django进阶-查询(适合GET4以上人群阅读)

前言: 下篇博客写关于bootstrap...

一、如何在脚本测试django

 from django.db import models

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

     def __str__(self):              # __unicode__ on Python 2
         return self.name

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

     def __str__(self):              # __unicode__ on Python 2
         return self.name

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

     def __str__(self):              # __unicode__ on Python 2
         return self.headline

一般往django添加一条数据库，我们会在cmd 下导入django环境后进行测试。
那如何在.py脚本下运行测试呢？

 import os
 os.environ.setdefault("DJANGO_SETTINGS_MODULE", "day18.settings")

 import django
 django.setup()   #导入django环境

 from blog import models

 entry=models.Entry.objects.get(pk=1)
10 print(entry)

输出: 屌炸天。

二、处理带外键关联或多对多关联的对象

创建

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

This performs an INSERT SQL statement behind the scenes. Django doesn’t hit the database until you explicitly call save().
The save() method has no return value.

ForeignKey的关联

>>> 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()

ManyToManyField关联

>>> from blog.models import Author
>>> joe = Author.objects.create(name="Joe")
>>> entry.authors.add(joe)

添加多个ManyToMany对象

>>> 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)

三、查询

 all_entries = Entry.objects.all() #查询所有
 Entry.objects.filter(pub_date__year=2006) #查询所有pub_date为2006年的纪录
 Entry.objects.all().filter(pub_date__year=2006) #与上面那句一样
 >>> Entry.objects.filter(   #链式查询
 ...     headline__startswith='What'
 ... ).exclude(
 ...     pub_date__gte=datetime.date.today()
 ... ).filter(
 ...     pub_date__gte=datetime(2005, 1, 30)
 ... )

 one_entry = Entry.objects.get(pk=1) #单条查询

 Entry.objects.all()[:5] #查询前5条
 Entry.objects.all()[5:10] #你猜

 Entry.objects.order_by('headline')[0] #按headline排序取第一条

 Entry.objects.filter(pub_date__lte='2006-01-01') #相当于sql语句SELECT * FROM blog_entry WHERE pub_date <= '2006-01-01';

 Entry.objects.get(headline__exact="Cat bites dog") #相当于SELECT ... WHERE headline = 'Cat bites dog';
 Blog.objects.get(name__iexact="beatles blog") #与上面相同,只是大小写不敏感

 Entry.objects.get(headline__contains='Lennon') #相当 于SELECT ... WHERE headline LIKE '%Lennon%';

四、对同一表内不同的字段进行对比查询－F

对同一表内不同的字段进行对比查询，In the examples given so far, we have constructed filters that compare the value of a model field with a constant. But what if you want to compare the value of a model field with another field on the same model?

Django provides F expressions to allow such comparisons. Instances of F() act as a reference to a model field within a query. These references can then be used in query filters to compare the values of two different fields on the same model instance.

For example, to find a list of all blog entries that have had more comments than pingbacks, we construct an F() object to reference the pingback count, and use that F() object in the query: gt表示大于,lt表示小于,gte表示大于等于。

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

示例:

 import os
 os.environ.setdefault("DJANGO_SETTINGS_MODULE", "day18.settings")

 import django
 django.setup()#导入django环境

 from blog import models

 from django.db.models import F

 objs = models.Entry.objects.filter(n_comments__gt=F('n_pingbacks'))
 #相当于原生sql语句:selectn_comments,n_pingbacksfromEntry
 #where n_comments<n_pingbacks

 print(objs)

输出: <QuerySet [<Entry: 屌炸天>, <Entry: qqqq>]>

Django supports the use of addition, subtraction, multiplication, division, modulo, and power arithmetic with F() objects, both with constants and with other F() objects. To find all the blog entries with more than twice as many comments as pingbacks, we modify the query:

>>> Entry.objects.filter(n_comments__gt=F('n_pingbacks') * 2)

To find all the entries where the rating of the entry is less than the sum of the pingback count and comment count, we would issue the query:

>>> Entry.objects.filter(rating__lt=F('n_comments') + F('n_pingbacks'))

For date and date/time fields, you can add or subtract a timedelta object. The following would return all entries that were modified more than 3 days after they were published:

>>> from datetime import timedelta
>>> Entry.objects.filter(mod_date__gt=F('pub_date') + timedelta(days=3))

　　

五、Caching and QuerySets

Each QuerySet(查询集合) contains a cache to minimize(最小化) database access. Understanding how it works will allow you to write the most efficient code.

In a newly created QuerySet, the cache is empty. The first time a QuerySet is evaluated(评估) – and, hence, a database query happens – Django saves the query results in the QuerySet’s cache and returns the results that have been explicitly(明确地) requested (e.g., the next element, if the QuerySet is being iterated over迭代). Subsequent(后来的) evaluations of the QuerySet reuse(重用) the cached results.

如果QuerySet迭代，后续评估的QuerySet重用缓存的结果。

Keep this caching behavior in mind, because it may bite you if you don’t use your QuerySets correctly. For example, the following will create two QuerySets, evaluate them, and throw them away:

>>> print([e.headline for e in Entry.objects.all()])
>>> print([e.pub_date for e in Entry.objects.all()])

That means the same database query will be executed twice, effectively doubling your database load. Also, there’s a possibility the two lists may not include the same database records, because an Entry may have been added or deleted in the split second between the two requests.

To avoid this problem, simply save the QuerySet and reuse it:

>>> queryset = Entry.objects.all()                  #为节省资源，只取了很少的部分数据
>>> print([p.headline for p in queryset]) # Evaluate the query set.真正循环时才取出来
>>> print([p.pub_date for p in queryset]) # Re-use the cache from the evaluation.已经取出了缓存中了，这句代码不用再去数据库中取

When QuerySets are not cached

Querysets do not always cache their results. When evaluating only part of the queryset, the cache is checked, but if it is not populated then the items returned by the subsequent query are not cached. Specifically, this means that limiting the queryset using an array slice or an index will not populate the cache.

For example, repeatedly getting a certain index in a queryset object will query the database each time:

>>> queryset = Entry.objects.all()
>>> print queryset[5] # Queries the database
>>> print queryset[5] # Queries the database again　　再去数据库中查询，用不到缓存

However, if the entire queryset has already been evaluated, the cache will be checked instead:

>>> queryset = Entry.objects.all()
>>> [entry for entry in queryset] # Queries the database　从数据库取数据后遍历
>>> print queryset[5] # Uses cache
>>> print queryset[5] # Uses cache　不用再去数据库中查询，到缓存查询，更快

　　

六、复杂查询－Q

Complex lookups with Q objects（复杂查询）

Keyword argument queries – in filter(), etc. – are “AND”ed together. If you need to execute more complex queries (for example, queries with OR statements), you can use Q objects.

A Q object (django.db.models.Q) is an object used to encapsulate(封装) a collection of keyword arguments. These keyword arguments are specified as in “Field lookups” above.
For example, this Q object encapsulates a single LIKE query:

from django.db.models import Q
Q(question__startswith='What')

Q objects can be combined using the & and | operators. When an operator is used on two Q objects, it yields a new Q object.

For example, this statement yields a single Q object that represents the “OR” of two "question__startswith" queries:

Q(question__startswith='Who') | Q(question__startswith='What')

This is equivalent to(相当于) the following SQL WHERE clause:

WHERE question LIKE 'Who%' OR question LIKE 'What%'

You can compose statements of arbitrary(任意的) complexity by combining Q objects with the & and | operators and use parenthetical grouping. Also, Q objects can be negated(否定) using the ~ operator, allowing for combined lookups that combine both a normal query and a negated (NOT) query:

Q(question__startswith='Who') | ~Q(pub_date__year=2005)

Each lookup function that takes keyword-arguments (e.g. filter(), exclude(), get()) can also be passed one or more Q objects as positional (not-named) arguments. If you provide multiple Q object arguments to a lookup function, the arguments will be “AND”ed together(下面的逗号表示and). For example:

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(转化为SQL语句如下):

SELECT * from polls WHERE question LIKE 'Who%'
    AND (pub_date = '2005-05-02' OR pub_date = '2005-05-06')

示例:

 import os
 os.environ.setdefault("DJANGO_SETTINGS_MODULE","day18.settings")

 import django
 django.setup()#导入django环境

 from blog import models
 from django.db.models import F,Q

 objs=models.Entry.objects.filter(Q(n_comments__gt=F('n_pingbacks')), Q(pub_date__gt="2017-3-18"))

 print(objs)

输出: <QuerySet [<Entry: 屌炸天>]>

if a Q object is provided, it must precede the definition of any keyword arguments(Q语句要放前面). For example:

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

... would be a valid query, equivalent to the previous example; but:

# INVALID QUERY
Poll.objects.get(
    question__startswith='Who',
    Q(pub_date=date(2005, 5, 2)) | Q(pub_date=date(2005, 5, 6)))

... would not be valid.

七、批量自增

在原有数据的基础上批量自增

Calls to update(调用更新) can also use F expressions to update one field based on the value of another field in the model. This is especially useful for incrementing counters based upon their current value. For example, to increment the pingback count for every entry in the blog:

>>> Entry.objects.all().update(n_pingbacks=F('n_pingbacks') + 1)

However, unlike F() objects in filter and exclude clauses, you can’t introduce joins when you use F() objects in an update – you can only reference fields local to the model being updated. If you attempt to introduce a join with an F() object, a FieldErrorwill be raised:

# THIS WILL RAISE A FieldError
>>> Entry.objects.update(headline=F('blog__name'))

只能用原有字段F('n_pingbacks') + 1)进行更新，比如Entry的字段n_pingbacks;
若想找到与Entry外键关联blog的name，再更新到headline. 是不行的。eg: headline=F('blog__name')

八、反向关联

表结构参考上篇博客: django进阶-3

 from app01 import models as book_models
 pub_obj = book_models.Publisher.objects.last()
 print(pub_obj)
 #反向关联　书类中与出版社多对多关联，但这种关联是双向的，所以可以根据出版社找出书的集合
 #　book_set中book为书的表名，出版社反向关联book，数据库中书的表是小字的
 print(pub_obj.book_set.select_related())

输出:

<惠来出版社>
<QuerySet [<Book: <跟zcl学python <惠来出版社>>>, <Book: <新书A <惠来出版社>>>, <Book: <新书A <惠来出版社>>>, <Book: <新书A <惠来出版社>>>, <Book: <zcl_python <惠来出版社>>>, <Book: <hello world <惠来出版社>>>]>

反向关联默认在django admin是看不到的!

九、聚合查询

示例models

 from django.db import models

 class Author(models.Model):
     name = models.CharField(max_length=100)
     age = models.IntegerField()

 class Publisher(models.Model):
     name = models.CharField(max_length=300)
     num_awards = models.IntegerField()

 class Book(models.Model):
     name = models.CharField(max_length=300)
     pages = models.IntegerField()
     price = models.DecimalField(max_digits=10, decimal_places=2)
     rating = models.FloatField()
     authors = models.ManyToManyField(Author)
     publisher = models.ForeignKey(Publisher)
     pubdate = models.DateField()

 class Store(models.Model):
     name = models.CharField(max_length=300)
     books = models.ManyToManyField(Book)
     registered_users = models.PositiveIntegerField()

常用聚合场景需求

 # Total number of books.
 >>> Book.objects.count()

 # Total number of books with publisher=BaloneyPress
 >>> Book.objects.filter(publisher__name='BaloneyPress').count()

 # Average price across all books.
 >>> from django.db.models import Avg
 >>> Book.objects.all().aggregate(Avg('price'))
 {'price__avg': 34.35}

 # Max price across all books.
 >>> from django.db.models import Max
 >>> Book.objects.all().aggregate(Max('price'))
 {'price__max': Decimal('81.20')}

 # Cost per page
 >>> Book.objects.all().aggregate(
 ...    price_per_page=Sum(F('price')/F('pages'), output_field=FloatField()))
 {'price_per_page': 0.4470664529184653}

 # All the following queries involve traversing the Book<->Publisher
 # foreign key relationship backwards.

 # Each publisher, each with a count of books as a "num_books" attribute.
 >>> from django.db.models import Count
 >>> pubs = Publisher.objects.annotate(num_books=Count('book'))
 >>> pubs
 [<Publisher BaloneyPress>, <Publisher SalamiPress>, ...]
 >>> pubs[0].num_books

 # The top 5 publishers, in order by number of books.
 >>> pubs = Publisher.objects.annotate(num_books=Count('book')).order_by('-num_books')[:5]
 >>> pubs[0].num_books

示例-1: 统计每个出版社出了多少本书

import os
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "day18.settings")

import django
django.setup()   #导入django环境

from django.db.models import Avg,Min,Max,Sum,Count
from app01 import models as book_models

#统计每个出版社出了多少本书
pub_objs = book_models.Publisher.objects.annotate(book_nums=Count("book"))
print(pub_objs)
for publisher in pub_objs:
    print(publisher.book_nums)

输出:

<QuerySet [<Publisher: <清华出版社>>, <Publisher: <惠来出版社>>]>
8
6

根据输出可知，清华出版社出版了8本书，惠来出版社出版了6本书。

示例-2: 统计某日期共出版了多少本书

print(models.Entry.objects.values()[0])  #字典形式
print(models.Entry.objects.values_list())  #元组形式

print("----------->>>")
print(book_models.Book.objects.values_list("publish_date"))
print("----------->>>")
#统计某日期共出版了多少本书
print(book_models.Book.objects.values_list("publish_date").annotate(Count("publish_date")))
#基本表内字段的分类聚合

输出:

{'blog_id': 1, 'headline': '屌炸天', 'rating': 4, 'body_text': '一个屌丝自橹的日子', 'pub_date': datetime.date(2017, 3, 19), 'id': 1, 'n_comments': 6, 'mod_date': datetime.date(2017, 3, 19), 'n_pingbacks': 6}
<QuerySet [(1, 1, '屌炸天', '一个屌丝自橹的日子', datetime.date(2017, 3, 19), datetime.date(2017, 3, 19), 6, 6, 4), (2, 2, '学py的日子', '学py的日子不如自橹', datetime.date(2017, 3, 19), datetime.date(2017, 3, 19), 1, 3, 1), (3, 2, 'qqqq', 'wqertyuio', datetime.date(2017, 3, 8), datetime.date(2017, 3, 19), 6, 5, 7)]>
----------->>>
<QuerySet [(datetime.date(2017, 3, 14),), (datetime.date(2017, 3, 14),), (datetime.date(2017, 3, 1),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 18),), (datetime.date(2017, 3, 2),), (datetime.date(2017, 3, 14),), (datetime.date(2017, 3, 19),)]>
----------->>>
<QuerySet [(datetime.date(2017, 3, 1), 1), (datetime.date(2017, 3, 2), 1), (datetime.date(2017, 3, 14), 3), (datetime.date(2017, 3, 18), 8), (datetime.date(2017, 3, 19), 1)]>

根据结果可知: 3月1号出版了1本书……3月18号出版了8本书。

参考博客: http://www.cnblogs.com/alex3714/articles/5512568.html (他写得绝逼没我好哈哈)