Foreign key relation without pk

[JasperAlexander]

How to make a foreign key relation using a field other than the primary key with a post request? For example, if you have users and items, how to make a new item with a relation to an existing user with only the username of the user, not the id (primary key)?

models.py

from django.db import models

class User(models.Model):
    username = models.CharField(max_length=100)

class Item(models.Model):
    name = models.CharField(max_length=256)
    owner = models.ForeignKey(User, null=True, on_delete=models.SET_NULL)

serializers.py

from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):
    id = serializers.CharField(read_only=True)
    username = serializers.CharField(read_only=True)

    class Meta:
        model = User
        fields = ["username"]

class ItemSerializer(serializers.ModelSerializer):
    owner = UserSerializer(read_only=True)

    class Meta:
        model = Item
        fields = ["name", "owner"]

Adding the following code to the ItemSerializer seems to allow me to add a new item with a relation to an existing user by providing the id (primary key) of a user in a post request. Changing owner_id to owner_username gives me a TypeError: 'Item() got unexpected keyword arguments: 'owner_username''.

def create(self, validated_data):
        request = self.context['request']
        validated_data['owner_id'] = request.data.get('owner')

        instance = super().create(validated_data)

        return instance