Django - How to prepopulate admin form fields

Question

I know that you can prepopulate admin form fields based on other fields. For example, I have a slug field that is automatically populated based on the title field.

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Answer 1

I tried a few of these answers and none of them worked. I simply wanted to prepulate a field with another field from a related model. Taking this answer as a starting point, I finally tried to manipulate the model instance object (here obj) directly and it worked for me.

class MyModelAdmin(models.ModelAdmin):

    def get_form(self, request, obj=None, **kwargs):
        form = super(MyModelAdmin, self).get_form(request, obj, **kwargs)
        if not obj.some_model_field:
            obj.some_model_field = obj.related_model.prepopulating_model_field

        return form