Making Friendly Template Contexts

制作�好的模�Context

You might have noticed that sample publisher list template stores all the books in a variable named object_list . While this works just fine, it isnt all that friendly to template authors: they have to just know that theyre dealing with books here. A better name for that variable would be publisher_list ; that variables content is pretty obvious.

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We can change the name of that variable easily with the template_object_name argument:

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publisher_info = {
    "queryset" : Publisher.objects.all(),
    **"template_object_name" : "publisher",**
}

urlpatterns = patterns('',
    (r'^publishers/$', list_detail.object_list, publisher_info)
)

Providing a useful template_object_name is always a good idea. Your coworkers who design templates will thank you.

使用有用的 template_object_name 总是个好想法。你的设计模�的�作伙伴会感谢你的。

Adding Extra Context

添加�外的Context

Often you simply need to present some extra information beyond that provided by the generic view. For example, think of showing a list of all the other publishers on each publisher detail page. The object_detail generic view provides the publisher to the context, but it seems theres no way to get a list of all publishers in that template.

你常常需�呈现比通用视图�供的更多的�外信�。例如，考虑一下在�个出版商页�实现所有其他 出版商列表。 object_detail 通用视图�供了出版商到context，但是看起�没有办法在模�中 获� 所有 出版商列表。

But there is: all generic views take an extra optional parameter, extra_context . This is a dictionary of extra objects that will be added to the templates context. So, to provide the list of all publishers on the detail detail view, wed use an info dict like this:

这是解决方法：所有的通用视图都有一个�外的�选�数 extra_context 。这个�数是一个字典数�类型，包��添加到模�的context中的�外的对象。所以��供所有的出版商明细给视图，我们就用这样的info字典：

publisher_info = {
    "queryset" : Publisher.objects.all(),
    "template_object_name" : "publisher",
    **"extra_context" : {"book_list" : Book.objects.all()}**
}

This would populate a {{ book_list }} variable in the template context. This pattern can be used to pass any information down into the template for the generic view. Its very handy.

这样就把一个 {{ book_list }} ��放到模�的context中。这个方法�以用�传递任�数� 到通用视图模�中去，�常方便。

However, theres actually a subtle bug here can you spot it?

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The problem has to do with when the queries in extra_context are evaluated. Because this example puts Publisher.objects.all() in the URLconf, it will be evaluated only once (when the URLconf is first loaded). Once you add or remove publishers, youll notice that the generic view doesnt reflect those changes until you reload the Web server (see Caching and QuerySets in Appendix C for more information about when QuerySets are cached and evaluated).

我们现在�看一下， extra_context 里包�数�库查询的问题。因为在这个例�中，我们把 Publisher.objects.all() 放在URLconf中，它�会执行一次（当URLconf第一次加载的时候）。当你添加或删除出版商，你会�现在��Web�务器之�，通用视图�会�映出这些修改的（有关QuerySet何时被缓存和赋值的更多信�请�考附录C中“缓存与查询集�一节）。

Note

备注

This problem doesnt apply to the queryset generic view argument. Since Django knows that particular QuerySet should never be cached, the generic view takes care of clearing the cache when each view is rendered.

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The solution is to use a callback in extra_context instead of a value. Any callable (i.e., a function) thats passed to extra_context will be evaluated when the view is rendered (instead of only once). You could do this with an explicitly defined function:

解决这个问题的办法是在 extra_context 中用一个回调（callback）� 代替使用一个��。任何�以调用的对象（例如一个函数）在传递给 extra_context �都会在�次视图渲染�执行（而�是�执行一次）。 你�以象这样定义一个函数：

def get_books():
    return Book.objects.all()

publisher_info = {
    "queryset" : Publisher.objects.all(),
    "template_object_name" : "publisher",
    "extra_context" : **{"book_list" : get_books}**
}

or you could use a less obvious but shorter version that relies on the fact that Publisher.objects.all is itself a callable:

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publisher_info = {
    "queryset" : Publisher.objects.all(),
    "template_object_name" : "publisher",
    "extra_context" : **{"book_list" : Book.objects.all}**
}

Notice the lack of parentheses after Book.objects.all ; this references the function without actually calling it (which the generic view will do later).

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Viewing Subsets of Objects

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Now lets take a closer look at this queryset key weve been using all along. Most generic views take one of these queryset arguments its how the view knows which set of objects to display (see Selecting Objects in Chapter 5 for an introduction to QuerySets, and see Appendix C for the complete details).

现在让我们�仔细看看这个 queryset 。大多数通用视图有一个 queryset �数，这个�数告诉视图�显示对象的集� （有关QuerySet的解释请看第五章的 “选择对象�章节，详细资料请�看附录C）。

To pick a simple example, we might want to order a list of books by publication date, with the most recent first:

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book_info = {
    "queryset" : Book.objects.all().order_by("-publication_date"),
}

urlpatterns = patterns('',
    (r'^publishers/$', list_detail.object_list, publisher_info),
    **(r'^books/$', list_detail.object_list, book_info),**
)

Thats a pretty simple example, but it illustrates the idea nicely. Of course, youll usually want to do more than just reorder objects. If you want to present a list of books by a particular publisher, you can use the same technique:

这是一个相当简�的例�，但是很说明问题。当然，你通常还想�比�新排�更多的事。 如果你想�呈现�个特定出版商出版的所有书�列表，你�以使用�样的技术：

**apress_books = {**
    **"queryset": Book.objects.filter(publisher__name="Apress Publishing"),**
    **"template_name" : "books/apress_list.html"**
**}**

urlpatterns = patterns('',
    (r'^publishers/$', list_detail.object_list, publisher_info),
    **(r'^books/apress/$', list_detail.object_list, apress_books),**
)

Notice that along with a filtered queryset , were also using a custom template name. If we didnt, the generic view would use the same template as the vanilla object list, which might not be what we want.

注� 在使用一个过滤的 queryset 的�时，我们还使用一个自定义的模��称。 如果我们�这么�，通用视图就会用以�的模�，这�能�是我们想�的结果。

Also notice that this isnt a very elegant way of doing publisher-specific books. If we want to add another publisher page, wed need another handful of lines in the URLconf, and more than a few publishers would get unreasonable. Well deal with this problem in the next section.

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Note

备注

If you get a 404 when requesting /books/apress/ , check to ensure you actually have a Publisher with the name Apress Publishing. Generic views have an allow_empty parameter for this case. See Appendix D for more details.

如果你在请求 /books/apress/ 时出现404错误，请检查以确�你的数�库中出版商 中有�为Apress Publishing的记录。通用视图有一个 allow_empty �数�以 用�处�这个情况，详情请看附录D。

Complex Filtering with Wrapper Functions

用函数包装�处���的数�过滤

Another common need is to filter down the objects given in a list page by some key in the URL. Earlier we hard-coded the publishers name in the URLconf, but what if we wanted to write a view that displayed all the books by some arbitrary publisher? We can wrap the object_list generic view to avoid writing a lot of code by hand. As usual, well start by writing a URLconf:

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urlpatterns = patterns('',
    (r'^publishers/$', list_detail.object_list, publisher_info),
    **(r'^books/(w+)/$', books_by_publisher),**
)

Next, well write the books_by_publisher view itself:

接下�，我们写 books_by_publisher 这个视图：（上�的代�中正则表达�有误，在 w ��加�斜線）

from django.http import Http404
from django.views.generic import list_detail
from mysite.books.models import Book, Publisher

def books_by_publisher(request, name):

    # Look up the publisher (and raise a 404 if it can't be found).
    try:
        publisher = Publisher.objects.get(name__iexact=name)
    except Publisher.DoesNotExist:
        raise Http404

    # Use the object_list view for the heavy lifting.
    return list_detail.object_list(
        request,
        queryset = Book.objects.filter(publisher=publisher),
        template_name = "books/books_by_publisher.html",
        template_object_name = "books",
        extra_context = {"publisher" : publisher}
    )

This works because theres really nothing special about generic views theyre just Python functions. Like any view function, generic views expect a certain set of arguments and return HttpResponse objects. Thus, its incredibly easy to wrap a small function around a generic view that does additional work before (or after; see the next section) handing things off to the generic view.

这是因为通用视图就是Python函数。和其他的视图函数一样，通用视图也是接�一些 �数并返回 HttpResponse 对象。因此，通过包装通用视图函数�以�更多的事。

Note

注�

Notice that in the preceding example we passed the current publisher being displayed in the extra_context . This is usually a good idea in wrappers of this nature; it lets the template know which parent object is currently being browsed.

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Performing Extra Work

处��外工作

The last common pattern well look at involves doing some extra work before or after calling the generic view.

我们��看看最�一个常用模�：在调用通用视图���些�外工作。

Imagine we had a last_accessed field on our Author object that we were using to keep track of the last time anybody looked at that author. The generic object_detail view, of course, wouldnt know anything about this field, but once again we could easily write a custom view to keep that field updated.

想象一下我们在 Author 对象里有一个 last_accessed 字段，我们用这个字段�更正对author的最近访问时间。当然通用视图 object_detail 并�能处� 这个问题，我们�以很容易的写一个自定义的视图�更新这个字段。

First, wed need to add an author detail bit in the URLconf to point to a custom view:

首先，我们需�在URL�置里设置指�到新的自定义视图：

from mysite.books.views import author_detail

urlpatterns = patterns('',
    #...
    **(r'^authors/(?P<author_id>d+)/$', author_detail),**
)

Then wed write our wrapper function:

接下�写包装函数：

import datetime
from mysite.books.models import Author
from django.views.generic import list_detail
from django.shortcuts import get_object_or_404

def author_detail(request, author_id):
    # Look up the Author (and raise a 404 if she's not found)
    author = get_object_or_404(Author, pk=author_id)

    # Record the last accessed date
    author.last_accessed = datetime.datetime.now()
    author.save()

    # Show the detail page
    return list_detail.object_detail(
        request,
        queryset = Author.objects.all(),
        object_id = author_id,
    )

Note

注�

This code wont actually work unless you add a last_accessed field to your Author model and create a books/author_detail.html template.

除�你添加 last_accessed 字段到你的 Author 模型并创建 books/author_detail.html 模�，�则这段代��能真正工作。

We can use a similar idiom to alter the response returned by the generic view. If we wanted to provide a downloadable plain-text version of the list of authors, we could use a view like this:

我们�以用�样的方法修改通用视图的返回值。如果我们想��供一个供下载用的 纯文本版本的author列表，我们�以用下�这个视图：

def author_list_plaintext(request):
    response = list_detail.object_list(
        request,
        queryset = Author.objects.all(),
        mimetype = "text/plain",
        template_name = "books/author_list.txt"
    )
    response["Content-Disposition"] = "attachment; filename=authors.txt"
    return response

This works because the generic views return simple HttpResponse objects that can be treated like dictionaries to set HTTP headers. This Content-Disposition business, by the way, instructs the browser to download and save the page instead of displaying it in the browser.

这个方法之所以工作是因为通用视图返回的 HttpResponse 对象�以象一个字典 一样的设置HTTP的头部。�便说一下，这个 Content-Disposition 的�义是 告诉�览器下载并�存这个页�，而�是在�览器中显示它。