Djangoqueryset

2019-11-14 17:35:15

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model对应数据库表，model的实例则对应表中的一条记录.

from blog.models import Blogb = Blog(name='Beatles Blog', tagline='All the latest Beatles news.')#调用save后才会在表中生成记录，b.id才会有值，因为id是数据库生成的#此处save相当于调用了insert语句b.save()b.name="Tom"#此处save相当于调用update语句b.save()

#create方法包含创建对象并save到数据库

Blog.objects.create(name='news Blog', tagline='All news.')

添加ForeignKey和ManyToManyField的field值

from blog.models import Entryentry = Entry.objects.get(pk=1)cheese_blog = Blog.objects.get(name="Cheddar Talk")#ForeighnKey成员，直接赋值entry.blog = cheese_blogentry.save()#ManyToManyField成员需要调用add方法joe = Author.objects.create(name="Joe")entry.authors.add(joe)#添加多个成员时paul = Author.objects.create(name="Paul")george = Author.objects.create(name="George")ringo = Author.objects.create(name="Ringo")entry.authors.add(paul,george,ringo)

每个model class都有一个Manager成员，默认名字为objects,注意是model class而不是model instance,Model.objects.all()返回所有QuerySet包含所有表中的对象.注意QuerySet是lazy的，创建时候并不会执行实际操作，只有evaluated时才真正执行操作。可以通过下面两个函数返回满足一定条件的QuerySet:
- filter(**kwargs):满足指定查询条件的QuerySet
- exclude(**kwargs):不满足查询条件的QuerySet

每个QuerySet都包含一个cache，以减少对数据库的访问，刚定义queryset时cache为空,evluated后cache就有值了，之后访问queryset都是访问cache中的值

#执行两次数据库查询PRint([e.headline for e in Entry.objects.all()])print([e.pub_date for e in Entry.objects.all()])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.queryset = Entry.objects.all()#只返回queryset中的一部分print queryset[5] # Queries the databaseprint queryset[5] # Queries the database againqueryset = Entry.objects.all()[entry for entry in queryset] # Queries the databaseprint queryset[5] # Uses cacheprint queryset[5] # Uses cache#如下操作都会导致queryset evaluated,填充cache[entry for entry in queryset]bool(queryset)entry in querysetlist(queryset)

可以返回QuerySet中的子集，相当于sql语句中的limit和offset

Entry.objects.all()[:5]Entry.objects.all()[5:10]Entry.objects.order_by('headline')[0]

返回单个对象使用get方法,如果对象不存在会抛出Model.DoesNotExist异常，如果有多个满足条件的对象则抛出Model.MultipleObjectsReturned异常。注意此处与QuerySet object[0]的区别，如果没有获取满足条件的QuerySet，用下标取对象会抛出IndexError的异常
field查询:filter,get等函数都使用field进行查询，如果field不存在则会抛TypeError异常。例外的是以ForeignKey类型的field作查询参数时，可以在该field后面加”_id”,
```
#查找外键blog id=4的所有EntryEntry.objects.filter(blog_id=4)
```
一些比较常用的查询关键字:
- exact:精确匹配，默认使用
```
#效果相同Blog.objects.get(id__exact=14) Blog.objects.get(id=14)
```
- iexact:匹配时忽略大小写
- contains:包含查询,大小写敏感
```
#相当于SELECT ... WHERE headline LIKE '%Lennon%';Entry.objects.get(headline__contains='Lennon')
```
- icontains:包含查询，大小写不敏感
- startswith,endswith:匹配开头或结尾，大小写敏感
- istartswith,iendswith:匹配开头或结尾，大小写不敏感
- in:是否在某个list中
- gt,lt,gte,lte等比较操作
- year,month,day,week_day,hour,minute,seconde等时间操作
- isnull:是否为空

跨relationship的field查询，

#entry与blog是多对一的关系Entry.objects.filter(blog__name='Beatles Blog')Blog.objects.filter(entry__headline__contains='Lennon')#entry与authors是多对多关系Blog.objects.filter(entry__authors__name='Lennon')Blog.objects.filter(entry__authors__name__isnull=True)Blog.objects.filter(entry__authors__isnull=False,   entry__authors__name__isnull=True)

注意下面代码的区别

#返回的blog 同时满足参数中的两个条件Blog.objects.filter(entry__headline__contains='Lennon',        entry__pub_date__year=2008)#blog包含多个entry,只要其中有两个分别满足下面两个条件，就返回blogBlog.objects.filter(entry__headline__contains='Lennon').filter(        entry__pub_date__year=2008)

F class:用于获取model field的值
Q class:用于定义重用查询条件，Q objects可以使用&和|进行组合，这样会生成新的Q object,field的查询关键字都可以在Q object的参数中使用

Q(question__startswith='Who') | ~Q(pub_date__year=2005)Poll.objects.get(    Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)),    question__startswith='Who')

reporter = Reporters.objects.get(name='Tintin')reporter.stories_filed += 1reporter.save()#与上面代码效果相同reporter = Reporters.objects.filter(name='Tintin')reporter.update(stories_filed=F('stories_filed') + 1)#更新所有stories_fieldReporter.objects.all().update(stories_filed=F('stories_filed') + 1)

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

删除对象使用delete()方法，model和queryset都包含此方法用于删除单个或者多个对象。对于外键对象，如果定义时设置了on_delete参数为true,那么删除外键对象时会把指向该外键对象的对象也删除
```
e.delete()Entry.objects.filter(pub_date__year=2005).delete()Entry.objects.all().delete()b = Blog.objects.get(pk=1)# This will delete the Blog and all of its Entry objects.b.delete()
```
复制对象:

简单情况

blog = Blog(name='My blog', tagline='Blogging is easy')blog.save() # blog.pk == 1# 设置pk为Noneblog.pk = Noneblog.save() # blog.pk == 2

继承类

class ThemeBlog(Blog):    theme = models.CharField(max_length=200)django_blog = ThemeBlog(name='Django', tagline='Django is easy', theme='python')django_blog.save() # django_blog.pk == 3#需要设置id和pk为Nonedjango_blog.pk = Nonedjango_blog.id = Nonedjango_blog.save() # django_blog.pk == 4

上面两种方式都不给related object赋值，赋值related object的代码如下:

entry = Entry.objects.all()[0] # some previous entryold_authors = entry.authors.all()entry.pk = Noneentry.save()entry.authors = old_authors # saves new many2many relations

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