How can I fix "ValueError: "<Klant: None - aCustomer>" needs to have a value for field "klant" before this many-to-many relationship can be used."

Here are the imports:

from django.db import models
from datetime import datetime, timedelta
from django.contrib.auth.models import User

This is the first class I defined. It is the status of the action (Actie) and it has a status-id and a status-name with a max_length attribute of 5 (todo, doing, done)

class Status(models.Model):
    id = models.IntegerField(primary_key=True)
    status_naam = models.CharField(max_length=5, default='todo')

    def __str__(self):
        return str(self.id) + " - " + self.status_naam

This is the second class Klant which means customer. It has an id, a customer-name and users from the customer which is a ManyToManyField referring to users from the the User-table django gives me.

class Klant(models.Model):
    id = models.IntegerField(primary_key=True)
    klant_naam = models.CharField(max_length=100, default='-')
    klant_gebruiker = models.ManyToManyField(User)

    def __str__(self):
        return str(self.id) + " - " + self.klant_naam

This is the class Actie (Action or the action the user determines) which has an id, an action-name, a action-status which refers to the table Status here above, an action-publish-date, an ending-date (the deadline) and a customer-id which refers to Klant.

class Actie(models.Model):
    id = models.IntegerField(primary_key=True)
    actie_naam = models.CharField(max_length=150, default='-')
    actie_status = models.ForeignKey(Status, default=1)
    actie_aanmaakdatum = models.DateTimeField(default=datetime.now())
    actie_einddatum = models.DateTimeField(default=datetime.now() + timedelta(days=1))
    actie_klant = models.ForeignKey(Klant, default=1)

    def __str__(self):
        return str(self.id) + " - " + self.actie_naam

This is what I'm doing in the shell:

(InteractiveConsole)
    >>> from MyApp.models import User, Klant
    >>> klant1 = Klant.objects.create(klant_naam='aCustomer')
    >>> user1 = User.objects.get(username='peterdevries')
    >>> klant1.klant_gebruiker.add(user1)
    Traceback (most recent call last):
      File "C:\shell.py", line 69, in handle
        self.run_shell(shell=options['interface'])
      File "C:\shell.py", line 61, in run_shell
        raise ImportError
    ImportError
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File "C:\related_descriptors.py", line 468, in __get__
        return self.related_manager_cls(instance)
      File "C:\related_descriptors.py", line 751, in __init__
        (instance, self.source_field_name))
    ValueError: "<Klant: None - aCustomer>" needs to have a value for field "klant" before this many-to-many relationship can be used.
    >>>

So my question now is what do I have to do to fix this ValueError?

Your models look like they were autogenerated via inspectdb. The problem is that all your primary keys are IntegerFields. This means that Django does not know that the database will give them a value automatically, so does not update the field on creation with that value - and therefore can't create the many-to-many relation, which requires inserting pks from both sides into the linking table.

You could change the fields to AutoField, but the simplest solution is just to delete those definitions altogether - Django will automatically set the primary key to an AutoField named id if another pk is not defined.