way you get the SQL query like this:
SELECT "testapp_post"."id", "testapp_post"."length"
FROM "testapp_post"
WHERE "testapp_post"."length" > (SELECT SUM(U0."length") AS "total"
FROM "testapp_comment" U0
WHERE U0."post_id" = "testapp_post"."id")
So you can even use aggregate subqueries in aggregate functions.
Example
You can count the number of workdays between two dates, excluding weekends and holidays, and aggregate and summarize them by employee:
class NonWorkDay(models.Model):
date = DateField()
class WorkPeriod(models.Model):
employee = models.ForeignKey(User, on_delete=models.CASCADE)
start_date = DateField()
end_date = DateField()
number_of_non_work_days = NonWorkDay.objects.filter(
date__gte=OuterRef('start_date'),
date__lte=OuterRef('end_date'),
).annotate(
cnt=Func('id', function='COUNT')
).values('cnt')
WorkPeriod.objects.values('employee').order_by().annotate(
number_of_word_days=Sum(F('end_date__year') - F('start_date__year') - number_of_non_work_days)
)
Hope this will help!
A solution which would work for any general aggregation could be implemented using Window classes from Django 2.0. I have added this to the Django tracker ticket as well.
This allows the aggregation of annotated values by calculating the aggregate over partitions based on the outer query model (in the GROUP BY clause), then annotating that data to every row in the subquery queryset. The subquery can then use the aggregated data from the first row returned and ignore the other rows.
Performance.objects.annotate(
reserved_seats=Subquery(
SeatReservation.objects.filter(
performance=OuterRef(name='pk'),
status__in=TAKEN_TYPES,
).annotate(
reserved_seat_count=Window(
expression=Count('pk'),
partition_by=[F('performance')]
),
).values('reserved_seat_count')[:1],
output_field=FloatField()
)
)
If I understand correctly, you are trying to count Spaces available in a Carpark. Subquery seems overkill for this, the good old annotate alone should do the trick:
Carpark.objects.annotate(Count('spaces'))
This will include a spaces__count value in your results.
OK, I have seen your note...
I was also able to run your same query with other models I had at hand. The results are the same, so the query in your example seems to be OK (tested with Django 1.11b1):
activities = Activity.objects.filter(event=OuterRef('pk')).values('event')
count_activities = activities.annotate(c=Count('*')).values('c')
Event.objects.annotate(spaces__count=Subquery(count_activities))
Maybe your "simplest real-world example" is too simple... can you share the models or other information?
"works for me" doesn't help very much. But.
I tried your example on some models I had handy (the Book -> Author type), it works fine for me in django 1.11b1.
Are you sure you're running this in the right version of Django? Is this the actual code you're running? Are you actually testing this not on carpark but some more complex model?
Maybe try to print(thequery.query) to see what SQL it's trying to run in the database. Below is what I got with my models (edited to fit your question):
SELECT (SELECT COUNT(U0."id") AS "c"
FROM "carparks_spaces" U0
WHERE U0."carpark_id" = ("carparks_carpark"."id")
GROUP BY U0."carpark_id") AS "space_count" FROM "carparks_carpark"
Not really an answer, but hopefully it helps.
How can I make an order_by like this ....
p = Product.objects.filter(vendornumber='403516006')\
.order_by('-created').distinct('vendor__name')
The problem is that I have multiple vendors with the same name, and I only want the latest product by the vendor ..
Hope it makes sense?
I got this DB error:
SELECT DISTINCT ON expressions must match initial ORDER BY expressions
LINE 1: SELECT DISTINCT ON ("search_vendor"."name")
"search_product"...
Based on your error message and this other question, it seems to me this would fix it:
p = Product.objects.filter(vendornumber='403516006')\
.order_by('vendor__name', '-created').distinct('vendor__name')
That is, it seems that the DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s). So by making the column you use in distinct as the first column in the order_by, I think it should work.
Just matching leftmost order_by() arg and distinct() did not work for me, producing the same error (Django 1.8.7 bug or a feature)?
qs.order_by('project').distinct('project')
however it worked when I changed to:
qs.order_by('project__id').distinct('project')
and I do not even have multiple order_by args.
In case you are hoping to use a separate field for distinct and order by another field you can use the below code
from django.db.models import Subquery
Model.objects.filter(
pk__in=Subquery(
Model.objects.all().distinct('foo').values('pk')
)
).order_by('bar')
I had a similar issue but then with related fields. With just adding the related field in distinct(), I didn't get the right results.
I wanted to sort by room__name keeping the person (linked to residency ) unique. Repeating the related field as per the below fixed my issue:
.order_by('room__name', 'residency__person', ).distinct('room__name', 'residency__person')
See also these related posts:
ProgrammingError: when using order_by and distinct together in django
django distinct and order_by
Postgresql DISTINCT ON with different ORDER BY