现在我们花点时间来探索关于模型的 API。首先，我们来改进主页：
有3个任务：
- 显示每个板块的总主题数
- 显示每个板块的总回复数
- 显示每个板块的最后发布者和日期
在实现这些功能前，我们先使用Python终端
因为我们要在Python终端尝试，所以，把所有的 models 定义一个 __str__ 方法是个好主意
boards/models.py(完整代码)
from django.db import models
from django.utils.text import Truncator
class Board(models.Model):
# ...
def __str__(self):
return self.name
class Topic(models.Model):
# ...
def __str__(self):
return self.subject
class Post(models.Model):
# ...
def __str__(self):
truncated_message = Truncator(self.message)
return truncated_message.chars(30)
在 Post 模型中，使用了 Truncator 工具类，这是将一个长字符串截取为任意长度字符的简便方法（这里我们使用30个字符）
现在打开 Python shell
python manage.py shell
from boards.models import Board
# First get a board instance from the database
board = Board.objects.get(name='Django')
这三个任务中最简单的一个就是获取当前版块的总主题数，因为 Topic 和 Baoard 是直接关联的。
board.topics.all()
<QuerySet [<Topic: Hello everyone!>, <Topic: Test>, <Topic: Testing a new post>, <Topic: Hi>]>
board.topics.count()
4
就这样子。
现在统计一个版块下面的回复数量有点麻烦，因为回复并没有和 Board 直接关联
from boards.models import Post
Post.objects.all()
<QuerySet [<Post: This is my first topic.. :-)>, <Post: test.>, <Post: Hi everyone!>,
<Post: New test here!>, <Post: Testing the new reply feature!>, <Post: Lorem ipsum dolor sit amet,...>,
<Post: hi there>, <Post: test>, <Post: Testing..>, <Post: some reply>, <Post: Random random.>
]>
Post.objects.count()
11
这里一共11个回复，但是它并不全部属于 "Django" 这个版块的。
我们可以这样来过滤
from boards.models import Board, Post
board = Board.objects.get(name='Django')
Post.objects.filter(topic__board=board)
<QuerySet [<Post: This is my first topic.. :-)>, <Post: test.>, <Post: hi there>,
<Post: Hi everyone!>, <Post: Lorem ipsum dolor sit amet,...>, <Post: New test here!>,
<Post: Testing the new reply feature!>
]>
Post.objects.filter(topic__board=board).count()
7
双下划线的topic__board用于通过模型关系来定位，在内部，Django 在 Board-Topic-Post之间构建了桥梁，构建SQL查询来获取属于指定版块下面的帖子回复。
最后一个任务是标识版块下面的最后一条回复
# order by the `created_at` field, getting the most recent first
Post.objects.filter(topic__board=board).order_by('-created_at')
<QuerySet [<Post: testing>, <Post: new post>, <Post: hi there>, <Post: Lorem ipsum dolor sit amet,...>,
<Post: Testing the new reply feature!>, <Post: New test here!>, <Post: Hi everyone!>,
<Post: test.>, <Post: This is my first topic.. :-)>
]>
# we can use the `first()` method to just grab the result that interest us
Post.objects.filter(topic__board=board).order_by('-created_at').first()
<Post: testing>
太棒了，现在我们来实现它
boards/models.py (完整代码)
from django.db import models
class Board(models.Model):
name = models.CharField(max_length=30, unique=True)
description = models.CharField(max_length=100)
def __str__(self):
return self.name
def get_posts_count(self):
return Post.objects.filter(topic__board=self).count()
def get_last_post(self):
return Post.objects.filter(topic__board=self).order_by('-created_at').first()
注意，我们使用的是self，因为这是Board的一个实例方法，所以我们就用这个Board实例来过滤这个 QuerySet
现在我们可以改进主页的HTML模板来显示这些新的信息
templates/home.html
{% extends 'base.html' %}
{% block breadcrumb %}
<li class="breadcrumb-item active">Boards</li>
{% endblock %}
{% block content %}
<table class="table">
<thead class="thead-inverse">
<tr>
<th>Board</th>
<th>Posts</th>
<th>Topics</th>
<th>Last Post</th>
</tr>
</thead>
<tbody>
{% for board in boards %}
<tr>
<td>
<a href="{% url 'board_topics' board.pk %}">{{ board.name }}</a>
<small class="text-muted d-block">{{ board.description }}</small>
</td>
<td class="align-middle">
{{ board.get_posts_count }}
</td>
<td class="align-middle">
{{ board.topics.count }}
</td>
<td class="align-middle">
{% with post=board.get_last_post %}
<small>
<a href="{% url 'topic_posts' board.pk post.topic.pk %}">
By {{ post.created_by.username }} at {{ post.created_at }}
</a>
</small>
{% endwith %}
</td>
</tr>
{% endfor %}
</tbody>
</table>
{% endblock %}
现在是这样的效果
运行测试：
python manage.py test
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
.......................................................EEE......................
======================================================================
ERROR: test_home_url_resolves_home_view (boards.tests.test_view_home.HomeTests)
----------------------------------------------------------------------
django.urls.exceptions.NoReverseMatch: Reverse for 'topic_posts' with arguments '(1, '')' not found. 1 pattern(s) tried: ['boards/(?P<pk>\\d+)/topics/(?P<topic_pk>\\d+)/$']
======================================================================
ERROR: test_home_view_contains_link_to_topics_page (boards.tests.test_view_home.HomeTests)
----------------------------------------------------------------------
django.urls.exceptions.NoReverseMatch: Reverse for 'topic_posts' with arguments '(1, '')' not found. 1 pattern(s) tried: ['boards/(?P<pk>\\d+)/topics/(?P<topic_pk>\\d+)/$']
======================================================================
ERROR: test_home_view_status_code (boards.tests.test_view_home.HomeTests)
----------------------------------------------------------------------
django.urls.exceptions.NoReverseMatch: Reverse for 'topic_posts' with arguments '(1, '')' not found. 1 pattern(s) tried: ['boards/(?P<pk>\\d+)/topics/(?P<topic_pk>\\d+)/$']
----------------------------------------------------------------------
Ran 80 tests in 5.663s
FAILED (errors=3)
Destroying test database for alias 'default'...
看起来好像有问题，如果没有回复的时候程序会崩溃
templates/home.html
{% with post=board.get_last_post %}
{% if post %}
<small>
<a href="{% url 'topic_posts' board.pk post.topic.pk %}">
By {{ post.created_by.username }} at {{ post.created_at }}
</a>
</small>
{% else %}
<small class="text-muted">
<em>No posts yet.</em>
</small>
{% endif %}
{% endwith %}
再次运行测试：
python manage.py test
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
................................................................................
----------------------------------------------------------------------
Ran 80 tests in 5.630s
OK
Destroying test database for alias 'default'...
我添加一个没有任何消息的版块，用于检查这个"空消息"
现在是时候来改进回复列表页面了。
现在，我将告诉你另外一种方法来统计回复的数量，用一种更高效的方式
和之前一样，首先在Python shell 中尝试
python manage.py shell
from django.db.models import Count
from boards.models import Board
board = Board.objects.get(name='Django')
topics = board.topics.order_by('-last_updated').annotate(replies=Count('posts'))
for topic in topics:
print(topic.replies)
2
4
2
1
这里我们使用annotate ，QuerySet将即时生成一个新的列，这个新的列，将被翻译成一个属性，可通过 topic.replies来访问，它包含了指定主题下的回复数。
我们来做一个小小的修复，因为回复里面不应该包括发起者的帖子
topics = board.topics.order_by('-last_updated').annotate(replies=Count('posts') - 1)
for topic in topics:
print(topic.replies)
1
3
1
0
很酷，对不对？
boards/views.py (完整代码)
from django.db.models import Count
from django.shortcuts import get_object_or_404, render
from .models import Board
def board_topics(request, pk):
board = get_object_or_404(Board, pk=pk)
topics = board.topics.order_by('-last_updated').annotate(replies=Count('posts') - 1)
return render(request, 'topics.html', {'board': board, 'topics': topics})
templates/topics.html(完整代码)
{% for topic in topics %}
<tr>
<td><a href="{% url 'topic_posts' board.pk topic.pk %}">{{ topic.subject }}</a></td>
<td>{{ topic.starter.username }}</td>
<td>{{ topic.replies }}</td>
<td>0</td>
<td>{{ topic.last_updated }}</td>
</tr>
{% endfor %}
下一步是修复主题的查看次数，但是，现在我们需要添加一个新的字段
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