django - 如何绑定另一个表中计算的计数值的表记录

Question

在我的应用程序中，我有一个培训列表。此列表中的一个字段应显示每个培训的预订数量。 为了说明我的意思我准备的SQL查询：

SELECT * 
FROM club_training a 
LEFT JOIN 
(SELECT training_id, count(*) 
FROM club_booking 
group by training_id) b 
ON a.id = b.training_id 

你能不能给我一些建议如何做到这一点在Django？ 我在我的代码中使用了Booking.objects.all().values('training_id').annotate(booked_amount=Count('training_id'))，但结果是所有培训的所有计数值都显示在列表上的每个培训中。应显示一个适合每次培训的计数值。

views.py

class HomePageView(TemplateView): 
    """Home Page with list of trainings""" 
    template_name = 'club/training_list.html' 


    def get_context_data(self, **kwargs): 
     now = datetime.datetime.now() 
     context = super(HomePageView, self).get_context_data(**kwargs) 
     context['trainings'] = Training.objects.filter(state="A", training_date__gte=now).order_by('training_date', 'start_time') 
     for each_training in context['trainings']:  
      each_training.diff = each_training.availability - each_training.counter 
      each_training.counter = Booking.objects.all().values('training_id').annotate(booked_amount=Count('training_id')) 
     return context 

models.py

class Training(models.Model): 
    """Class for plan training""" 
    STATE = (
     ('A', 'Active'), 
     ('I', 'Inactive'), 
    ) 
    name = models.ForeignKey('TrnDesc') 
    instructor = models.ForeignKey('Instructor') 
    start_time = models.TimeField(blank=True) 
    end_time = models.TimeField(default='00:00:00') 
    availability = models.PositiveIntegerField(default=15) 
    state = models.CharField(max_length=1, choices=STATE, default='A') 
    training_date = models.DateField(default=date.today) 
    counter = models.PositiveIntegerField(default=0) 
    def __str__(self): 
     return self.name.name 

class Booking(models.Model): 
    """Data of people which book fitness classes""" 
    first_name = models.CharField(max_length=50) 
    last_name = models.CharField(max_length=50) 
    email = models.CharField(max_length=50) 
    phone = models.CharField(max_length=10) 
    training = models.ForeignKey('Training') 
    def __str__(self): 
     return self.training.name.name 

training_list.html

{% extends 'club/base.html' %} 

{% block content %} 

     <ul class="nav nav-pills"> 
      <li role="presentation" class="active"><a href="#">Fitness Classes</a></li> 
      <li role="presentation"><a href="#">Join Us</a></li> 
      <li role="presentation"><a href="#">Contact Us</a></li> 
     </ul> 
     <br></br> 
    {% regroup trainings by training_date as date_list %} 
    {% for date in date_list %} 
     <div class="panel panel-default"> 


      <div class="panel-heading">{{date.grouper|date:"l, d F o"}}</div> 


      <table class="table"> 
       <tr> 
        <th style="width: 20%">Training name</th> 
        <th style="width: 30%">Training description</th> 
        <th style="width: 10%">Instructor</th> 
        <th style="width: 10%">Start time</th> 
        <th style="width: 10%">End time</th> 
        <th style="width: 10%">Left</th> 
        <th style="width: 10%">Test_counter</th> 
        <th style="width: 10%">Actions</th> 
       </tr> 


       {% for training in date.list %} 

       <tr> 
        <td>{{training.name}}</td> 
        <td>{{training.name.desc}}</td> 
        <td>{{training.instructor}}</td> 
        <td>{{training.start_time|time:"H:i"}}</td> 
        <td>{{training.end_time|time:"H:i"}}</td> 
        <td>{{training.diff}}</td> 
        <td>{{training.counter}}</td> 
        <td><a href="{% url 'book' training_id=training.pk%}"><button type="button" class="btn btn-primary">Book</button></a></td> 
       </tr> 

       {% endfor %}  
      </table> 
     </div> 
    {% endfor %} 

{% endblock %} 

Answer 1

for each_training in context['trainings']:  
     each_training.diff = each_training.availability - each_training.counter 
     each_training.counter = Booking.objects.filter(training_id=each_training.id).count() # just modify this line 
    return context 

Answer 2

我想从另一个方向的方法：each_training.booking_set.count()

而且，我不知道它适合你的数据库的目标，但如果我明白你在找什么，你可以设置它是这样的：

models.py

class Training(models.Model): 
    ... 
    <fields> 
    ... 
    def __str__(self): 
     return self.name.name 

    @property 
    def counter(self): 
     return self.booking_set.count() 

    @property 
    def diff(self): 
     return self.availability - self.counter 

像这样，值可以直接从模型转到模板。

我能看到的唯一问题是我认为它阻止了这些字段成为查询集的一部分。例如，Training.objects.filter(counter__gt=0)将不起作用。如果你需要的话，我认为你仍然必须找到一个机会来保存和更新数据库中的值，possibly using a signal，这样每次调用视图时都不必一次又一次地保存值。

它也像你可以使用一个经理来处理一些您可以在视图都在做逻辑：

managers.py

from django.db.models import Manager 

class Active(Manager): 

    def by_date(self, date) 
     Training.objects.filter(state="A", training_date__gte=date).order_by('training_date', 'start_time') 

然后你可以经理添加到您的模型（即一定要保持你的香草经理）：

models.py

from .managers import Courses 

class Training(models.Model): 
    ... 
    <fields> 
    objects = models.Manager() 
    active = Active() 
    ... 

    def __str__(self): 
     return self.name.name 

现在，您可以使用纤细的ListView分发所有信息。

from django.views.generic import ListView 

class HomePageView(ListView): 
    """Home Page with list of trainings""" 
    template_name = 'club/training_list.html' 
    context_object_name = "trainings" 

    def get_queryset(self): 
     now = datetime.datetime.now() 
     return Trainings.active.by_date(now) 

我不熟悉你在模板中做的一些事情，但是一切都应该像现在一样工作。

或许我的路要走，但我希望它的一些好的精神食粮:)