[Fixed]-Django: Cannot validate a form without an initial foreignkey value

If you are setting the patient in the view, then just leave the patient form out of the list of fields for you model form:

DiagnosisForm(forms.ModelForm):
    class Meta:
        model = Diagnosis
        fields = ('myfield1', 'myfield2', ...)

You can use exclude if you prefer:

DiagnosisForm(forms.ModelForm):
    class Meta:
        model = Diagnosis
        exclude = ('author', 'patient',)

The problem in your current form is that you have

    exclude = ['patient']
    exclude = ('author',)

The second exclude replaces the first. You should have:

    exclude = ['author', 'patient']

See the model form docs on selecting the fields to use for more info.