Django入门与实践-第20章：QuerySets（查询结果集）（完结）

http://127.0.0.1:8000/boards/1/

#boards/models.py
from django.utils.text import Truncator
 
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)

python manage.py shell
from boards.models import Board
# First get a board instance from the database
board = Board.objects.get(name='Django')
board.topics.all()
board.topics.count()
#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()
Post.objects.count()
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)
Post.objects.filter(topic__board=board).count()
最后一个任务是标识版块下面的最后一条回复
Post.objects.filter(topic__board=board).order_by('-created_at')
Post.objects.filter(topic__board=board).order_by('-created_at').first()#boards/models.py
class Board(models.Model):
    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()<--templates/home.html-->
<!--以下代码应该在<small></small>中但是未通过-->
<a href="{% url 'topic_posts' board.pk post.topic.pk %}" rel="external nofollow"  rel="external nofollow" >
　　 By {{ post.created_by.username }} at {{ post.created_at }}
</a>{% 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 %}" rel="external nofollow" >{{ 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>
                    </small>
                {% endwith %}
            </td>
            </tr>
            {% endfor %}
        </tbody>
    </table>
{% endblock %}

<!--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 %}" rel="external nofollow"  rel="external nofollow" >
                By {{ post.created_by.username }} at {{ post.created_at }}
            </a>
        </small>
    {% else %}
        <small class="text-muted">
            <em>No posts yet.</em>
        </small>
    {% endif %}
{% endwith %}

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)#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)

#boards/views.py 更新
from django.db.models import Count
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-->
{% for topic in topics %}
    <tr>
        <td><a href="{% url 'topic_posts' board.pk topic.pk %}" rel="external nofollow" >{{ topic.subject }}</a></td>
        <td>{{ topic.starter.username }}</td>
        <td>{{ topic.replies }}</td>
        <td>0</td>
        <td>{{ topic.last_updated }}</td>
    </tr>
{% endfor %}