Django ForeignKey Error: How to Assign a Doctor Instance via Dropdown?

Hi all,
I have a Django model setup where each Patient is linked to a Doctor using a ForeignKey. My models are defined as follows:

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from django.contrib.auth.models import User
class Doctor(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)

class Patient(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    assignedDoctorId = models.ForeignKey(Doctor, on_delete=models.CASCADE)

In the UI, I've implemented a dropdown menu for selecting a doctor for a patient using a ModelChoiceField:

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assignedDoctorId = forms.ModelChoiceField(queryset=models.Doctor.objects.all().filter(status=True), empty_label="Name and Department", to_field_name="user_id")

However, when I select a doctor from the dropdown, I receive the following error:

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ValueError: Cannot assign "'2'": "Patient.assignedDoctorId" must be a "Doctor" instance.

I intended to use a ForeignKey to associate each patient with a doctor. What am I doing wrong?