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Django入门与实践-第23章：分页实现

我们可以非常容易地使用 CBV 来实现分页功能。但首先我想手工分页，这样就更有助于我们理解背后的机制，这样它就不那么神秘了。

实际上对 boards 列表视图分页并没有意义，因为我们不期望有很多 boards。但无疑对于主题列表和帖子列表来说是需要一些分页的。

从现在起，我们将在 board_topics 这个视图中来操作。

首先，我们添加一些帖子。我们可以直接使用应用程序的用户界面来添加几个帖子，或者打开 python shell 编写一个小脚本来为我们完成：

python manage.py shell
from django.contrib.auth.models import User
from boards.models import Board, Topic, Post

user = User.objects.first()

board = Board.objects.get(name='Django')

for i in range(100):
    subject = 'Topic test #{}'.format(i)
    topic = Topic.objects.create(subject=subject, board=board, starter=user)
    Post.objects.create(message='Lorem ipsum...', topic=topic, created_by=user)

很好，现在我们有一些数据可以玩了。

在我们返回去写代码之前，让我们用 python shell 来做一些更多的实验：

python manage.py shell
from boards.models import Topic

# All the topics in the app
Topic.objects.count()
107

# Just the topics in the Django board
Topic.objects.filter(board__name='Django').count()
104

# Let's save this queryset into a variable to paginate it
queryset = Topic.objects.filter(board__name='Django').order_by('-last_updated')

定义一个你要分页的查询集(QuerySet)的排序是很重要的。否则，会返回给你错误的结果。

现在让我们导入 Paginator 工具：

from django.core.paginator import Paginator

paginator = Paginator(queryset, 20)

这里我们告诉Django将查询集按照每页20个元素分页。现在让我们来研究一些 paginator 的属性：

# count the number of elements in the paginator
paginator.count
104

# total number of pages
# 104 elements, paginating 20 per page gives you 6 pages
# where the last page will have only 4 elements
paginator.num_pages
6

# range of pages that can be used to iterate and create the
# links to the pages in the template
paginator.page_range
range(1, 7)

# returns a Page instance
paginator.page(2)
<Page 2 of 6>

page = paginator.page(2)

type(page)
django.core.paginator.Page

type(paginator)
django.core.paginator.Paginator

这里我们必须注意，因为如果我们试图找到一个不存在的页面，分页器会抛出一个异常:

paginator.page(7)
EmptyPage: That page contains no results

或者如果我们随意传递进去一个不是页码数字的参数，也会报错：

paginator.page('abc')
PageNotAnInteger: That page number is not an integer

我们必须在设计用户界面时牢记这些细节。

我们来简单看一下 Page 类提供的属性和方法：

page = paginator.page(1)

# Check if there is another page after this one
page.has_next()
True

# If there is no previous page, that means this one is the first page
page.has_previous()
False

page.has_other_pages()
True

page.next_page_number()
2

# Take care here, since there is no previous page,
# if we call the method `previous_page_number() we will get an exception:
page.previous_page_number()
EmptyPage: That page number is less than 1

FBV 分页

这里是我们如何使用 FBV 来实现分页：

boards/views.py 查看完整文件

from django.db.models import Count
from django.core.paginator import Paginator, EmptyPage, PageNotAnInteger
from django.shortcuts import get_object_or_404, render
from django.views.generic import ListView
from .models import Board

def board_topics(request, pk):
    board = get_object_or_404(Board, pk=pk)
    queryset = board.topics.order_by('-last_updated').annotate(replies=Count('posts') - 1)
    page = request.GET.get('page', 1)

    paginator = Paginator(queryset, 20)

    try:
        topics = paginator.page(page)
    except PageNotAnInteger:
        # fallback to the first page
        topics = paginator.page(1)
    except EmptyPage:
        # probably the user tried to add a page number
        # in the url, so we fallback to the last page
        topics = paginator.page(paginator.num_pages)

    return render(request, 'topics.html', {'board': board, 'topics': topics})

这部分的实现是使用了 Bootstrap 的四个分页组件来正确的渲染页面。你需要花时间阅读代码，看看它是否适合你。我们在这里使用的是我们之前用过的方法。在这种情况下，topics 不再是一个查询集（QuerySet），而是一个 paginator.page 的实例。

在 topics HTML列表的基础上，我们可以渲染分页组件：

templates/topics.html 查看完整文件

{% if topics.has_other_pages %}
  <nav aria-label="Topics pagination" class="mb-4">
    <ul class="pagination">
      {% if topics.has_previous %}
        <li class="page-item">
          <a class="page-link" href="?page={{ topics.previous_page_number }}">Previous</a>
        </li>
      {% else %}
        <li class="page-item disabled">
          <span class="page-link">Previous</span>
        </li>
      {% endif %}

      {% for page_num in topics.paginator.page_range %}
        {% if topics.number == page_num %}
          <li class="page-item active">
            <span class="page-link">
              {{ page_num }}
              <span class="sr-only">(current)</span>
            </span>
          </li>
        {% else %}
          <li class="page-item">
            <a class="page-link" href="?page={{ page_num }}">{{ page_num }}</a>
          </li>
        {% endif %}
      {% endfor %}

      {% if topics.has_next %}
        <li class="page-item">
          <a class="page-link" href="?page={{ topics.next_page_number }}">Next</a>
        </li>
      {% else %}
        <li class="page-item disabled">
          <span class="page-link">Next</span>
        </li>
      {% endif %}
    </ul>
  </nav>
{% endif %}

GCBV 分页

下面，相同的实现，但这次使用ListView。

boards/views.py 查看完整文件

class TopicListView(ListView):
    model = Topic
    context_object_name = 'topics'
    template_name = 'topics.html'
    paginate_by = 20

    def get_context_data(self, **kwargs):
        kwargs['board'] = self.board
        return super().get_context_data(**kwargs)

    def get_queryset(self):
        self.board = get_object_or_404(Board, pk=self.kwargs.get('pk'))
        queryset = self.board.topics.order_by('-last_updated').annotate(replies=Count('posts') - 1)
        return queryset

在使用基于类的视图分页时，我们与模板中paginator进行交互的方式有点不同。它会在模板中提供以下变量:paginator,page_obj,is_paginated,object_list,还有一个我们在 context_object_name 中定义名字的变量。在我们的例子中，这个额外的变量将被命名为 topics ，并且它将等同于 object_list。

关于这个 get_context_data ，其实，它就是我们在扩展 GCBV 时向请求上下文添加内容的方式。

但这里的主要是 paginate_by 属性。一般情况下，只需添加它就足够了。

要记得更新 urls.py 哦：

myproject/urls.py 查看完整文件

from django.conf.urls import url
from boards import views

urlpatterns = [
    # ...
    url(r'^boards/(?P<pk>\d+)/$', views.TopicListView.as_view(), name='board_topics'),
]

现在我们来修改一下模板：

templates/topics.html 查看完整文件

{% block content %}
  <div class="mb-4">
    <a href="{% url 'new_topic' board.pk %}" class="btn btn-primary">New topic</a>
  </div>

  <table class="table mb-4">
    <!-- table content suppressed -->
  </table>

  {% if is_paginated %}
    <nav aria-label="Topics pagination" class="mb-4">
      <ul class="pagination">
        {% if page_obj.has_previous %}
          <li class="page-item">
            <a class="page-link" href="?page={{ page_obj.previous_page_number }}">Previous</a>
          </li>
        {% else %}
          <li class="page-item disabled">
            <span class="page-link">Previous</span>
          </li>
        {% endif %}

        {% for page_num in paginator.page_range %}
          {% if page_obj.number == page_num %}
            <li class="page-item active">
              <span class="page-link">
                {{ page_num }}
                <span class="sr-only">(current)</span>
              </span>
            </li>
          {% else %}
            <li class="page-item">
              <a class="page-link" href="?page={{ page_num }}">{{ page_num }}</a>
            </li>
          {% endif %}
        {% endfor %}

        {% if page_obj.has_next %}
          <li class="page-item">
            <a class="page-link" href="?page={{ page_obj.next_page_number }}">Next</a>
          </li>
        {% else %}
          <li class="page-item disabled">
            <span class="page-link">Next</span>
          </li>
        {% endif %}
      </ul>
    </nav>
  {% endif %}

{% endblock %}

现在花点时间运行一下测试代码，如果有需要调整的地方就修一下。

boards/tests/test_view_board_topics.py

from django.test import TestCase
from django.urls import resolve
from ..views import TopicListView

class BoardTopicsTests(TestCase):
    # ...
    def test_board_topics_url_resolves_board_topics_view(self):
        view = resolve('/boards/1/')
        self.assertEquals(view.func.view_class, TopicListView)

可复用的分页模板

就像我们在 form.html 中封装模板时做的一样，我们也可以为分页的HTML代码片创建类似的东西。

我们来对主题帖子页面进行分页，进而找到一种复用分页组件的方法。

boards/views.py 查看完整文件

class PostListView(ListView):
    model = Post
    context_object_name = 'posts'
    template_name = 'topic_posts.html'
    paginate_by = 2

    def get_context_data(self, **kwargs):
        self.topic.views += 1
        self.topic.save()
        kwargs['topic'] = self.topic
        return super().get_context_data(**kwargs)

    def get_queryset(self):
        self.topic = get_object_or_404(Topic, board__pk=self.kwargs.get('pk'), pk=self.kwargs.get('topic_pk'))
        queryset = self.topic.posts.order_by('created_at')
        return queryset

更新一下 url.py [查看完整文件]

from django.conf.urls import url
from boards import views

urlpatterns = [
    # ...
    url(r'^boards/(?P<pk>\d+)/topics/(?P<topic_pk>\d+)/$', views.PostListView.as_view(), name='topic_posts'),
]

现在，我们从topics.html模板中获取分页部分的html代码片，并在 templates/includes 文件夹下面创建一个名为 pagination.html 的新文件，和 forms.html 同级目录:

myproject/
 |-- myproject/
 |    |-- accounts/
 |    |-- boards/
 |    |-- myproject/
 |    |-- static/
 |    |-- templates/
 |    |    |-- includes/
 |    |    |    |-- form.html
 |    |    |    +-- pagination.html  <-- here!
 |    |    +-- ...
 |    |-- db.sqlite3
 |    +-- manage.py
 +-- venv/

templates/includes/pagination.html

{% if is_paginated %}
  <nav aria-label="Topics pagination" class="mb-4">
    <ul class="pagination">
      {% if page_obj.has_previous %}
        <li class="page-item">
          <a class="page-link" href="?page={{ page_obj.previous_page_number }}">Previous</a>
        </li>
      {% else %}
        <li class="page-item disabled">
          <span class="page-link">Previous</span>
        </li>
      {% endif %}

      {% for page_num in paginator.page_range %}
        {% if page_obj.number == page_num %}
          <li class="page-item active">
            <span class="page-link">
              {{ page_num }}
              <span class="sr-only">(current)</span>
            </span>
          </li>
        {% else %}
          <li class="page-item">
            <a class="page-link" href="?page={{ page_num }}">{{ page_num }}</a>
          </li>
        {% endif %}
      {% endfor %}

      {% if page_obj.has_next %}
        <li class="page-item">
          <a class="page-link" href="?page={{ page_obj.next_page_number }}">Next</a>
        </li>
      {% else %}
        <li class="page-item disabled">
          <span class="page-link">Next</span>
        </li>
      {% endif %}
    </ul>
  </nav>
{% endif %}

现在，我们在 topic_posts.html 文件中来使用它：

templates/topic_posts.html 查看完整文件

{% block content %}

  <div class="mb-4">
    <a href="{% url 'reply_topic' topic.board.pk topic.pk %}" class="btn btn-primary" role="button">Reply</a>
  </div>

  {% for post in posts %}
    <div class="card {% if forloop.last %}mb-4{% else %}mb-2{% endif %} {% if forloop.first %}border-dark{% endif %}">
      {% if forloop.first %}
        <div class="card-header text-white bg-dark py-2 px-3">{{ topic.subject }}</div>
      {% endif %}
      <div class="card-body p-3">
        <div class="row">
          <div class="col-2">
            <img src="{% static 'img/avatar.svg' %}" alt="{{ post.created_by.username }}" class="w-100">
            <small>Posts: {{ post.created_by.posts.count }}</small>
          </div>
          <div class="col-10">
            <div class="row mb-3">
              <div class="col-6">
                <strong class="text-muted">{{ post.created_by.username }}</strong>
              </div>
              <div class="col-6 text-right">
                <small class="text-muted">{{ post.created_at }}</small>
              </div>
            </div>
            {{ post.message }}
            {% if post.created_by == user %}
              <div class="mt-3">
                <a href="{% url 'edit_post' post.topic.board.pk post.topic.pk post.pk %}"
                   class="btn btn-primary btn-sm"
                   role="button">Edit</a>
              </div>
            {% endif %}
          </div>
        </div>
      </div>
    </div>
  {% endfor %}

  {% include 'includes/pagination.html' %}

{% endblock %}

别忘了修改主循环为 {% for post in posts %}。

我们同样也可以更新一下先前的模板，topics.html 模板同样也可以这个封装的分页模板。

templates/topics.html 查看完整文件

{% block content %}
  <div class="mb-4">
    <a href="{% url 'new_topic' board.pk %}" class="btn btn-primary">New topic</a>
  </div>

  <table class="table mb-4">
    <!-- table code suppressed -->
  </table>

  {% include 'includes/pagination.html' %}

{% endblock %}

为了测试目的，你需要添加一些帖子（或者通过 python shell 去创建），然后修改代码中的 ** paginate_by** 到一个较小的数字，比如 2 ，然后看看页面会发生什么变化。

（查看完整文件）

更新一下测试用例：

boards/tests/test_view_topic_posts.py

from django.test import TestCase
from django.urls import resolve
from ..views import PostListView

class TopicPostsTests(TestCase):
    # ...
    def test_view_function(self):
        view = resolve('/boards/1/topics/1/')
        self.assertEquals(view.func.view_class, PostListView)