Often people add an _id suffix to the ForeignKeys or OneToOneFields in their models, for example:

from django.db import models

class MyModel(models.Model):
    other_model_id = models.ForeignKey(OtherModel, on_delete=models.CASCADE)

Why is it a problem?

The main reason why it is a problem is because the .other_model itself does not store the id. Indeed, Django makes an implicit twin-field with an _id suffix that stores the primary key (or another unique column if you specify that) to the target model. The ForeignKey itself is designed to (lazily) load the related model.

It furthermore means that the twin-field Django has constructed is now named other_model_id_id, which is also the default name of the column where it stores the primary key. This name is rather "ugly".

What can be done to resolve the problem?

Renaming the field without the _id suffix, so change it to:

from django.db import models

class MyModel(models.Model):
    other_model = models.ForeignKey(OtherModel, on_delete=models.CASCADE)

You will need to run makemigrations to rename the field. The Django makemigrations tool will likely detect that you renamed the field and ask:

Did you rename mymodel.other_model_id to activity.other_model (a ForeignKey)? [y/N]

A question that should be answered with yes.