how can I query a list of instances of a model without direct relationship field

Using Django Using the ORM
1

following is my code snippet:

class Captain(models.Model):
    name = models.OneToOneField(User, on_delete=models.CASCADE)
    date_of_birth = models.DateField(null=True, blank=True)
    joined_date = models.DateField(null=True, blank=True)

class Boat(models.Model):
    name = models.CharField(max_length=200)
    contact = models.CharField(max_length=200)
    captain = models.OneToOneField(Captain,on_delete=models.CASCADE, null=True, blank=True)

First there is a captain instance created, lets say name=‘John’.
Then there are 3 boat instances, named, “Boat A”, “Boat B” and “Boat C”,

My question is, is there a way “John” can query all the boat instances(specifically the name fields), so he could update, which boat he is on, at any given moment?

I have tried several ways to achieve this, but could only get the instances associated with the captain only, not the whole list of boats, (Boat A, Boat B, Boat C)

what is the best way to approach this problem without altering the models, the problem can be solved by altering the models, however when I alter the models I, have to fix a lot of templates work.

2

Hey there!
If you want to get all boats, that is:

Boat.objects.all()

But, if for example you want to get all boats that does not have a captain in the moment:

Boat.objects.filter(captain__isnull=True)

But, if for example, you want to get all boats that match a specific name:

boat_names = ["Abeerden 13", "The locust"]
Boat.objects.filter(name__in=boat_names)

This is all common operations, if you’re struggling with database queries using the ORM, i suggest that you review the section Making queries in the docs

3

Thanks, yes with the query Boat.objects.all() I do get all the instances of the Boat model,
However I cannot seems to update the boat instance through the captain instance.

Lets say, in early morning captain instance ‘John’ was in in Boat instance “Boat A”, and in the afternoon if he wanted to be in “Boat B” I could not make it update, followin is my forms.py and views.py

#forms.py
class BoatNameChange(forms.ModelForm):
    boat_name = forms.ModelChoiceField(queryset=Boat.objects.all())
    class Meta:
        model = Boat
        fields = ['boat_name']

#views.py
class BoatPickView(LoginRequiredMixin, generic.UpdateView):
    model = Boat
    form_class = BoatNameChange
    template_name = 'captain/boat_change.html'

I hope I make myslef clear.

4

If you have an instance of Captain named captain, then because of the OneToOne relationship between Captain and Boat, the Boat that captain is related to is captain.boat. (See the docs at Model field reference | Django documentation | Django) There is no need for a query at this point.

I believe (but have not verified this) that you should be able to reassign captain from boat_a to boat_b by:
captain.boat.captain = None
boat_b.captain = captain

(It’s also possible that the first statement may also work as captain.boat = None. Again, conjecture on my part. You may also need to save captain.boat after the first statement and boat_b after the second.)

5

Thanks will look in to it.