When you are using models related by a Foreign Key, by default you have
access to the "children" models from the parent model. For example, your
Sensor model gets a "measurment_set" property just because of such foreign
key:
>>> dir(Sensor.objects.all()[0])
>
> [other properties, '*measurment_set*', [more properties]
>
>
>
which means you can access your measurements from a Sensor model:
>>> sensor = Sensor.objects.all()[0]
> >>> sensor
> <Sensor: id:1 / 2018-05-15>
> >>> sensor.measurment_set.all()
> <QuerySet [<Measurment: sens_id:1, time:2018-05-16 13:20:57.989089+00:00>,
> <Measurment: sens_id:1, time:2018-05-15 13:18:50.656182+00:00>]>
>
You can change the default name in the ForeignKey field declaration, using
the *related_name* parameter.
You can also access the children from the templates:
<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>
>
> {% *with sensor.measurment_set.all.0 as last_measurment* %}
>
> <td>{{ last_measurment.temperature1 }}</td><td>{{
>> last_measurment.temperature2 }}</td><td>{{ last_measurment.temperature3
>> }}</td>
>
> {% endwith %}
>
> </tr>
>
> {% endfor %}
>
> </table>
>
> {% else %}
>
> <p>There are no sensors in the DB.</p>
>
> {% endif %}
>
>
Using *with* limits your children to a single item, and telling the
template *all.0* you refer to the FIRST measurement for such sensor. The
only thing you need to do is tell the Measurment model to list itself by
time_of_measurment *reversed*, so the "first" object in the list will be
the latest measurement:
> class Measurment(models.Model):
> ....
> class Meta:
> ordering = ['-time_of_measurment',]
The way I have done it is a bit hacky (and I guess there are cleaner ways
to do it, using get_context or similars), but hey!
El mar., 15 may. 2018 a las 13:21, Anton Smirnov (<
antonsmirnov1...@gmail.com>) escribió:
> There are two models "Sensors" with information about them 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 latest measurement data from
> the "Measurments" model corresponding for each sensor.
>
> for displaying data about sensor I used ListView:
>
> view:
>
> from .models import Sensor, Measurmentclass 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 max of time_of_measurment for each id_sens.
>
> How it can be done?
>
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--
Fidel Leon
fi...@flm.cat
Phone: +34 622 26 44 92
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