django關於model查詢

阿新 • • 發佈：2019-01-20

Python程式碼

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()
authors = models.ManyToManyField(Author)
def __unicode__(self):
return

self.headline

這是model，有blog，author，以及entry；其中entry分別與blog與author表關聯，entry與blog表是通過外來鍵(models.ForeignKey())相連，屬於一對多的關係，即一個entry對應多個blog，entry與author是多對多的關係，通過modles.ManyToManyField()實現。
一、插入資料庫，用save()方法實現，如下：
>>> from mysite.blog.models import Blog
>>> b = Blog(name='Beatles Blog', tagline='All the latest Beatles news.')

>>> b.save()

二、更新資料庫，也用save()方法實現，如下：
>> b5.name = 'New name'
>> b5.save()

儲存外來鍵和多對多關係的欄位，如下例子：
更新外來鍵欄位和普通的欄位一樣，只要指定一個物件的正確型別。
>>> cheese_blog = Blog.objects.get(name="Cheddar Talk")
>>> entry.blog = cheese_blog
>>> entry.save()

更新多對多欄位時又一點不太一樣，使用add()方法新增相關聯的欄位的值。
>> joe = Author.objects.create(name="Joe")
>> entry.authors.add(joe)

三、檢索物件

>>> 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."

1、檢索所有的物件

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

使用all()方法返回資料庫中的所有物件。

2、檢索特定的物件
使用以下兩個方法：
fileter(**kwargs)
返回一個與引數匹配的QuerySet,相當於等於(=).
exclude(**kwargs)
返回一個與引數不匹配的QuerySet,相當於不等於(!=)。

Entry.objects.filter(pub_date__year=2006)
不使用Entry.objects.all().filter(pub_date__year=2006),雖然也能執行，all()最好再獲取所有的物件時使用。
上面的例子等同於的sql語句：
slect * from entry where pub_date_year='2006'

連結過濾器：
>>> Entry.objects.filter(
...     headline__startswith='What'
... ).exclude(
...     pub_date__gte=datetime.now()
... ).filter(
...     pub_date__gte=datetime(2005, 1, 1)
... )

最後返回的QuerySet是headline like 'What%' and put_date<now() and pub_date>2005-01-01

另外一種方法：
>> q1 = Entry.objects.filter(headline__startswith="What")
>> q2 = q1.exclude(pub_date__gte=datetime.now())
>> q3 = q1.filter(pub_date__gte=datetime.now())
這種方法的好處是可以對q1進行重用。

QuerySet是延遲載入
只在使用的時候才會去訪問資料庫，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時才會訪問資料庫。

其他的QuerySet方法
>>> Entry.objects.all()[:5]
這是查詢前5個entry表裡的資料

>>> Entry.objects.all()[5:10]
這是查詢從第5個到第10個之間的資料。

>>> Entry.objects.all()[:10:2]
這是查詢從第0個開始到第10個，步長為2的資料。

>>> Entry.objects.order_by('headline')[0]
這是取按headline欄位排序後的第一個物件。

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

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

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

>>> Blog.objects.get(id__exact=14) # Explicit form
>>> Blog.objects.get(id=14)         # __exact is implied
這兩種方式是等同的，都是查詢id=14的物件。

>>> Blog.objects.get(name__iexact="beatles blog")
查詢name="beatles blog"的物件，不去飯大小寫。

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

startswith 等同於sql語句中的 name like 'Lennon%',
endswith等同於sql語句中的 name like '%Lennon'.

>>> Entry.objects.filter(blog__name__exact='Beatles Blog')
查詢entry表中外來鍵關係blog_name='Beatles Blog'的Entry物件。

>>> Blog.objects.filter(entry__headline__contains='Lennon')
查詢blog表中外來鍵關係entry表中的headline欄位中包含Lennon的blog資料。

Blog.objects.filter(entry__author__name='Lennon')
查詢blog表中外來鍵關係entry表中的author欄位中包含Lennon的blog資料。

Blog.objects.filter(entry__author__name__isnull=True)
Blog.objects.filter(entry__author__isnull=False,entry__author__name__isnull=True)
查詢的是author_name為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)
這兩種查詢在某些情況下是相同的，某些情況下是不同的。第一種是限制所有的blog資料的，而第二種情況則是第一個filter是
限制blog的，而第二個filter則是限制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
等同於select * from where id=14

# Get blogs entries with id 1, 4 and 7
>>> Blog.objects.filter(pk__in=[1,4,7])
等同於select * from where id in{1,4,7}
# Get all blog entries with id > 14
>>> Blog.objects.filter(pk__gt=14)
等同於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
這三種情況是相同的

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

Caching and QuerySets

>>> print [e.headline for e in Entry.objects.all()]
>>> print [e.pub_date for e in Entry.objects.all()]
應改寫為：
>> queryset = Poll.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.、
這樣利用快取，減少訪問資料庫的次數。

四、用Q物件實現複雜的查詢

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

五、比較物件

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

六、刪除

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()
刪除所有

七、一次更新多個值

# Update all the headlines with pub_date in 2007.
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)

如果用save()方法，必須一個一個進行儲存，需要對其就行遍歷，如下：
for item in my_queryset:
    item.save()

關聯物件

one-to-many
>>> e = Entry.objects.get(id=2)
>>> e.blog # Returns the 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() # "UPDATE blog_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; it must be accessed from an instance:
>>> Blog.entry_set

add(obj1, obj2, ...)
    Adds the specified model objects to the related object set.
create(**kwargs)
    Creates a new object, saves it and puts it in the related object set. Returns the newly created object.
remove(obj1, obj2, ...)
    Removes the specified model objects from the related object set.
clear()
    Removes all objects from the related object set.

many-to-many型別：
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 this Author.

one-to-one 型別：
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語句進行查詢：

def my_custom_sql(self):
    from django.db import connection
    cursor = connection.cursor()
    cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
    row = cursor.fetchone()
    return row

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