2：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

高级操作

F类：我的理解是返回特定字段的值

例如我想找到所有评论比广播次数多的entry，就可以按照下面这样写

>>> from django.db.models import F
>>> Entry.objects.filter(n_comments__gt=F('n_pingbacks'))

F类返回的是该字段的值，当值是数值的是，可以做算术运算，当时字符串是，可以比较是否相等等运算

pk ：代表primary key

Q类：封装查询已形成更复杂的查询，每个Q查询之间可以用&（and）或者|（or）相连，用逗号相隔表示关系and，Q查询也可以使用~（not）

Poll.objects.get(
    Q(question__startswith='Who'),
    Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))
)
#roughly translates into the SQL:
SELECT * from polls WHERE question LIKE 'Who%'
    AND (pub_date = '2005-05-02' OR pub_date = '2005-05-06')

注意：Q查询一定要在关键字查询之前，否则查询可能无效

关系查询

1：外键查询，一个类可以通过属性的方法来访问外键的类

>>> e = Entry.objects.get(id=2)
>>> e.blog = some_blog
>>> e.save()

2：一个被外键的类可以通过FOO_set的方法来访问源类，其中Foo是源类

>>> b = Blog.objects.get(id=1)
>>> b.entry_set.all() # Returns all Entry objects related to Blog.

# b.entry_set is a Manager that returns QuerySets.
>>> b.entry_set.filter(headline__contains='Lennon')
>>> b.entry_set.count()

3：其他的多对一，一对多，一对一的情况可以类比上面这两种情况

如果你不想使用django提供的api接口，那么你可以通过下面两种方式直接运行sql语句

第一种，通过Manager.raw(raw_query, params=None, translations=None)的方法运行sql语句

class Person(models.Model):
    first_name = models.CharField(...)
    last_name = models.CharField(...)
    birth_date = models.DateField(...)
>>> Person.objects.raw('SELECT id, first_name, last_name, birth_date FROM myapp_person')
...
>>> Person.objects.raw('SELECT last_name, birth_date, first_name, id FROM myapp_person')
...
>>> Person.objects.raw('''SELECT first AS first_name,
...                              last AS last_name,
...                              bd AS birth_date,
...                              pk as id,
...                       FROM some_other_table''')

第二种：完全不用django提供的api，自己建立数据库连接，运行sql查询

def my_custom_sql():
    from django.db import connection, transaction
    cursor = connection.cursor()

    # Data modifying operation - commit required
    cursor.execute("UPDATE bar SET foo = 1 WHERE baz = %s", [self.baz])
    transaction.commit_unless_managed()

    # Data retrieval operation - no commit required
    cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
    row = cursor.fetchone()

    return row