Django之QuerySet 創建對象

在前面的模型介紹中設置了3個對象，出版商(publisher)，作者(Authro)，書籍(book)。首先我們在網頁中添加各個對象信息填寫的界面。填寫后點擊提交。將會傳遞給后端。傳遞方式采用post

后端生出處理代碼如下：

def show_all_infor(request):
    if request.method == 'POST':
        publish=request.POST['pubisher']
        address=request.POST['address']
        city=request.POST['city']
        state_province=request.POST['state_province']
        coutry=request.POST['coutry']
        web=request.POST['web']
        ret1=Publisher.objects.get_or_create(name=publish,address=address,city=city,state_province=state_province,country=coutry,website=web)
        firstname=request.POST['firstname']
        secondname=request.POST['secondname']
        email=request.POST['email']
        ret2=Author.objects.get_or_create(first_name=firstname,last_name=secondname,email=email)
        title=unicode(request.POST['title'])
        publish_date=unicode(request.POST['publish_date'])
        if len(Book.objects.filter(title=title)) > 0:
           return HttpResponse('already exist')
        else:
            ret3=Book()
            ret3.title=title
            ret3.publisher=Publisher.objects.all()[1]
            ret3.publish_date=publish_date
            ret3.save()
            ret3.authors.add(Author.objects.all()[1])
            return HttpResponse(Book.objects.all())

首先判斷數據傳遞方式是否是post. 如果是的話則取出post的數據。然后進行對象創建。在這里用的是Publisher.objects.get_or_create方法。其實對象創建總共有4種方法：

方法一：

Publisher.objects.create(name=publish,address=address,city=city,state_province=state_provicen,coutry=coutry,website=web)

方法二：

ret=Publisher(name=publish,address=address,city=city,state_province=state_provicen,coutry=coutry,website=web)

ret.save()

方法三：

ret=Publisher()

ret.name=publish

ret.address=address

ret.save()

方法四：首先嘗試獲取。不存在就創建，可以防止重復。就是這里采取的方法。

ret1=Publisher.objects.get_or_create(name=publish,address=address,city=city,state_province=state_province,country=coutry,website=web)

 

Publisher和Author創建的方法都是get_or_create的方法，這里需要重點講下Book的創建方法。在Book中有個字段authors和publisher分別是ManyToManyField和ForeignKey

authors=models.ManyToManyField(Author)

publisher=models.ForeignKey(Publisher)

首先來看下publisher的創建。由於是ForeignKey的關系，也就是一對多。因此直接從Publisher對象中選擇一個進行賦值就可以了

ret3.publisher=Publisher.objects.all()[1]

接下來看下authors的創建，authors是ManyToManyField，也就是多對多的關系。在創建的authors的時候要注意兩點：

1 首先是要創建一個Book對象，然后進行保存。才能進行添加authors。如果代碼是這樣的：

ret3=Book()

ret3.authors.add(Author.objects.all()[1])
 ret3.title=title
 ret3.publisher=Publisher.objects.all()[1]
 ret3.publish_date=publish_date
 ret3.save()

則會報如下的錯誤"<Book: >" needs to have a value for field "book" before this many-to-many relationship can be used.

這個錯誤的意思就是需要先創建一個book的對象

因此正確的代碼就是：

ret3=Book()

ret3.title=title

ret3.publisher=Publisher.objects.all()[1]
 ret3.publish_date=publish_date
 ret3.save()
 ret3.authors.add(Author.objects.all()[1])

2 在創建Book對象的時候需要將書名，出版商，出版日期都包含進來，否則會報錯。

如果代碼是下面的這樣：

ret3=Book()

ret3.save()

ret3.title=title

ret3.publisher=Publisher.objects.all()[1]
 ret3.publish_date=publish_date
 ret3.authors.add(Author.objects.all()[1])

就會報如下的錯誤IntegrityError：site_prj_book.publisher_id may not be NULL

因此正確的代碼如下：

ret3=Book()

ret3.title=title

ret3.publisher=Publisher.objects.all()[1]

ret3.publish_date=publish_date

ret3.save()

ret3.authors.add(Author.objects.all()[1])

另外在創建Book對象的時候，並沒有用到get_or_create的方法。這是因為Book有ManyToManyField和ForeignKey的關系，需要首先創建一個對象。但是存在一個問題，如果輸入的書名已經有記錄了，那么如何避免重復記錄呢，這里用到了filter方法：

Book.objects.filter(title=title)

過濾該名字的書名，如果存在，則返回already exist的界面。就不會繼續新建對象。

if len(Book.objects.filter(title=title)) > 0:

   return HttpResponse('already exist')

來看下幾個查詢界面的后端處理代碼：首先是獲取所有的對象。然后在網頁上顯示

def result1(request):
    result=Publisher.objects.all()
    return HttpResponse(result)

def result2(request):
    result=Author.objects.all()
    return HttpResponse(result)

def result3(request):
    result=Book.objects.all()
    return HttpResponse(result)

其實相對於在網頁上顯示，在Django工程中也有查詢的方法。在pycharm中進入終端，然后輸入python manage.py shell進入管理界面。

D:\django_test2>python manage.py shell

Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 20:32:19) [MSC v.1500 32 bit (Intel)]

Type "copyright", "credits" or "license" for more information.

IPython 5.3.0 -- An enhanced Interactive Python.

?         -> Introduction and overview of IPython's features.

%quickref -> Quick reference.

help      -> Python's own help system.

object?   -> Details about 'object', use 'object??' for extra details.

然后從模型中引入各個模塊

In [1]: from site_prj.models import Publisher,Author,Book

輸入查詢語句：

In [2]: Publisher.objects.all()

得到結果：

Out[2]: <QuerySet [<Publisher: 成都日報>

 

另外我們還可以在shell中打印出對應的SQL語句便於問題定位和查看：

In [5]: print Publisher.objects.all().query

SELECT "site_prj_publisher"."id", "site_prj_publisher"."name", "site_prj_publisher"."address", "site_prj_publisher"."city", "site_prj_publisher"."state_province", "site_prj_publisher"."country", "site_prj_publishe

r"."website" FROM "site_prj_publisher"

 

這一章介紹了對象創建的基本方法，下一章將介紹更對對象的高級應用。