Count number of records by date in Django

问题:

I have a model similar to the following:

class Review(models.Model):
    venue = models.ForeignKey(Venue, db_index=True)
    review = models.TextField()  
    datetime_created = models.DateTimeField(default=datetime.now)

I'd like to query the database to get the total number of reviews for a venue grouped by day. The MySQL query would be:

SELECT DATE(datetime_created), count(id) 
FROM REVIEW 
WHERE venue_id = 2
GROUP BY DATE(datetime_created);

What is the best way to accomplish this in Django? I could just use

Review.objects.filter(venue__pk=2)

and parse the results in the view, but that doesn't seem right to me.

回答1:

This should work (using the same MySQL specific function you used):

Review.objects.filter(venue__pk=2)
    .extra({'date_created' : "date(datetime_created)"})
    .values('date_created')
    .annotate(created_count=Count('id'))

回答2:

Just for completeness, since extra() is aimed for deprecation, one could use this approach:

from django.db.models.expressions import DateTime

Review.objects.all().\
    annotate(month=DateTime("timestamp", "month", pytz.timezone("Etc/UTC"))).\
    values("month").\
    annotate(created_count=Count('id')).\
    order_by("-month")

It worked for me in django 1.8, both in sqlite and MySql databases.

回答3:

If you were storing a date field, you could use this:

from django.db.models import Count

Review.objects.filter(venue__pk = 2)
    .values('date').annotate(event_count = Count('id'))

Because you're storing datetime, it's a little more complicated, but this should offer a good starting point. Check out the aggregation docs here.

回答4:

Now that Extra() is being depreciated a more appropriate answer would use Trunc such as this accepted answer

Now the OP's question would be answered as follows

from django.db.models.functions import TruncDay

Review.objects.all()
    .annotate(date=TruncDay('datetime_created')
    .values("date")
    .annotate(created_count=Count('id'))
    .order_by("-date")

回答5:

Also you can define custom function:

from django.db.models.expressions import Func

# create custom sql function
class ExtractDateFunction(Func):
    function = "DATE" # thats the name of function, the way it mapped to sql

# pass this function to annotate
Review.objects.filter(venue__pk=2)
      .annotate(date_created=ExtractDateFunction("datetime_created"))
      .values('date_created')
      .annotate(created_count=Count('id'))

Just make sure that your DB engine supports DATE function