Is it possible to force django to make a subquery when I try to insert some values?
This produces two separate queries:
CommunicationLog.objects.create(
device='asdfasdf',
date_request=CommunicationLog.objects.get(pk=343).date_request,
content_obj_id=338, type_request=1, type_change=2
)
You definitely cannot do it by using create. There's no available API that will let you do it, since this is very unusual use case. You have to fall back to raw sql.
Even if the API won't let you do what you want, you can still improve the performance a little bit (which I assume is your intention) by using the .only method when querying the date:
CommunicationLog.objects.create(
device='asdfasdf',
date_request=CommunicationLog.objects.only('date_request').get(343).date_request,
content_obj_id=338, type_request=1, type_change=2
)
Related
I'm thinking of using a raw query to quickly get around limitations with either my brain or the Django ORM, but I don't want to redevelop the infrastructure required to support the existing ORM code such as filters. Right now I'm stuck with two dead ends:
Writing an inner raw query and reusing that like any other query set. Even though my raw query selects the correct columns, I can't filter on it:
AttributeError: 'RawQuerySet' object has no attribute 'filter'
This is corroborated by another answer, but I'm still hoping that that information is out of date.
Getting the SQL and parameters from the query set and wrapping that in a raw query. It seems the raw SQL should be retrievable using queryset.query.get_compiler(DEFAULT_DB_ALIAS).as_sql() - how would I get the parameters as well (obviously without actually running the query)?
One option for dealing with complex queries is to write a VIEW that encapsulates the query, and then stick a model in front of that. You will still be able to filter (and depending upon your view, you may even get push-down of parameters to improve query performance).
All you need to do to get a model that is backed by a view is have it as "unmanaged", and then have the view created by a migration operation.
It's better to try to write a QuerySet if you can, but at times it is not possible (because you are using something that cannot be expressed using the ORM, for instance, or you need to to something like a LATERAL JOIN).
I found that the objects could be duplicate in a queryset. However, when I try to access each of the object and do nothing, it changes and seems to be right.
Here are the commands I have typed into the shell
At first I gained a queryset orderby the field 'receiveTime'. Then it seems that ds[1996] equals to ds[1997]. And I try to use the loop:
for d in ds:
pass
Then the ds[1996] isn't equal to ds[1997], but what have I done?
Maybe it is a feature of the lazy search?
plus 1:I have reproduced it just now. I didn't do any inserting or deleting just now.
These are the commands I just typed into the shell.
plus 2:I have seen the raw sql queries when I call the ds[0] and ds[1] which I have shown in the picture 2. The sql queries are correct but the answer seems to be wrong. I think maybe the reason is that the sorting parameter receiveTime of two objects are the same, which lead to the disorder of the objects?
Here are the raw sql queries
Replace order_by("receive_time") with order_by("receive_time", "id"). PostgreSQL uses qsort which is an unstable sort. Given only receive_time, if values are the same, the order is not guaranteed.
Don't post code or logs in images. Ever.
I want to get all the questinos with no answers.I use this:
all_questions=[q for q in Question.objects.all() if not q.answer_set.all()]
It works. But then I need to invoke order_by method with all_questions, so I need to change it to a queryset, how?
Or, is there a standard method like Question.objects.filter(answer_count=0) ? I find hard but no results.
Solution: Change answer_count__gt=0 to answer_count=0.
all_questions=Question.objects.annotate(answer_count=Count('answer')).filter(answer_count=0)
You should be able to use an annotation much more efficiently than doing one query per question.
Question.objects.annotate(answer_count=Count('answer')).filter(answer_count=0)
That said, you could just add the order_by directly into your Questions.objects.all() query. But like I said, it's much less efficient to do a query per question.
I have some MySQL code that looks like this:
SELECT
visitor AS team,
COUNT(*) AS rg,
SUM(vscore>hscore) AS rw,
SUM(vscore<hscore) AS rl
FROM `gamelog` WHERE status='Final'
AND date(start_et) BETWEEN %s AND %s GROUP BY visitor
I'm trying to translate this into a Django version of that query, without making multiple queries. Is this possible? I read up on how to do Sum(), and Count(), but it doesn't seem to work when I want to compare two fields like I'm doing.
Here's the best I could come up with so far, but it didn't work...
vrecord = GameLog.objects.filter(start_et__range=[start,end],visitor=i['id']
).aggregate(
Sum('vscore'>'hscore'),
Count('vscore'>'hscore'))
I also tried using 'vscore>hscore' in there, but that didn't work either. Any ideas? I need to use as few queries as possible.
Aggregation only works on single fields in the Django ORM. I looked at the code for the various aggregation functions, and noticed that the single-field restriction is hardwired. Basically, when you use, say, Sum(field), it just records that for later, then it passes it to the database-specific backend for conversion to SQL and execution. Apparently, aggregation and annotation are not standardized in SQL.
Anyway, you probably need to use a raw SQL query.
I am hoping someone can help me out with a quick question I have regarding chaining Django querysets. I am noticing a slow down because I am evaluating many data points in the database to create data trends. I was wondering if there was a way to have the chained filters evaluated locally instead of hitting the database. Here is a (crude) example:
pastries = Bakery.objects.filter(productType='pastry') # <--- will obviously always hit DB, when evaluated
cannoli = pastries.filter(specificType='cannoli') # <--- can this be evaluated locally instead of hitting the DB when evaluated, as long as pastries was evaluated?
I have checked the docs and I do not see anything specifying this, so I guess it's not possible, but I wanted to check with the 'braintrust' first ;-).
BTW - I know that I can do this myself by implementing some methods to loop through these datapoints and evaluate the criteria, but there are so many datapoints that my deadline does not permit me manually implementing this.
Thanks in advance.
QuerySet methods always produce SQL that returns the desired expression. This is why you cannot e.g. call various methods after slicing; SQL does not support that syntax. The ORM does nothing more than assemble said SQL. If you want fancier processing then you will need to perform it in Python code yourself.