How to operate with two models in one view?

[Anton Smirnov]

I have two models: "Sensors" with information about its and "Measurments" .

class Sensor(models.Model):
    date_start = models.DateField()
    Latitude =  models.DecimalField(max_digits=18, decimal_places=15)
    Longitude = models.DecimalField(max_digits=18, decimal_places=15)

    def __str__(self):
        return 'id:%s / %s' % (self.id, self.date_start)

class Measurment(models.Model):
    sens = models.ForeignKey(Sensor, on_delete=models.PROTECT)
    time_of_measurment = models.DateTimeField()
    humidity = models.PositiveSmallIntegerField()
    temperature1 = models.DecimalField(max_digits=5, decimal_places=2)
    temperature2 = models.DecimalField(max_digits=5, decimal_places=2)
    temperature3 = models.DecimalField(max_digits=5, decimal_places=2)

    def __str__(self):
        return 'sens_id:%s, time:%s' % (self.sens.id, self.time_of_measurment)

From each sensor I serially recive measurments. On the page, it is necessary to display the sensor data and the one latest measurement data from the "Measurments" model corresponding for each sensor.

for displaying sensors data I used ListView:

view:

from .models import Sensor, Measurment
class SenorsListView(generic.ListView):
    model = Sensor, Measurment
    context_object_name = 'sensors_list'
    template_name = 'sensors_list.html'
    queryset = Sensor.objects.all().order_by('-date_start')

template "sensors_list.html":

{% extends "base_generic.html" %}
{% block content %}
    <h1>Sensors List</h1>
    {% if sensors_list %}
    <table class="table">
  <tr>
    <td><h4>ID sens<h4></td>
    <td><h4>Date of install<h4></td>
    <td><h4>Latitude<h4></td>
    <td><h4>Longitude<h4></td>
    <td><h4>Data from last measurments<h4></td>
  </tr>
      {% for sensor in sensors_list %}
      <tr>
    <td>{{sensor.id}}</td>
    <td>{{sensor.date_start}}</td>
   <td>{{sensor.Latitude}}</td>
    <td>{{sensor.Longitude}}</td>
    <td>{{}}</td>
      </tr>
      {% endfor %}
    </table>
    {% else %}
      <p>There are no sensors in the DB.</p>
    {% endif %} 
{% endblock %}

But dont know How to display data about last measurments. I think need using aggregate to calculate last_time how Max(time_of_measurment) for each id_sens. And then get it whith filter(sens=id_sens, time_of_measurment = last_time )

How it can be done right?

[Daniel Roseman]

An easy way to do this would be to define the default ordering for Measurment to be by reverse time; then you can access `sensor.measurment_set.first` directly in the template.

class Measurment:

    ...

    class Meta:

         ordering = ['-time_of_measurment']

...

{% for sensor in sensors_list %}

  ...

  {% with sensor.measurment_set.first as last_measurment %}

    {{ last_measurment.humidity }}  # etc

  {% endwith %}

{% endfor %}

--

DR.