CODEWITHSUNDEEP
pythondjangodjango-querysetmany-to-one
Gonzalo Dambra
Gonzalo Dambra

Reputation: 980

Django models' best solution for this reverse ForeignKey

I'm making a personal project to manage restaurants. Two of my models are facing a problem, these models are DiningRoom and Table.

DiningRoom is the representation of any area that the restaurant could have (e.g. we could have one area inside and other area in the terrace of the building). And in every DiningRoom we can set a layout of Tables.

So, the more object-oriented way I find to map this is by many-to-one relationship (ForeignKey). Since one DiningRoom can have many Tables, and one Table can be only in one DiningRoom. Right?

So my models are:

class DiningRoom(models.Model):
    account = models.ForeignKey(Account, on_delete=models.CASCADE, null=False)
    name = models.CharField(max_length=50, null=False, blank=False)
    rows = models.IntegerField(max=15, null=False)
    cols = models.IntegerField(max=15, null=False)  # rows and columns are for the room's grid layout.


class Table(models.Model):
    row = models.IntegerField(max=15, null=False)  # The row in the room's grid where the table is at.
    col = models.IntegerField(max=15, null=False)  # the column in the room's grid where the table is at.
    dining_room = models.ForeignKey(DiningRoom, on_delete=models.CASCADE, null=False)  # Here is the problem.

The problem is that when I am querying DiningRooms of the account in the view, I need to fetch also the Tables that are related to each DiningRoom in the queryset result.

def dining_rooms(request):
    try:
        account = Account.objects.get(id=request.session['account_id'])
    except Account.DoesNotExists:
        return response(request, "error.html", {'error': 'Account.DoesNotExists'})

    dining_rooms = DiningRoom.objects.filter(account=account)

But also I need the Tables of the results in dining_rooms!

I found two possible solutions but none seem to be "correct" to me. One is to make a Many-to-many relationship and validate that any Table is only in one DiningRoom in the view. And the second and worse one could be fetching the Tables once for each DiningRoom obtained in the queryset (but imagine a restaurant with 5 or 6 different areas (DiningRooms), it would be needed to fetch the database six times for every time).

Doing it in vice-versa and fetching all the Tables and select_related DiningRooms is not possible since it's possible to have a DiningRoom with no Tables in it (and in this case we will have missing DiningRooms).

What could be the best way to handle this? Thanks!

Upvotes: 1

Views: 65

Answers (1)

José Eras
José Eras

Reputation: 90

You can use the related_name or relationships backwards, an acceptable solution would be to create a method in the DiningRoom model that is called associated_tables() and return using the related_name (modelname_set, in this case it would be table_set). It is the name of the lowercase child model followed by the suffix _set

class DiningRoom(models.Model):
    #your fields

    def associated_tables(self):
        return self.table_set.all()

In addition, this video tutorial could clear your days and give you a better idea about reverse relationships: https://youtu.be/7tAZdYRA8Sw

Upvotes: 1

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