Django 1.11中外键为何需默认值？迁移遇非空报错

Let me break down why you're seeing this error and how to fix it—it's a common gotcha when working with non-nullable ForeignKeys in Django.

Why the default value is required #

Django asks for a default value because if your Credit table already exists in the database, there are existing records that don't have a user value set, and you're making the user field non-nullable. When running makemigrations, Django needs to populate this field for all existing rows to satisfy the database's non-null constraint—it can't leave those fields empty.

This has nothing to do with your goal of associating new Credit instances with the current active user; it's purely a database consistency requirement for existing data.

How to resolve this #

You have a few options depending on whether you already have Credit records in your database:

Option 1: No existing Credit records yet #

If you haven't created any Credit entries and this is the first time adding this field, double-check your migration history. If the Credit table doesn't exist yet, Django won't ask for a default value because there's no data to populate. Just run makemigrations and migrate normally.

Option 2: You have existing Credit records #

Sub-option A: Use a temporary default value

Pick an existing user (like your superuser) to assign to all existing Credit records temporarily:

1. Update your Credit model's user field to include a default (replace 1 with the ID of a valid user in your database):

   user = models.ForeignKey(settings.AUTH_USER_MODEL, default=1)
   

2. Run makemigrations and migrate—this will populate all existing Credit rows with the user ID you specified.
3. Once the migration is done, remove the default argument from the field:

   user = models.ForeignKey(settings.AUTH_USER_MODEL)
   

4. Run makemigrations and migrate again to remove the default constraint from the database.

Sub-option B: Temporarily allow null values (then backfill)

If you don't want to assign a random user to existing records, you can temporarily make the field nullable:

1. Update the user field to allow nulls:

   user = models.ForeignKey(settings.AUTH_USER_MODEL, null=True)
   

2. Run makemigrations and migrate—this adds the field without enforcing non-null yet.
3. Manually backfill the user field for all existing Credit records (you can do this via the Django admin, a management command, or the shell).
4. Once all records have a user value, remove the null=True argument:

   user = models.ForeignKey(settings.AUTH_USER_MODEL)
   

5. Run makemigrations and migrate again to enforce the non-null constraint.

Associating new Credit instances with the current active user #

As for your original goal of linking new Credit entries to the current logged-in user, that's handled in your view logic, not the model's default. When creating a Credit instance, use request.user (assuming you're in a view with access to the request):

from django.shortcuts import render, redirect
from .models import Credit

def add_credit(request):
    if request.method == 'POST':
        # Create Credit instance linked to current user
        Credit.objects.create(
            user=request.user,
            name=request.POST['name'],
            payment_reference=request.POST['payment_reference'],
            amount=request.POST['amount'],
            # Fill in other required fields
        )
        return redirect('credit_list')
    return render(request, 'add_credit.html')

This way, every new Credit you create will automatically be tied to the active user, and the migration-time default was just a one-time fix for existing data.

内容的提问来源于stack exchange，提问作者Phillip Smith