Django 检查查询是否存在

在 django 中如何检查查询是否存在任何条目

sc=scorm.objects.filter(Header__id=qp.id)

在 php 中就是这样做的

if(mysql_num_rows($resultn)) {
// True condition
}
else {
// False condition
}
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小开
最佳答案

You can use exists():

if scorm.objects.filter(Header__id=qp.id).exists():
....

Returns True if the QuerySet contains any results, and False if not. This tries to perform the query in the simplest and fastest way possible, but it does execute nearly the same query as a normal QuerySet query.

Older versions: (<1.2)

Use count():

sc=scorm.objects.filter(Header__id=qp.id)


if sc.count() > 0:
...

The advantage over e.g. len() is, that the QuerySet is not yet evaluated:

count() performs a SELECT COUNT(*) behind the scenes, so you should always use count() rather than loading all of the record into Python objects and calling len() on the result.

Having this in mind, When QuerySets are evaluated can be worth reading.

If you use get(), e.g. scorm.objects.get(pk=someid), and the object does not exists, an ObjectDoesNotExist exception is raised:

from django.core.exceptions import ObjectDoesNotExist
try:
sc = scorm.objects.get(pk=someid)
except ObjectDoesNotExist:
print ...
小开

As of Django 1.2, you can use exists():

https://docs.djangoproject.com/en/dev/ref/models/querysets/#exists

if some_queryset.filter(pk=entity_id).exists():
print("Entry contained in queryset")
小开

this worked for me!

if some_queryset.objects.all().exists(): print("this table is not empty")

小开

Django provides a method called exists() to check if results exists for our query. exists() method return 'True' or 'False'

Class Company(models.Model):
name = models.CharField(max_length=100)
year_established = models.DateField()


Class Car(models.Model):
name = models.CharField(max_length=100)
company = models.ForeignKey(Company,related_name='car_company',on_delete=models.CASCADE)

following are the queries with exists() method

1. Car.objects.filter(name='tesla').exists()
2. Company.objects.filter(year_established='date').exists()

using exists in related field - here foreign key

  1. Car.objects.filter(company__name='tesla').exists()

you can give any filter available and use exists() method

小开

len(queryset) also works.

sc = scorm.objects.filter(Header__id=qp.id)


if len(sc):
....

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