1 =============

     2 Generic views

     3 =============

     4 

     5 Writing Web applications can be monotonous, because we repeat certain patterns

     6 again and again. Django tries to take away some of that monotony at the model

     7 and template layers, but Web developers also experience this boredom at the view

     8 level.

     9 

    10 Django's *generic views* were developed to ease that pain. They take certain

    11 common idioms and patterns found in view development and abstract them so that

    12 you can quickly write common views of data without having to write too much

    13 code.

    14 

    15 We can recognize certain common tasks, like displaying a list of objects, and

    16 write code that displays a list of *any* object. Then the model in question can

    17 be passed as an extra argument to the URLconf.

    18 

    19 Django ships with generic views to do the following:

    20 

    21     * Perform common "simple" tasks: redirect to a different page and

    22       render a given template.

    23 

    24     * Display list and detail pages for a single object. If we were creating an

    25       application to manage conferences then a ``talk_list`` view and a

    26       ``registered_user_list`` view would be examples of list views. A single

    27       talk page is an example of what we call a "detail" view.

    28 

    29     * Present date-based objects in year/month/day archive pages,

    30       associated detail, and "latest" pages. The Django Weblog's

    31       (http://www.djangoproject.com/weblog/) year, month, and

    32       day archives are built with these, as would be a typical

    33       newspaper's archives.

    34 

    35     * Allow users to create, update, and delete objects -- with or

    36       without authorization.

    37 

    38 Taken together, these views provide easy interfaces to perform the most common

    39 tasks developers encounter.

    40 

    41 Using generic views

    42 ===================

    43 

    44 All of these views are used by creating configuration dictionaries in

    45 your URLconf files and passing those dictionaries as the third member of the

    46 URLconf tuple for a given pattern.

    47 

    48 For example, here's a simple URLconf you could use to present a static "about"

    49 page::

    50 

    51     from django.conf.urls.defaults import *

    52     from django.views.generic.simple import direct_to_template

    53 

    54     urlpatterns = patterns('',

    55         ('^about/$', direct_to_template, {

    56             'template': 'about.html'

    57         })

    58     )

    59 

    60 Though this might seem a bit "magical" at first glance  -- look, a view with no

    61 code! --, actually the ``direct_to_template`` view simply grabs information from

    62 the extra-parameters dictionary and uses that information when rendering the

    63 view.

    64 

    65 Because this generic view -- and all the others -- is a regular view function

    66 like any other, we can reuse it inside our own views. As an example, let's

    67 extend our "about" example to map URLs of the form ``/about/<whatever>/`` to

    68 statically rendered ``about/<whatever>.html``. We'll do this by first modifying

    69 the URLconf to point to a view function:

    70 

    71 .. parsed-literal::

    72 

    73     from django.conf.urls.defaults import *

    74     from django.views.generic.simple import direct_to_template

    75     **from books.views import about_pages**

    76 

    77     urlpatterns = patterns('',

    78         ('^about/$', direct_to_template, {

    79             'template': 'about.html'

    80         }),

    81         **('^about/(\\w+)/$', about_pages),**

    82     )

    83 

    84 Next, we'll write the ``about_pages`` view::

    85 

    86     from django.http import Http404

    87     from django.template import TemplateDoesNotExist

    88     from django.views.generic.simple import direct_to_template

    89 

    90     def about_pages(request, page):

    91         try:

    92             return direct_to_template(request, template="about/%s.html" % page)

    93         except TemplateDoesNotExist:

    94             raise Http404()

    95 

    96 Here we're treating ``direct_to_template`` like any other function. Since it

    97 returns an ``HttpResponse``, we can simply return it as-is. The only slightly

    98 tricky business here is dealing with missing templates. We don't want a

    99 nonexistent template to cause a server error, so we catch

   100 ``TemplateDoesNotExist`` exceptions and return 404 errors instead.

   101 

   102 .. admonition:: Is there a security vulnerability here?

   103 

   104     Sharp-eyed readers may have noticed a possible security hole: we're

   105     constructing the template name using interpolated content from the browser

   106     (``template="about/%s.html" % page``). At first glance, this looks like a

   107     classic *directory traversal* vulnerability. But is it really?

   108 

   109     Not exactly. Yes, a maliciously crafted value of ``page`` could cause

   110     directory traversal, but although ``page`` *is* taken from the request URL,

   111     not every value will be accepted. The key is in the URLconf: we're using

   112     the regular expression ``\w+`` to match the ``page`` part of the URL, and

   113     ``\w`` only accepts letters and numbers. Thus, any malicious characters

   114     (dots and slashes, here) will be rejected by the URL resolver before they

   115     reach the view itself.

   116 

   117 Generic views of objects

   118 ========================

   119 

   120 The ``direct_to_template`` certainly is useful, but Django's generic views

   121 really shine when it comes to presenting views on your database content. Because

   122 it's such a common task, Django comes with a handful of built-in generic views

   123 that make generating list and detail views of objects incredibly easy.

   124 

   125 Let's take a look at one of these generic views: the "object list" view. We'll

   126 be using these models::

   127 

   128     # models.py

   129     from django.db import models

   130 

   131     class Publisher(models.Model):

   132         name = models.CharField(max_length=30)

   133         address = models.CharField(max_length=50)

   134         city = models.CharField(max_length=60)

   135         state_province = models.CharField(max_length=30)

   136         country = models.CharField(max_length=50)

   137         website = models.URLField()

   138 

   139         def __unicode__(self):

   140             return self.name

   141 

   142         class Meta:

   143             ordering = ["-name"]

   144 

   145     class Book(models.Model):

   146         title = models.CharField(max_length=100)

   147         authors = models.ManyToManyField('Author')

   148         publisher = models.ForeignKey(Publisher)

   149         publication_date = models.DateField()

   150 

   151 To build a list page of all publishers, we'd use a URLconf along these lines::

   152 

   153     from django.conf.urls.defaults import *

   154     from django.views.generic import list_detail

   155     from books.models import Publisher

   156 

   157     publisher_info = {

   158         "queryset" : Publisher.objects.all(),

   159     }

   160 

   161     urlpatterns = patterns('',

   162         (r'^publishers/$', list_detail.object_list, publisher_info)

   163     )

   164 

   165 That's all the Python code we need to write. We still need to write a template,

   166 however. We could explicitly tell the ``object_list`` view which template to use

   167 by including a ``template_name`` key in the extra arguments dictionary, but in

   168 the absence of an explicit template Django will infer one from the object's

   169 name. In this case, the inferred template will be

   170 ``"books/publisher_list.html"`` -- the "books" part comes from the name of the

   171 app that defines the model, while the "publisher" bit is just the lowercased

   172 version of the model's name.

   173 

   174 .. highlightlang:: html+django

   175 

   176 This template will be rendered against a context containing a variable called

   177 ``object_list`` that contains all the publisher objects. A very simple template

   178 might look like the following::

   179 

   180     {% extends "base.html" %}

   181 

   182     {% block content %}

   183         <h2>Publishers</h2>

   184         <ul>

   185             {% for publisher in object_list %}

   186                 <li>{{ publisher.name }}</li>

   187             {% endfor %}

   188         </ul>

   189     {% endblock %}

   190 

   191 That's really all there is to it. All the cool features of generic views come

   192 from changing the "info" dictionary passed to the generic view. The

   193 :doc:`generic views reference</ref/generic-views>` documents all the generic

   194 views and all their options in detail; the rest of this document will consider

   195 some of the common ways you might customize and extend generic views.

   196 

   197 Extending generic views

   198 =======================

   199 

   200 .. highlightlang:: python

   201 

   202 There's no question that using generic views can speed up development

   203 substantially. In most projects, however, there comes a moment when the

   204 generic views no longer suffice. Indeed, the most common question asked by new

   205 Django developers is how to make generic views handle a wider array of

   206 situations.

   207 

   208 Luckily, in nearly every one of these cases, there are ways to simply extend

   209 generic views to handle a larger array of use cases. These situations usually

   210 fall into a handful of patterns dealt with in the sections that follow.

   211 

   212 Making "friendly" template contexts

   213 -----------------------------------

   214 

   215 You might have noticed that our sample publisher list template stores all the

   216 books in a variable named ``object_list``. While this works just fine, it isn't

   217 all that "friendly" to template authors: they have to "just know" that they're

   218 dealing with publishers here. A better name for that variable would be

   219 ``publisher_list``; that variable's content is pretty obvious.

   220 

   221 We can change the name of that variable easily with the ``template_object_name``

   222 argument:

   223 

   224 .. parsed-literal::

   225 

   226     publisher_info = {

   227         "queryset" : Publisher.objects.all(),

   228         **"template_object_name" : "publisher",**

   229     }

   230 

   231     urlpatterns = patterns('',

   232         (r'^publishers/$', list_detail.object_list, publisher_info)

   233     )

   234 

   235 Providing a useful ``template_object_name`` is always a good idea. Your

   236 coworkers who design templates will thank you.

   237 

   238 Adding extra context

   239 --------------------

   240 

   241 Often you simply need to present some extra information beyond that provided by

   242 the generic view. For example, think of showing a list of all the books on each

   243 publisher detail page. The ``object_detail`` generic view provides the

   244 publisher to the context, but it seems there's no way to get additional

   245 information in that template.

   246 

   247 But there is: all generic views take an extra optional parameter,

   248 ``extra_context``. This is a dictionary of extra objects that will be added to

   249 the template's context. So, to provide the list of all books on the detail

   250 detail view, we'd use an info dict like this:

   251 

   252 .. parsed-literal::

   253 

   254     from books.models import Publisher, **Book**

   255 

   256     publisher_info = {

   257         "queryset" : Publisher.objects.all(),

   258         "template_object_name" : "publisher",

   259         **"extra_context" : {"book_list" : Book.objects.all()}**

   260     }

   261 

   262 This would populate a ``{{ book_list }}`` variable in the template context.

   263 This pattern can be used to pass any information down into the template for the

   264 generic view. It's very handy.

   265 

   266 However, there's actually a subtle bug here -- can you spot it?

   267 

   268 The problem has to do with when the queries in ``extra_context`` are evaluated.

   269 Because this example puts ``Book.objects.all()`` in the URLconf, it will

   270 be evaluated only once (when the URLconf is first loaded). Once you add or

   271 remove books, you'll notice that the generic view doesn't reflect those

   272 changes until you reload the Web server (see :ref:`caching-and-querysets`

   273 for more information about when QuerySets are cached and evaluated).

   274 

   275 .. note::

   276 

   277     This problem doesn't apply to the ``queryset`` generic view argument. Since

   278     Django knows that particular QuerySet should *never* be cached, the generic

   279     view takes care of clearing the cache when each view is rendered.

   280 

   281 The solution is to use a callback in ``extra_context`` instead of a value. Any

   282 callable (i.e., a function) that's passed to ``extra_context`` will be evaluated

   283 when the view is rendered (instead of only once). You could do this with an

   284 explicitly defined function:

   285 

   286 .. parsed-literal::

   287 

   288     def get_books():

   289         return Book.objects.all()

   290 

   291     publisher_info = {

   292         "queryset" : Publisher.objects.all(),

   293         "template_object_name" : "publisher",

   294         "extra_context" : **{"book_list" : get_books}**

   295     }

   296 

   297 or you could use a less obvious but shorter version that relies on the fact that

   298 ``Book.objects.all`` is itself a callable:

   299 

   300 .. parsed-literal::

   301 

   302     publisher_info = {

   303         "queryset" : Publisher.objects.all(),

   304         "template_object_name" : "publisher",

   305         "extra_context" : **{"book_list" : Book.objects.all}**

   306     }

   307 

   308 Notice the lack of parentheses after ``Book.objects.all``; this references

   309 the function without actually calling it (which the generic view will do later).

   310 

   311 Viewing subsets of objects

   312 --------------------------

   313 

   314 Now let's take a closer look at this ``queryset`` key we've been using all

   315 along. Most generic views take one of these ``queryset`` arguments -- it's how

   316 the view knows which set of objects to display (see :doc:`/topics/db/queries` for

   317 more information about ``QuerySet`` objects, and see the

   318 :doc:`generic views reference</ref/generic-views>` for the complete details).

   319 

   320 To pick a simple example, we might want to order a list of books by

   321 publication date, with the most recent first:

   322 

   323 .. parsed-literal::

   324 

   325     book_info = {

   326         "queryset" : Book.objects.all().order_by("-publication_date"),

   327     }

   328 

   329     urlpatterns = patterns('',

   330         (r'^publishers/$', list_detail.object_list, publisher_info),

   331         **(r'^books/$', list_detail.object_list, book_info),**

   332     )

   333 

   334 

   335 That's a pretty simple example, but it illustrates the idea nicely. Of course,

   336 you'll usually want to do more than just reorder objects. If you want to

   337 present a list of books by a particular publisher, you can use the same

   338 technique:

   339 

   340 .. parsed-literal::

   341 

   342     **acme_books = {**

   343         **"queryset": Book.objects.filter(publisher__name="Acme Publishing"),**

   344         **"template_name" : "books/acme_list.html"**

   345     **}**

   346 

   347     urlpatterns = patterns('',

   348         (r'^publishers/$', list_detail.object_list, publisher_info),

   349         **(r'^books/acme/$', list_detail.object_list, acme_books),**

   350     )

   351 

   352 Notice that along with a filtered ``queryset``, we're also using a custom

   353 template name. If we didn't, the generic view would use the same template as the

   354 "vanilla" object list, which might not be what we want.

   355 

   356 Also notice that this isn't a very elegant way of doing publisher-specific

   357 books. If we want to add another publisher page, we'd need another handful of

   358 lines in the URLconf, and more than a few publishers would get unreasonable.

   359 We'll deal with this problem in the next section.

   360 

   361 .. note::

   362 

   363     If you get a 404 when requesting ``/books/acme/``, check to ensure you

   364     actually have a Publisher with the name 'ACME Publishing'.  Generic

   365     views have an ``allow_empty`` parameter for this case.  See the

   366     :doc:`generic views reference</ref/generic-views>` for more details.

   367 

   368 Complex filtering with wrapper functions

   369 ----------------------------------------

   370 

   371 Another common need is to filter down the objects given in a list page by some

   372 key in the URL. Earlier we hard-coded the publisher's name in the URLconf, but

   373 what if we wanted to write a view that displayed all the books by some arbitrary

   374 publisher? We can "wrap" the ``object_list`` generic view to avoid writing a lot

   375 of code by hand. As usual, we'll start by writing a URLconf:

   376 

   377 .. parsed-literal::

   378 

   379     from books.views import books_by_publisher

   380 

   381     urlpatterns = patterns('',

   382         (r'^publishers/$', list_detail.object_list, publisher_info),

   383         **(r'^books/(\\w+)/$', books_by_publisher),**

   384     )

   385 

   386 Next, we'll write the ``books_by_publisher`` view itself::

   387 

   388     from django.http import Http404

   389     from django.views.generic import list_detail

   390     from books.models import Book, Publisher

   391 

   392     def books_by_publisher(request, name):

   393 

   394         # Look up the publisher (and raise a 404 if it can't be found).

   395         try:

   396             publisher = Publisher.objects.get(name__iexact=name)

   397         except Publisher.DoesNotExist:

   398             raise Http404

   399 

   400         # Use the object_list view for the heavy lifting.

   401         return list_detail.object_list(

   402             request,

   403             queryset = Book.objects.filter(publisher=publisher),

   404             template_name = "books/books_by_publisher.html",

   405             template_object_name = "books",

   406             extra_context = {"publisher" : publisher}

   407         )

   408 

   409 This works because there's really nothing special about generic views -- they're

   410 just Python functions. Like any view function, generic views expect a certain

   411 set of arguments and return ``HttpResponse`` objects. Thus, it's incredibly easy

   412 to wrap a small function around a generic view that does additional work before

   413 (or after; see the next section) handing things off to the generic view.

   414 

   415 .. note::

   416 

   417     Notice that in the preceding example we passed the current publisher being

   418     displayed in the ``extra_context``. This is usually a good idea in wrappers

   419     of this nature; it lets the template know which "parent" object is currently

   420     being browsed.

   421 

   422 Performing extra work

   423 ---------------------

   424 

   425 The last common pattern we'll look at involves doing some extra work before

   426 or after calling the generic view.

   427 

   428 Imagine we had a ``last_accessed`` field on our ``Author`` object that we were

   429 using to keep track of the last time anybody looked at that author::

   430 

   431     # models.py

   432 

   433     class Author(models.Model):

   434         salutation = models.CharField(max_length=10)

   435         first_name = models.CharField(max_length=30)

   436         last_name = models.CharField(max_length=40)

   437         email = models.EmailField()

   438         headshot = models.ImageField(upload_to='/tmp')

   439         last_accessed = models.DateTimeField()

   440 

   441 The generic ``object_detail`` view, of course, wouldn't know anything about this

   442 field, but once again we could easily write a custom view to keep that field

   443 updated.

   444 

   445 First, we'd need to add an author detail bit in the URLconf to point to a

   446 custom view:

   447 

   448 .. parsed-literal::

   449 

   450     from books.views import author_detail

   451 

   452     urlpatterns = patterns('',

   453         #...

   454         **(r'^authors/(?P<author_id>\\d+)/$', author_detail),**

   455     )

   456 

   457 Then we'd write our wrapper function::

   458 

   459     import datetime

   460     from books.models import Author

   461     from django.views.generic import list_detail

   462     from django.shortcuts import get_object_or_404

   463 

   464     def author_detail(request, author_id):

   465         # Look up the Author (and raise a 404 if she's not found)

   466         author = get_object_or_404(Author, pk=author_id)

   467 

   468         # Record the last accessed date

   469         author.last_accessed = datetime.datetime.now()

   470         author.save()

   471 

   472         # Show the detail page

   473         return list_detail.object_detail(

   474             request,

   475             queryset = Author.objects.all(),

   476             object_id = author_id,

   477         )

   478 

   479 .. note::

   480 

   481     This code won't actually work unless you create a

   482     ``books/author_detail.html`` template.

   483 

   484 We can use a similar idiom to alter the response returned by the generic view.

   485 If we wanted to provide a downloadable plain-text version of the list of

   486 authors, we could use a view like this::

   487 

   488     def author_list_plaintext(request):

   489         response = list_detail.object_list(

   490             request,

   491             queryset = Author.objects.all(),

   492             mimetype = "text/plain",

   493             template_name = "books/author_list.txt"

   494         )

   495         response["Content-Disposition"] = "attachment; filename=authors.txt"

   496         return response

   497 

   498 This works because the generic views return simple ``HttpResponse`` objects

   499 that can be treated like dictionaries to set HTTP headers. This

   500 ``Content-Disposition`` business, by the way, instructs the browser to

   501 download and save the page instead of displaying it in the browser.