[Django]-Django Admin ManyToManyField

Hmm, I don’t think you want inlines here.

You want to be using the Django admin’s filter_horizontal:

https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.filter_horizontal

class ProfileAdmin(UserAdmin)
    filter_horizontal = ('opetest',)

That will give you the widget that you’re describing, used to add/remove Groups on the User Change page.

Ok, based on your edits, updated answer – basically, what we have is a UserProfile, linked to each user.

The UserProfile contains a m2m relationship to opetest – which we show in the admin with a filter_horizontal. End result is something like this:

models.py

from django.db import models
from django.contrib.auth.models import User

class opetest(models.Model):
    name = models.CharField(max_length=200)
    author = models.ForeignKey(User, related_name='author')
    description = models.TextField(u'Test description', help_text = u'Some words about quiz')
    pub_date = models.DateTimeField('date published', blank=False)
    #vacancies = models.ManyToManyField(Vacancy, blank=True)
    students = models.ManyToManyField(User, blank=True, related_name='opetests') #This field I want to edit on "User change page"
    estimate = models.IntegerField(default = 0, help_text = u'Estimate time in hours. \'0\' - unlimited')

class UserProfile(models.Model):
    user = models.OneToOneField(User, unique=True)
    ope = models.ManyToManyField(opetest)
    test_flag = models.BooleanField()

admin.py

from django.contrib import admin
from django.contrib.auth.admin import UserAdmin
from django.contrib.auth.models import User
from secondapp.models import UserProfile, opetest

admin.site.unregister(User)

class opetestAdmin(admin.ModelAdmin):
    pass

class UserProfileInline(admin.StackedInline):
    model = UserProfile
    filter_horizontal = ('ope',)

class CustomUserAdmin(UserAdmin):
    #filter_horizontal = ('user_permissions', 'groups', 'ope')
    save_on_top = True
    list_display = ('username', 'email', 'first_name', 'last_name', 'is_staff', 'last_login')
    inlines = [UserProfileInline]

admin.site.register(User, CustomUserAdmin)
admin.site.register(opetest, opetestAdmin)

Let me know if you have any questions, or need anything further.