Django form not working for ManyToMany (auth.users) [duplicate] - django

Question

This question already has an answer here:
Why doesn't save() automatically call save_m2m()?
(1 answer)
Closed 6 years ago.
I'm trying to create a Debt model using Django. It has a debtors field which link a debt to many users.
It behaves the way I want through Django's administration panel, but I can't get it to work using a django form. I can create debts but the the debtors field is emptying, despite the browser showing an accurate list of users to choose from.
What's even more surprising is that I thought that using blank=False in the model definition shouldn't allow that.
models.py:
from django.db import models
from django.utils import timezone
class Debt(models.Model):
author = models.ForeignKey('auth.User', related_name='author')
name = models.CharField(max_length=200)
amount = models.FloatField()
debtors = models.ManyToManyField('auth.User', blank=False)
invoice = models.FileField(blank=True)
created_date = models.DateTimeField(default=timezone.now)
def __str__(self):
return self.name
forms.py:
from django import forms
from .models import Debt
class NewDebtForm(forms.ModelForm):
class Meta:
model = Debt
fields = ('name', 'amount', 'invoice', 'created_date', 'debtors')
views.py:
from django.shortcuts import render, redirect
from .models import Debt
from .forms import NewDebtForm
def debt_list(request):
if request.method == "POST":
form = NewDebtForm(request.POST)
if form.is_valid():
debt = form.save(commit=False)
debt.author = request.user
debt.save()
debts = Debt.objects.order_by('created_date')
form = NewDebtForm()
return render(request, 'debt_share/debt_list.html',
{'debts': debts,
'form': form})
debt_list.html:
{% extends 'debt_share/base.html' %}
{% block content %}
{% for debt in debts %}
<h1>{{ debt.name }}</h1>
<h2>payé par {{ debt.author }}</h2>
<h2>{{ debt.amount }} {{ debt.invoice }}</h2>
<p>
Concerne
{% for user in debt.debtors.all %}
{{ user }}
{% endfor %}
</p>
{% endfor %}
<form method="POST">{% csrf_token %}
{{ form.as_p }}
<button type="submit">Save</button>
</form>
{% endblock content %}

Answer 1

When using ModelForms for many-to-many relationships, and use form.save(commit=False), you have to also call form.save_m2m(). Many-to-many relationships have to be saved in a second step, after the original object has been created; otherwise, its ID would not be known.
For more details, refer to the explanation in the docs: https://docs.djangoproject.com/en/1.10/topics/forms/modelforms/#the-save-method