django 的数据库操作

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本文详细介绍了如何使用Python ORM与Django模型进行数据库操作，包括数据的插入、更新、检索以及关联对象的管理。从创建模型类、实例化对象、执行SQL语句到复杂的查询操作和多对多关系的处理，全面覆盖了Django框架下模型操作的实用技巧。

Python代码

class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def__unicode__(self):
returnself.name
class Author(models.Model):
name = models.CharField(max_length=50)
email = models.EmailField()
def__unicode__(self):
returnself.name
class Entry(models.Model):
blog = models.ForeignKey(Blog)
headline =models.CharField(max_length=255)
body_text = models.TextField()
pub_date = models.DateTimeField()
authors = models.ManyToManyField(Author)
def__unicode__(self):
returnself.headline

这是model，有blog，author，以及entry；其中entry分别与blog与author表关联，entry与blog表是通过外键(models.ForeignKey())相连，属于一对多的关系，即一个entry对应多个blog，entry与author是多对多的关系，通过modles.ManyToManyField()实现。
一、插入数据库，用save()方法实现，如下：
>>> frommysite.blog.models import Blog
>>> b =Blog(name='Beatles Blog', tagline='All the latest Beatlesnews.')
>>> b.save()

二、更新数据库，也用save()方法实现，如下：
>> b5.name = 'New name'
>> b5.save()

保存外键和多对多关系的字段，如下例子：
更新外键字段和普通的字段一样，只要指定一个对象的正确类型。
>>> cheese_blog =Blog.objects.get(name="Cheddar Talk")
>>> entry.blog =cheese_blog
>>> entry.save()

更新多对多字段时又一点不太一样，使用add()方法添加相关联的字段的值。
>> joe =Author.objects.create(name="Joe")
>> entry.authors.add(joe)

三、检索对象

>>> Blog.objects
<django.db.models.manager.Managerobject at ...>
>>> b =Blog(name='Foo', tagline='Bar')
>>> b.objects
Traceback:
    ...
AttributeError: "Manager isn't accessible via Bloginstances."

1、检索所有的对象

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

使用all()方法返回数据库中的所有对象。

2、检索特定的对象
使用以下两个方法：
fileter(**kwargs)
返回一个与参数匹配的QuerySet,相当于等于(=).
exclude(**kwargs)
返回一个与参数不匹配的QuerySet,相当于不等于(!=)。

Entry.objects.filter(pub_date__year=2006)
不使用Entry.objects.all().filter(pub_date__year=2006),虽然也能运行，all()最好再获取所有的对象时使用。
上面的例子等同于的sql语句：
slect * from entry where pub_date_year='2006'

链接过滤器：
>>>Entry.objects.filter(
...    headline__startswith='What'
... ).exclude(
...    pub_date__gte=datetime.now()
... ).filter(
...    pub_date__gte=datetime(2005, 1, 1)
... )

最后返回的QuerySet是headline like 'What%' andput_date<now() andpub_date>2005-01-01

另外一种方法：
>> q1 =Entry.objects.filter(headline__startswith="What")
>> q2 =q1.exclude(pub_date__gte=datetime.now())
>> q3 =q1.filter(pub_date__gte=datetime.now())
这种方法的好处是可以对q1进行重用。

QuerySet是延迟加载
只在使用的时候才会去访问数据库，如下：
>>> q =Entry.objects.filter(headline__startswith="What")
>>> q =q.filter(pub_date__lte=datetime.now())
>>> q =q.exclude(body_text__icontains="food")
>>> print q
在print q时才会访问数据库。

其他的QuerySet方法
>>>Entry.objects.all()[:5]
这是查找前5个entry表里的数据

>>>Entry.objects.all()[5:10]
这是查找从第5个到第10个之间的数据。

>>>Entry.objects.all()[:10:2]
这是查询从第0个开始到第10个，步长为2的数据。

>>>Entry.objects.order_by('headline')[0]
这是取按headline字段排序后的第一个对象。

>>>Entry.objects.order_by('headline')[0:1].get()
这和上面的等同的。

>>>Entry.objects.filter(pub_date__lte='2006-01-01')
等同于SELECT * FROM blog_entry WHERE pub_date <='2006-01-01';

>>>Entry.objects.get(headline__exact="Man bites dog")
等同于SELECT ... WHERE headline = 'Man bites dog';

>>>Blog.objects.get(id__exact=14) # Explicit form
>>>Blog.objects.get(id=14)        # __exact is implied
这两种方式是等同的，都是查找id=14的对象。

>>>Blog.objects.get(name__iexact="beatles blog")
查找name="beatles blog"的对象，不去饭大小写。

Entry.objects.get(headline__contains='Lennon')
等同于SELECT ... WHERE headline LIKE '%Lennon%';

startswith 等同于sql语句中的 name like 'Lennon%',
endswith等同于sql语句中的 name like '%Lennon'.

>>>Entry.objects.filter(blog__name__exact='Beatles Blog')
查找entry表中外键关系blog_name='Beatles Blog'的Entry对象。

>>>Blog.objects.filter(entry__headline__contains='Lennon')
查找blog表中外键关系entry表中的headline字段中包含Lennon的blog数据。

Blog.objects.filter(entry__author__name='Lennon')
查找blog表中外键关系entry表中的author字段中包含Lennon的blog数据。

Blog.objects.filter(entry__author__name__isnull=True)
Blog.objects.filter(entry__author__isnull=False,entry__author__name__isnull=True)
查询的是author_name为null的值

Blog.objects.filter(entry__headline__contains='Lennon',entry__pub_date__year=2008)
Blog.objects.filter(entry__headline__contains='Lennon').filter(entry__pub_date__year=2008)
这两种查询在某些情况下是相同的，某些情况下是不同的。第一种是限制所有的blog数据的，而第二种情况则是第一个filter是
限制blog的，而第二个filter则是限制entry的

>>>Blog.objects.get(id__exact=14) # Explicit form
>>>Blog.objects.get(id=14) # __exact is implied
>>>Blog.objects.get(pk=14) # pk implies id__exact
等同于select * from where id=14

# Get blogs entries with id 1, 4 and 7
>>>Blog.objects.filter(pk__in=[1,4,7])
等同于select * from where id in{1,4,7}
# Get all blog entries with id > 14
>>>Blog.objects.filter(pk__gt=14)
等同于select * from id>14

>>>Entry.objects.filter(blog__id__exact=3) # Explicit form
>>>Entry.objects.filter(blog__id=3)       # __exact is implied
>>>Entry.objects.filter(blog__pk=3)       # __pk implies __id__exact
这三种情况是相同的

>>>Entry.objects.filter(headline__contains='%')
等同于SELECT ... WHERE headline LIKE '%\%%';

Caching and QuerySets

>>> print [e.headlinefor e in Entry.objects.all()]
>>> print [e.pub_datefor e in Entry.objects.all()]
应改写为：
>> queryset =Poll.objects.all()
>>> print [p.headlinefor p in queryset] # Evaluate the query set.
>>> print [p.pub_datefor p in queryset] # Re-use the cache from the evaluation.、
这样利用缓存，减少访问数据库的次数。

四、用Q对象实现复杂的查询

Q(question__startswith='Who') |Q(question__startswith='What')
等同于WHERE question LIKE 'Who%' OR question LIKE 'What%'

Poll.objects.get(
   Q(question__startswith='Who'),
   Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6))
)
等同于SELECT * from polls WHERE question LIKE 'Who%' AND (pub_date ='2005-05-02' OR pub_date = '2005-05-06')

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

五、比较对象

>>> some_entry ==other_entry
>>> some_entry.id ==other_entry.id

六、删除

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()
删除所有

七、一次更新多个值

# Update all the headlines with pub_date in 2007.
Entry.objects.filter(pub_date__year=2007).update(headline='Everythingis the same')

>>> b =Blog.objects.get(pk=1)
# Change every Entry so that it belongs to this Blog.
>>>Entry.objects.all().update(blog=b)

如果用save()方法，必须一个一个进行保存，需要对其就行遍历，如下：
for item in my_queryset:
   item.save()

关联对象

one-to-many
>>> e =Entry.objects.get(id=2)
>>> e.blog # Returnsthe related Blog object.

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

>>> e =Entry.objects.get(id=2)
>>> e.blog =None
>>> e.save() # "UPDATEblog_entry SET blog_id = NULL ...;"

>>> e =Entry.objects.get(id=2)
>>> print e.blog #Hits the database to retrieve the associated Blog.
>>> print e.blog #Doesn't hit the database; uses cached version.

>>> e =Entry.objects.select_related().get(id=2)
>>> print e.blog #Doesn't hit the database; uses cached version.
>>> print e.blog #Doesn't hit the database; uses cached version

>>> 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()

>>> b =Blog.objects.get(id=1)
>>> b.entries.all() #Returns all Entry objects related to Blog.
# b.entries is a Manager that returns QuerySets.
>>>b.entries.filter(headline__contains='Lennon')
>>>b.entries.count()

You cannot access a reverse ForeignKey Manager from the class; itmust be accessed from an instance:
>>>Blog.entry_set

add(obj1, obj2, ...)
    Adds thespecified model objects to the related object set.
create(**kwargs)
    Creates anew object, saves it and puts it in the related object set. Returnsthe newly created object.
remove(obj1, obj2, ...)
    Removes thespecified model objects from the related object set.
clear()
    Removes allobjects from the related object set.

many-to-many类型：
e = Entry.objects.get(id=3)
e.authors.all() # Returns all Author objects for this Entry.
e.authors.count()
e.authors.filter(name__contains='John')
a = Author.objects.get(id=5)
a.entry_set.all() # Returns all Entry objects for thisAuthor.

one-to-one 类型：
class EntryDetail(models.Model):
    entry =models.OneToOneField(Entry)
    details =models.TextField()

ed = EntryDetail.objects.get(id=2)
ed.entry # Returns the related Entry object

使用sql语句进行查询：

def my_custom_sql(self):
    fromdjango.db importconnection
    cursor =connection.cursor()
   cursor.execute("SELECT foo FROM bar WHERE baz = %s",[self.baz])
    row =cursor.fetchone()
    returnrow