django models Making queries

这是后面要用到的类

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

    def __unicode__(self):
        return self.name

class Author(models.Model):
    name = models.CharField(max_length=50)
    email = models.EmailField()

    def __unicode__(self):
        return self.name

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

    def __unicode__(self):
        return self.headline

Creating objects

>>> from blog.models import Blog
>>> b = Blog(name=‘Beatles Blog‘, tagline=‘All the latest Beatles news.‘)
>>> b.save()

Saving ForeignKey and ManyToManyField fields

>>> from blog.models import Entry
>>> entry = Entry.objects.get(pk=1)
>>> cheese_blog = Blog.objects.get(name="Cheddar Talk")
>>> entry.blog = cheese_blog
>>> entry.save()
>>> from blog.models import Author
>>> joe = Author.objects.create(name="Joe")
>>> entry.authors.add(joe)
>>> 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)

Retrieving objects

>>> Blog.objects
<django.db.models.manager.Manager object at ...>
>>> b = Blog(name=‘Foo‘, tagline=‘Bar‘)
>>> b.objects
Traceback:
    ...
AttributeError: "Manager isn‘t accessible via Blog instances."

Retrieving all objects

>>> all_entries = Entry.objects.all()

filter(**kwargs)

Returns a new QuerySet containing objects that match the given lookup parameters.

exclude(**kwargs)

Returns a new QuerySet containing objects that do not match the given lookup parameters.

Entry.objects.filter(pub_date__year=2006)

Chaining filters

>>> Entry.objects.filter(
...     headline__startswith=‘What‘
... ).exclude(
...     pub_date__gte=datetime.now()
... ).filter(
...     pub_date__gte=datetime(2005, 1, 1)
... )

get,get 方法会返回符合条件的那个元素，当没有符合筛选条件的元素是get方法会抛出一个 DoesNotExist的异常，所以要慎用

>>> one_entry = Entry.objects.get(pk=1)

Field lookups

筛选属性的时候的一些限制条件，基本格式是 field__lookuptype=value，注意是双下划线，下面是lookuptype

exact：Exact match，准确匹配，可以匹配到null（None） iexact： 不区分大小写的准确匹配

contains：包含                                                        icontains：不区分大小写的包含

in：在（一个筛选的列表）里面

gt ：Greater than                                                    gte：Greater than or equal to.

lt：Less than.                                                          lte：Less than or equal to.

startswith，istartswith，endswith，iendswith

range：pub_date__range=(start_date, end_date))

year，month，day，weekday，

regex:正则表达式匹配，iregex，不区分大小写的

Aggregation functions

聚合函数，针对查询集合的一些操作

Avg：计算平均值，返回值类型float

Count：计算集合的元素个数，返回值类型int

Max，Min，Sum

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