django orm 多对多操作

class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def __str__(self):
        return self.name

class Author(models.Model):
    name = models.CharField(max_length=200)
    email = models.EmailField()

    def __str__(self):
        return self.name

class Entry(models.Model):
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
    headline = models.CharField(max_length=255)
    body_text = models.TextField()
    pub_date = models.DateField()
    mod_date = models.DateField()
    authors = models.ManyToManyField(Author)
    n_comments = models.IntegerField()
    n_pingbacks = models.IntegerField()
    rating = models.IntegerField()

    def __str__(self):
        return self.headline

1，一对多关系的添加

>>> entry = Entry.objects.get(pk=1)
>>> cheese_blog = Blog.objects.get(name="Cheddar Talk")
>>> entry.blog = cheese_blog
>>> entry.save()

2，多对多的关系的添加

>>> john = Author.objects.create(name="John")
>>> paul = Author.objects.create(name="Paul")
>>> george = Author.objects.create(name="George")
>>> ringo = Author.objects.create(name="Ringo")
>>> entry.authors.add(john, paul, george, ringo)

3，链接式过滤查询子集

>>> Entry.objects.filter(
...     headline__startswith='What'
... ).exclude(
...     pub_date__gte=datetime.date.today()
... ).filter(
...     pub_date__gte=datetime.date(2005, 1, 30)
... )

4，get请求

one_entry = Entry.objects.get(pk=1)当get请求访问时，如果没有当前数据，那么会报DoesNotExist的错误。在实际应用中可以，try捕捉异常.ps:如果get请求捕捉到多个值，那么也会报错，错误为MultipleObjectsReturned,

二，查找方式 PS:exact,startwith,endwith,contains 等查找只要前面加上i，那么大小写是一样的。

1，exact   精确查找
Entry.objects.get(headline__exact="Cat bites dog")2,iexact 只要字母对，大小写不影响结果

Entry.objects.get(headline__iexact="Cat bites dog")3,isnull  是否为空

F参数

>>> from django.db.models import F
>>> Entry.objects.filter(n_comments__gt=F('n_pingbacks'))
>>> Entry.objects.filter(n_comments__gt=F('n_pingbacks') * 2)
>>> Entry.objects.filter(rating__lt=F('n_comments') + F('n_pingbacks'))

Entry.objects.filter(authors__name=F('blog__name'))

>>> from datetime import timedelta
>>> Entry.objects.filter(mod_date__gt=F('pub_date') + timedelta(days=3))

Q参数

Q(question__startswith='Who') | Q(question__startswith='What')

Q(question__startswith='Who') | ~Q(pub_date__year=2005)       ～表示的是非

Poll.objects.get(
    Q(question__startswith='Who'),
    Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))     这里的Q表示and
)

三，queryset缓存 print的时候：不会调用缓存，所以每次print都会引起数据库的再次调用

>>> queryset = Entry.objects.all()
>>> print([p.headline for p in queryset]) # Evaluate the query set.
>>> print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation.切片会引起queryset的再次操作

>>> queryset = Entry.objects.all()
>>> print(queryset[5]) # Queries the database
>>> print(queryset[5]) # Queries the database again如果先遍历后，那么后续切片就会有限运用缓存

>>> queryset = Entry.objects.all()
>>> [entry for entry in queryset] # Queries the database
>>> print(queryset[5]) # Uses cache
>>> print(queryset[5]) # Uses cache

 下面的例子也是应用缓存

>>> [entry for entry in queryset]
>>> bool(queryset)
>>> entry in queryset
>>> list(queryset)

四，删除delete

e.delete()
Entry.objects.filter(pub_date__year=2005).delete()
b = Blog.objects.get(pk=1)
# This will delete the Blog and all of its Entry objects.
b.delete()
Entry.objects.all().delete()

五，更新update

Entry.objects.filter(pub_date__year=2007).update(headline='Everything is the same')

>>> b = Blog.objects.get(pk=1)

# Change every Entry so that it belongs to this Blog.
>>> Entry.objects.all().update(blog=b)

来源：https://www.cnblogs.com/52forjie/p/9636570.html

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