加入多个模型以便在 Django 中获取 orm 查询？

Question

我必须返回特定时间内最畅销曲目的平均长度，使用 3 个模型我设法形成一个看起来像这样的 sql 查询

select  AVG(t.Milliseconds) as lengthTrack
from invoice_items as il
    join invoices as i on i.invoiceid = il.invoiceid
    join tracks as t on t.TrackId = il.TrackId 
where i.InvoiceDate between '2012-01-01' and '2013-12-31'
order by il.Quantity DESC LIMIT 10

我设法想出了这个，因为我真的很困惑如何加入这两个表以获得 10 个最畅销的表的平均长度跟踪：

start_date = datetime.date(2012, 01, 01)
end_date=datetime.date(2013, 12, 31)

InvoiceItem .objects.filter(ino_date__range=(start_date, end_date),)

模型.py

class Invoice(models.Model):
    id                = models.AutoField(primary_key=True, db_column='InvoiceId')
    customerid        = models.ForeignKey(Customer, db_column='CustomerId', on_delete=models.CASCADE)
    invoicedate       = models.DateTimeField(db_column='InvoiceDate')
    billingaddress    = models.TextField(max_length=70, db_column='BillingAddress', blank=True, null=True)
    billingcity       = models.TextField(max_length=40, db_column='BillingCity', blank=True, null=True)
    billingstate      = models.TextField(max_length=40, db_column='BillingState', blank=True, null=True)
    billingcountry    = models.TextField(max_length=40, db_column='BillingCountry', blank=True, null=True)
    billingpostalcode = models.TextField(max_length=10, db_column='BillingPostalCode', blank=True, null=True)
    total             = models.TextField(db_column='Total')

class Track(models.Model):
    id           = models.AutoField(primary_key=True, db_column='TrackId')
    playlist     = models.ManyToManyField(Playlist, through='PlaylistTrack')
    name         = models.TextField(db_column='Name', max_length=200)
    albumid      = models.ForeignKey(Album, db_column='AlbumId', blank=True, null=True, on_delete=models.CASCADE)
    mediatypeid  = models.ForeignKey(MediaType, db_column='MediaTypeId', on_delete=models.CASCADE)
    genreid      = models.ForeignKey(Genre, db_column='GenreId', blank=True, null=True, on_delete=models.CASCADE)
    composer     = models.TextField(db_column='Composer', blank=True, null=True, max_length=220)
    milliseconds = models.IntegerField(db_column='Milliseconds')
    bytes        = models.IntegerField(db_column='Bytes', blank=True, null=True)
    unitprice    = models.DecimalField(db_column='UnitPrice', max_digits=5, decimal_places=2)

class InvoiceItem(models.Model):
    id        = models.AutoField(primary_key=True, db_column='InvoiceLineId')
    invoiceid = models.ForeignKey(Invoice, db_column='InvoiceId', on_delete=models.CASCADE)
    trackid   = models.ForeignKey(Track, db_column='TrackId', on_delete=models.CASCADE)
    unitprice = models.DecimalField(db_column='UnitPrice', max_digits=5, decimal_places=2)
    quantity  = models.IntegerField(db_column='Quantity')

serialerzs.py

class InvoiceSerialer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Invoice
        fields = ('id', 'total')


class InvoiceItemSerialer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = InvoiceItem
        fields = '__all__'


class TrackSerialer(serializers.HyperlinkedModelSerializer):
    class Meta:
        model = Track
        fields = '__all__'

在视图中我是否需要在 erializer_class 中添加 3 个序列化程序？我也想问一下我的查询是否正确？

Answer 1

在两个日期之间获得最多的销量将是一个注释轨道在这些日期之间售出多少次的问题。

from django.db.models import Count, Q
top_ten_selling_tracks = Track.objects.annotate(
   sales_count=Count( 
      'invoice_item', # count the number of invoices that point to this track
      filter=Q(invoiceid__invoicedate__range=(start_date, end_date)) # only count them if the associated invoice falls in the appropriate range
   )).order_by(
      '-sales_count' # order them by sales descending, to get the top 10
   )[:10] # take the top ten tracks

平均值可以在 python 中完成（Django 可以轻松地聚​​合查询集，但由于您的限制约束，它可能更容易只是

return avg(t.milliseconds for t in top_ten_selling_tracks)