I've multiple fields in my model, and I need to remove the average of only the columns user inputs
Could be
How can I do it dynamically?
I know I can do
mean = results.aggregate(Avg("student_score"))
This is one, I want to add multiple Avg statements dynamically
I tried making a loop as well to get all names and add all fields given by user one by one
eg - Avg('students'), Avg('playtime'), Avg('grade'), Avg('sales')
But I get
QuerySet.aggregate() received non-expression(s): <class 'django.db.models.aggregates.Avg'>('students'), <class 'django.db.models.aggregates.Avg'>('sales').
I've even tried raw query, but it needs a unique ID because of which that isn't working
Any workaround ideas?
I am using MySQL DB
Aggregate return single result from the list of objects. You need to annotate if you need multiple result like following,
YourModel.objects.values("YOUR GROUP BY VALUES HERE").annotate(Avg('students'), Avg('playtime'), Avg('grade'), Avg('sales')
Related
I have a queryset like this:
predicts = Prediction.objects.select_related('match').filter(match_id=pk)
I need to annotate this with a new field is_correct. I need to compare two string fields and the result should be annotated in this new field. the fields that I want to compare are:
predict from Prediction table
result from Match table (that has been joined through select_related)
I need to know what expression should I put inside my annotate function; below I have my current code which throughs a TypeError exception:
predicts = predicts.annotate(is_correct=(F('predict') == F('result')))
all help will be greatly appreciated.
UPDATE:
I found an alternative solution that does the job for me (filtering the Prediction based on Match result using filter and exclude), but I still like to know how to address this specific case where the new annotated field is the result of the comparison between two other fields of the queryset. For those who may need it, in Django 2.2 and later the Nullif database function does a comparison between two fields.
You can use the extra function, a hook for injecting specific clauses into the SQL.
First of all, we must know the names of the apps and the models, or the name of the tables in the database.
Assuming that in your case, the two tables are called "app_prediction" and "app_match".
The sentence would be as follows:
Prediction.objects.select_related('match').extra(
select={'is_correct': "app_prediction.predict = app_match.result"}
)
This will add a field called is_correct in your result,
in the database, the fields and tables must be called in the same way.
It would be best to see the models.
Can anyone help, I want to return an ordered list based on forloop in Django using a field in the model that contains both integer and string in the format MM/1234. The loop should return the values with the least interger(1234) in ascending order in the html template.
Ideally you want to change the model to have two fields, one integer and one string, so you can code a queryset with ordering based on the integer one. You can then define a property of the model to return the self.MM+"/"+str( self.nn) composite value if you often need to use that. But if it's somebody else's database schema, this may not be an option.
In which case you'll have to convert your queryset into a list (which reads all the data rows at once) and then sort the list in Python rather than in the database. You can run out of memory or bring your server to its knees if the list contains millions of objects. count=qs.count() is a DB operation that won't.
qs = Foo.objects.filter( your_selection_criteria)
# you might want to count this before the next step, and chicken out if too many
# also this simple key function will crash if there's ever no "/" in that_field
all_obj = sorted( list( qs),
key = lambda obj: obj.that_field.split('/')[1] )
i come with this doubt abobt how to make a linked dates list based on existing objects, first of all i have a model with a DateTimeField which stores the date and hour that the object was added.
I have something like:
|pk|name|date
|1|name 1|2016-08-02 16:14:30.405305
|2|name 2|2016-08-02 16:15:30.405305
|3|name 3|2016-08-03 16:46:29.532976
|4|name 4|2016-08-04 16:46:29.532976
And i have some records with the same day but different hour, what i want is to make a list displaying only the unique days:
2016-08-02
2016-08-03
2016-08-04
And also because i'm using the CBV DayArchiveView i want to add a link to that elements to list them per day with a url pattern like this:
url(r'^archive/(?P<year>[0-9]{4})/(?P<month>[-\w]+)/(?P<day>[0-9]+)/$', ArticleDayArchiveView.as_view(), name="archive_day"),
The truth is that i don't have a clue of how to achieve that, can you help me with that?
Extracting unique dates
instances = YourModel.objects.all()
unique_dates = list(set(map(lambda x: x.date.strftime("%Y-%m-%d"), instances)))
About listing them, your url pattern looks ok. You need to define a view in order to retrieve them and wire up with that url.
UPDATE:
If you want to order them, just:
sorted_dates = sorted(unique_dates)
I have a Result object that is tagged with "one" and "two". When I try to query for objects tagged "one" and "two", I get nothing back:
q = Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
print len(q)
# prints zero, was expecting 1
Why does it not work with Q? How can I make it work?
The way django-taggit implements tagging is essentially through a ManytoMany relationship. In such cases there is a separate table in the database that holds these relations. It is usually called a "through" or intermediate model as it connects the two models. In the case of django-taggit this is called TaggedItem. So you have the Result model which is your model and you have two models Tag and TaggedItem provided by django-taggit.
When you make a query such as Result.objects.filter(Q(tags__name="one")) it translates to looking up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has the name="one".
Trying to match for two tag names would translate to looking up up rows in the Result table that have a corresponding row in the TaggedItem table that has a corresponding row in the Tag table that has both name="one" AND name="two". You obviously never have that as you only have one value in a row, it's either "one" or "two".
These details are hidden away from you in the django-taggit implementation, but this is what happens whenever you have a ManytoMany relationship between objects.
To resolve this you can:
Option 1
Query tag after tag evaluating the results each time, as it is suggested in the answers from others. This might be okay for two tags, but will not be good when you need to look for objects that have 10 tags set on them. Here would be one way to do this that would result in two queries and get you the result:
# get the IDs of the Result objects tagged with "one"
query_1 = Result.objects.filter(tags__name="one").values('id')
# use this in a second query to filter the ID and look for the second tag.
results = Result.objects.filter(pk__in=query_1, tags__name="two")
You could achieve this with a single query so you only have one trip from the app to the database, which would look like this:
# create django subquery - this is not evaluated, but used to construct the final query
subquery = Result.objects.filter(pk=OuterRef('pk'), tags__name="one").values('id')
# perform a combined query using a subquery against the database
results = Result.objects.filter(Exists(subquery), tags__name="two")
This would only make one trip to the database. (Note: filtering on sub-queries requires django 3.0).
But you are still limited to two tags. If you need to check for 10 tags or more, the above is not really workable...
Option 2
Query the relationship table instead directly and aggregate the results in a way that give you the object IDs.
# django-taggit uses Content Types so we need to pick up the content type from cache
result_content_type = ContentType.objects.get_for_model(Result)
tag_names = ["one", "two"]
tagged_results = (
TaggedItem.objects.filter(tag__name__in=tag_names, content_type=result_content_type)
.values('object_id')
.annotate(occurence=Count('object_id'))
.filter(occurence=len(tag_names))
.values_list('object_id', flat=True)
)
TaggedItem is the hidden table in the django-taggit implementation that contains the relationships. The above will query that table and aggregate all the rows that refer either to the "one" or "two" tags, group the results by the ID of the objects and then pick those where the object ID had the number of tags you are looking for.
This is a single query and at the end gets you the IDs of all the objects that have been tagged with both tags. It is also the exact same query regardless if you need 2 tags or 200.
Please review this and let me know if anything needs clarification.
first of all, this three are same:
Result.objects.filter(tags__name="one", tags__name="two")
Result.objects.filter(Q(tags__name="one") & Q(tags__name="two"))
Result.objects.filter(tags__name_in=["one"]).filter(tags__name_in=["two"])
i think the name field is CharField and no record could be equal to "one" and "two" at same time.
in python code the query looks like this(always false, and why you are geting no result):
from random import choice
name = choice(["abtin", "shino"])
if name == "abtin" and name == "shino":
we use Q object for implement OR or complex queries
Into the example that works you do an end on two python objects (query sets). That gets applied to any record not necessarily to the same record that has one AND two as tag.
ps: Why do you use the in filter ?
q = Result.objects.filter(tags_name_in=["one"]).filter(tags_name_in=["two"])
add .distinct() to remove duplicates if expecting more than one unique object
Suppose I have a model Order, which has a column num -- an order number. Now I want to filter several rows from this model in admin view. Having 1 value, I do:
http://bla-bla-bla/admin/app/order/?num__exact=11534
How can I do this when I have several values?
Or should I use queryset()? How then I should send a list of values to request?
in should work, try this in the url
http://bla-bla-bla/admin/app/order/?num__in=11534,11535,11536
Don't forget that whatever you put in the query string has to be allowed for the admin interface. You can't put in filters that weren't defined there - ever since this security release https://www.djangoproject.com/weblog/2010/dec/22/security/