Django admin interface: using horizontal_filter with ManyToMany field with intermediate table

Question

I am trying to enhance the django admin interface similar to what has been done in the accepted answer of this SO post. I have a many-to-many relationship between a User table and a Project table. In the django admin, I would like to be able to assign users to a project as in the image below:

It works fine with a simple ManyToManyField but the problem is that my model uses the through parameter of the ManyToManyField to use an intermediary table. I cannot use the save_m2m() and set() function and I am clueless on how to adapt the code below to make it work.

The model:

class UserProfile(models.Model):
    user = models.OneToOneField(User, unique=True)
    projects = models.ManyToManyField(Project, through='Membership')

class Project(models.Model):
    name = models.CharField(max_length=100, unique=True)
    application_identifier = models.CharField(max_length=100)
    type = models.IntegerField(choices=ProjectType)
    ...

class Membership(models.Model):
    project = models.ForeignKey(Project,on_delete=models.CASCADE)
    user = models.ForeignKey(UserProfile,on_delete=models.CASCADE)

    # extra fields
    rating = models.IntegerField(choices=ProjectType)
    ...

The code used for the widget in admin.py:

from django.contrib.admin.widgets import FilteredSelectMultiple

class ProjectAdminForm(forms.ModelForm):
    class Meta:
        model = Project
        fields = "__all__" # not in original SO post

    userprofiles = forms.ModelMultipleChoiceField(
        queryset=UserProfile.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name='User Profiles',
            is_stacked=False
        )
    )

    def __init__(self, *args, **kwargs):
        super(ProjectAdminForm, self).__init__(*args, **kwargs)
            if self.instance.pk:
                self.fields['userprofiles'].initial = self.instance.userprofile_set.all()

    def save(self, commit=True):
        project = super(ProjectAdminForm, self).save(commit=False)  
        if commit:
            project.save()

        if project.pk:
            project.userprofile_set = self.cleaned_data['userprofiles']
            self.save_m2m()

        return project

class ProjectAdmin(admin.ModelAdmin):
    form = ProjectAdminForm
    ...

Note: all the extra fields from the intermediary model do not need to be changed in the Project Admin view (they are automatically computed) and they all have a default value.

Thanks for your help!

Answer 1

I could find a way of solving this issue. The idea is:

1. Create new entries in the Membership table if and only if they are new (otherwise it would erase the existing data for the other fields in the Membership table)
2. Remove entries that were deselected from the Membership table

To do this, I replaced:

if project.pk:
    project.userprofile_set = self.cleaned_data['userprofiles']
    self.save_m2m()

By:

if project.pk:
    # Get the existing relationships
    current_project_selections = Membership.objects.filter(project=project)
    current_selections = [o.userprofile for o in current_project_selections]

    # Get the submitted relationships
    submitted_selections = self.cleaned_data['userprofiles']

    # Create new relation in Membership table if they do not exist
    for userprofile in submitted_selections :
        if userprofile not in current_selections:
            Membership(project=project,userprofile=userprofile).save()

    # Remove the relations that were deselected from the Membership table
    for project_userprofile in current_project_selections:
        if project_userprofile.userprofile not in submitted_selections :
            project_userprofile.delete()