Django manytomany accessing related column - django

Question

I'm trying to get my head around how Django understands m2m relationships, in SQL you would just add some joins through the intermediate table.
I have a Container which contains various Samples. A Sample can be spread over various Containers.
So in my container I add a alias samples m2m field (essentially a book mark to the other table).
What I can do is get a single Container and display the form information, I would like to add the Sample columns to the form, if I do this for the samples m2m field it returns a multifield, but how do I access the other related fields through the m2m sample_id >=< container_id ?
class Container(models.Model):
container_id = models.AutoField(primary_key=True)
samples = models.ManyToManyField(Sample, through='JoinSampleContainer', through_fields=('container_id', 'sample_id'), related_name='container')
location_id = models.ForeignKey(Location, db_column='location_id', on_delete = models.PROTECT)
icon_desc = models.ForeignKey(Icon, db_column='icon_desc', null=True, blank=True, default='Box',on_delete = models.PROTECT)
container_name = models.CharField(max_length=50, blank=True, null=True)
container_type = models.CharField(max_length=50, blank=True, null=True)
In my Sample table I add the containers alias to act as a bookmark to the other table
class Sample(models.Model):
sample_id = models.AutoField(primary_key=True)
containers = models.ManyToManyField(Container, through='JoinSampleContainer', through_fields=('sample_id', 'container_id'), related_name='sample')
sample_number = models.IntegerField()
material_type = models.CharField(max_length=200, default='', blank=True, null=True, choices = MATERIALS)
weight = models.DecimalField(max_digits=6, decimal_places=2)
description = models.CharField(max_length=500, default='', blank=True, null=True)
recovery_method = models.CharField(max_length=200, default='', blank=True, null=True, choices = RECOVERY_METHODS)
comments = models.CharField(max_length=1000, default='', blank=True, null=True)
In this case I am managing the through table:
class JoinSampleContainer(models.Model):
id = models.AutoField(primary_key=True)
container_id = models.ForeignKey(Container, db_column='container_id', on_delete = models.PROTECT)
sample_id = models.ForeignKey(Sample, db_column='sample_id', on_delete = models.PROTECT)
So now I want to display the contents of a single container through a form. I have the url's setup to pass the container_id.
# views.py
def containercontents(request, pk):
post = get_object_or_404(Container, pk=pk)
# objects = Container.samples.all()
if request.method == "POST":
form = ContainerContentsForm(request.POST, instance=post)
if form.is_valid():
post = form.save(commit=False)
#post.user = request.user
#post.datetime = datetime.datetime.now()
post.save()
return redirect('allcontainer')
#, pk=post.pk)
else:
form = ContainerContentsForm(instance=post)
return render(request, 'container/containercontents.html', {'form': form})
The Form
# form.py
class ContainerContentsForm(forms.ModelForm):
class Meta:
model = Container
fields = (
'location_id',
'container_name',
'container_type',
'icon_desc',
'samples',
)
N.B. The samples seems to list everything regardless of the container.
Then the html
# html
contents
which passes to:
# html
<div class="">
{{ form }}
</div>

Answer 1

Your models are defined wrongly: You should not define the ManyToManyField on both models, only on one of them. So remove the containers field on Sample and only keep it on Container. Set the related_name to "containers" (plural). That way the relationship Container -> Sample is container.samples.all() and the reverse one is sample.containers.all().
Now the purpose of a form is to allow you to select which Samples you want to associate to a Container. So by default the field will be represented by a ModelMultipleChoiceField. The already associated Samples should be pre-selected when you initialise the form with a Container instance.
You can narrow the samples to select from by specifying the queryset for the field, by overriding the default field in the form:
class ContainerContentsForm(forms.ModelForm):
class Meta:
# same code here
samples = forms.ModelMultipleChoiceField(
queryset = Sample.objects.filter(...)
)
You say you want to "display the contents of a Container through the form", if you just want to display, why use a form? To just display the contents, loop through the related samples and display them:
{% for sample in form.instance.samples.all %}
{{ sample.sample_id }}
{% endfor %}
Note: You should rename your ids to id. sample.sample_id is bad programming style. But I already told you that.