CODEWITHSUNDEEP
pythondjangodatabase
ZarifS
ZarifS

Reputation: 477

Linking one models with a set of another in Django

I am currently working on developing a database and API system where users can create a portfolio which contains a list of coins. I am using Django and I searched everywhere but I kept seeing foreign keys but I'm not sure that's what I need in this situation.

I want two models, one for portfolios which a user will be able to query on, and another coin model which the user will be able to also query on. However in the portfolio there should be a list of coins. I know how to do this in Java using objects but not sure the method in Django.

Here is my model class:

from django.db import models


class Portfolio(models.Model):
    name = models.CharField(max_length=250)

    def __str__(self):
        return self.name


class Coin(models.Model):
    name = models.CharField(max_length=100)
    symbol = models.CharField(max_length=5)
    price = models.DecimalField(max_digits=20, decimal_places=9)
    info = models.TextField()
    website = models.TextField()
    rank = models.IntegerField()

    def __str__(self):
        return self.name + " - " + self.symbol

Now I would ideally have something like coins = list of Coins model if I was using java to make the objects, but since this is for a database and in Django I'm not sure how I should link the two.

I've seen related objects but did not understand the explanations for my issue. How should I go about setting up these models? Thanks.

Upvotes: 1

Views: 5050

Answers (1)

VMatić
VMatić

Reputation: 996

It sounds like you want to have a number of Portfolio objects each of which can have varying investments in Coin objects. In this case, you'd want to use a ManyToManyField:

class Portfolio(models.Model):
    name = models.CharField(max_length=250)
    coins = models.ManyToManyField(Coin)

The database would then store the two dimensional table of which Portfolio holds which coin.

However an alternate approach you could try is to create an object that separately represents the investment:

class Investment(models.Model):
    portfolio = models.ForeignKey(Portfolio)
    coin = models.ForeignKey(Coin)
    bought = models.DateTimeField() # date the investment was made
    sold = models.DateTimeField() # date the investment was sold
    amount = models.DecimalField() # number of coins held

You could then add a property to Portfolio:

class Portfolio(models.Model):
    name = models.CharField(max_length=250)

    @property
    def coins(self):
        return Investment.objects.filter(portfolio=self)

In this way you can not only keep track of which portfolio holds which coins, buy also the entire historical positions too.

Upvotes: 4

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