Navigating many2many relationships in both directions - django

Question

I'm trying to understand how Django returns columns from foreign keys, particularly the m2m situation, easy in SQL but I'm trying to get into Django.
In this example I have 3 models, Sample which has a m2m with Container
and Location which has a 1-to-many with Container.
Scenario 1a: From the Sample table get the Containers that sample is in(return sample_number and container_name).
Scenario 1b: From the Container get the related Samples (return container_number and sample_number).
Scenario 2a: From the Location model get the containers (location_name and container_names).
Scenario 2b: From the Container model get the location (Container_name and location_name).
Hopefully this will serve as a good overall reference for others.
# models.py
class Location(models.Model):
location_id = models.AutoField(primary_key=True)
location_name = models.CharField(max_length=100, blank=True, null=True)
class Sample(models.Model):
sample_id = models.AutoField(primary_key=True)
sample_number = models.IntegerField()
class Container(models.Model): #like a friend
container_id = models.AutoField(primary_key=True)
container_name = models.CharField(max_length=50, blank=True, null=True)
location_id = models.ForeignKey(Location, db_column='location_id', on_delete = models.PROTECT, related_name = 'location')
samples = models.ManyToManyField('Sample', through='ContainerSamples', related_name='containers')
# views.py - Implements a filter
def detailcontainer(request, container_id):
container = get_object_or_404(Container, pk=container_id)
samples = container.samples.all()
container_contents = container.samples.all()
unassigned_samples = Sample.objects.all()
qs = Sample.objects.all()
context = {
'queryset': qs,
'container':container,
'container_contents': container_contents,
'unassigned_samples': unassigned_samples,
}
return render(request, 'container/detailcontainer.html', context)
# templates
{% for unassigned in unassigned_samples %}
# 1a [solved]
{% for unassigned in unassigned_samples %}
{{ unassigned.sample_number }}
{% for container in unassigned.containers.all %}
{{ container.location_id }}.{{ container.container_name }}
{% endfor %}
{% endfor %}
# 1b
{{ unassigned.____________ }} # the container_name
{{ unassigned.____________ }} # the related samples (sample_number)
# 2a
{{ unassigned.____________ }} # the location_name
{{ unassigned.____________ }} # the related container names (container_name)
# 2b
{{ unassigned.____________ }} # the container_name
{{ unassigned.____________ }} # the location_name
{% endfor %}

Answer 1

Scenario 1a: From the Sample table get the Containers that sample is in(return sample_number and container_name).
container_set = sample.containers.all()
for container in container_set:
print([container.container_name, sample.sample_name])
Scenario 1b: From the Container get the related Samples (return container_number and sample_number).
sample_set = container.samples.all()
for sample in sample_set:
print([sample.sample_number, container.container_number])
Scenario 2a: From the Location model get the containers (location_name and container_names).
container_set = location.location.all() # You have Container.location_id.related_name = 'location', I don't know why.
for container in container_set:
print([location.location_name, container.container_name])
Scenario 2b: From the Container model get the location (Container_name and location_name).
print([container.container_name, container.location_id.location_name])
Also your models should be written as follows:
# models.py
class Location(models.Model):
name = models.CharField(max_length=100, blank=True, null=True)
class Sample(models.Model):
number = models.IntegerField()
class Container(models.Model): #like a friend
container = models.AutoField(primary_key=True)
name = models.CharField(max_length=50, blank=True, null=True)
location = models.ForeignKey(Location, on_delete = models.PROTECT, related_name='containers')
samples = models.ManyToManyField(Sample, through='ContainerSamples', related_name='containers')
If you're using the same definition for the autofield on the models, there' no reason to add it. When you're defining properties on a model such as a name, you shouldn't prefix it with the model name. It should be understood that it's the model's name. And finally related_name is the field on the referenced model for the relationship to traverse back to the current model.