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Create An Object That Has A ForeignKey With Another Object

- 1 answer

Assume that I have two models:

class Category(models.Model):
    title = models.CharField(max_length=255)


class Product(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    title = models.CharField(max_length=255)

What I wanna accomplish is creating a Product object.

Assume I'll receive category_id from the POST request

>>> category = Category.objects.get(id=category_id)

>>> Product.objects.create(category=category, title='Send the category object')

>>> Product.objects.create(category_id=category_id, title='Send only category id')

As you can see, there are two options, the first is to send category instance to create() method and the second is to send category_id, so my question is which approach is better for performance?

I know that I need to check if that category exists in the DB or not but this is not the case that I'm talking about in the question.

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Answer

If you have the primary key (category_id) there is no need to fetch the category first, you thus can use:

Product.objects.create(category_id=category_id, title='Send only category id')

which will thus only make one query: a query to create the Product, and thus will avoid querying for the category.

The two .create(…)s will make exactly the same query since Django will simply retrieve the .id of the category object, and make the query with that id.

I know that I need to check if that category exists in the DB or not but this is not the case that I'm talking about in the question.

No, if one would make a request with a category_id for which there is no record for the table behind the Category model, it will raise an IntegrityError: the database will normally ensure that ForeignKeys always point to valid items, so the Category.objects.get(id=category_id) is not necessary: if you are only fetching it to fetch the category, you can omit that.

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source: stackoverflow.com
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