diff --git a/kfet/auth/fields.py b/kfet/auth/fields.py
index 28ba1c9e..045667a8 100644
--- a/kfet/auth/fields.py
+++ b/kfet/auth/fields.py
@@ -1,20 +1,26 @@
 from django import forms
-from django.contrib.auth.models import Permission
-from django.contrib.contenttypes.models import ContentType
 from django.forms import widgets
 
+from .models import Group, Permission
 
-class KFetPermissionsField(forms.ModelMultipleChoiceField):
 
-    def __init__(self, *args, **kwargs):
-        queryset = Permission.objects.filter(
-            content_type__in=ContentType.objects.filter(app_label="kfet"),
-        )
-        super().__init__(
-            queryset=queryset,
-            widget=widgets.CheckboxSelectMultiple,
-            *args, **kwargs
-        )
+class GroupsField(forms.ModelMultipleChoiceField):
+    def __init__(self, **kwargs):
+        kwargs.setdefault('queryset', Group.objects.all())
+        kwargs.setdefault('widget', widgets.CheckboxSelectMultiple)
+        super().__init__(**kwargs)
+
+
+class BasePermissionsField(forms.ModelMultipleChoiceField):
+    def __init__(self, **kwargs):
+        kwargs.setdefault('widget', widgets.CheckboxSelectMultiple)
+        super().__init__(**kwargs)
 
     def label_from_instance(self, obj):
         return obj.name
+
+
+class CorePermissionsField(BasePermissionsField):
+    def __init__(self, **kwargs):
+        kwargs.setdefault('queryset', Permission.kfetcore.all())
+        super().__init__(**kwargs)
diff --git a/kfet/auth/forms.py b/kfet/auth/forms.py
index 0c9fa53b..0de47664 100644
--- a/kfet/auth/forms.py
+++ b/kfet/auth/forms.py
@@ -1,43 +1,28 @@
 from django import forms
-from django.contrib.auth.models import Group, User
+from django.contrib.auth.models import User
+from django.utils.translation import ugettext_lazy as _
 
-from .fields import KFetPermissionsField
+from utils.forms import KeepUnselectableModelFormMixin
 
 
-class GroupForm(forms.ModelForm):
-    permissions = KFetPermissionsField()
+from .fields import GroupsField, CorePermissionsField
+from .models import Group
 
-    def clean_name(self):
-        name = self.cleaned_data['name']
-        return 'K-Fêt %s' % name
 
-    def clean_permissions(self):
-        kfet_perms = self.cleaned_data['permissions']
-        # TODO: With Django >=1.11, the QuerySet method 'difference' can be
-        # used.
-        # other_groups = self.instance.permissions.difference(
-        #     self.fields['permissions'].queryset
-        # )
-        other_perms = self.instance.permissions.exclude(
-            pk__in=[p.pk for p in self.fields['permissions'].queryset],
-        )
-        return list(kfet_perms) + list(other_perms)
+class GroupForm(KeepUnselectableModelFormMixin, forms.ModelForm):
+    permissions = CorePermissionsField(label=_("Permissions"), required=False)
+
+    keep_unselectable_fields = ['permissions']
 
     class Meta:
         model = Group
         fields = ['name', 'permissions']
 
 
-class UserGroupForm(forms.ModelForm):
-    groups = forms.ModelMultipleChoiceField(
-            Group.objects.filter(name__icontains='K-Fêt'),
-            label='Statut équipe',
-            required=False)
+class UserGroupForm(KeepUnselectableModelFormMixin, forms.ModelForm):
+    groups = GroupsField(label=_("Statut équipe"), required=False)
 
-    def clean_groups(self):
-        kfet_groups = self.cleaned_data.get('groups')
-        other_groups = self.instance.groups.exclude(name__icontains='K-Fêt')
-        return list(kfet_groups) + list(other_groups)
+    keep_unselectable_fields = ['groups']
 
     class Meta:
         model = User
diff --git a/kfet/auth/migrations/0001_initial.py b/kfet/auth/migrations/0001_initial.py
index 061570a8..036a30c6 100644
--- a/kfet/auth/migrations/0001_initial.py
+++ b/kfet/auth/migrations/0001_initial.py
@@ -21,4 +21,26 @@ class Migration(migrations.Migration):
                 ('token', models.CharField(unique=True, max_length=50)),
             ],
         ),
+        migrations.CreateModel(
+            name='Group',
+            fields=[
+                ('group_ptr', models.OneToOneField(parent_link=True, serialize=False, primary_key=True, auto_created=True, to='auth.Group')),
+            ],
+            options={
+                'verbose_name': 'Groupe',
+                'verbose_name_plural': 'Groupes',
+            },
+            bases=('auth.group',),
+        ),
+        migrations.CreateModel(
+            name='Permission',
+            fields=[
+            ],
+            options={
+                'verbose_name': 'Permission',
+                'verbose_name_plural': 'Permissions',
+                'proxy': True,
+            },
+            bases=('auth.permission',),
+        ),
     ]
diff --git a/kfet/auth/migrations/0002_existing_groups.py b/kfet/auth/migrations/0002_existing_groups.py
new file mode 100644
index 00000000..f816429e
--- /dev/null
+++ b/kfet/auth/migrations/0002_existing_groups.py
@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+def existing_groups(apps, schema_editor):
+    Group = apps.get_model('auth', 'Group')
+    KFetGroup = apps.get_model('kfetauth', 'Group')
+
+    for group in Group.objects.filter(name__icontains='K-Fêt'):
+        kf_group = KFetGroup(group_ptr=group, pk=group.pk, name=group.name)
+        kf_group.save()
+
+
+class Migration(migrations.Migration):
+    """
+    Data migration.
+
+    Previously, K-Fêt groups were identified with the presence of 'K-Fêt' in
+    their name.
+    """
+
+    dependencies = [
+        ('kfetauth', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunPython(existing_groups),
+    ]
diff --git a/kfet/auth/models.py b/kfet/auth/models.py
index ecd40091..0dfd26fd 100644
--- a/kfet/auth/models.py
+++ b/kfet/auth/models.py
@@ -1,5 +1,9 @@
+from django.contrib.auth.models import (
+    Group as DjangoGroup, Permission as DjangoPermission,
+)
 from django.db import models
 from django.utils.crypto import get_random_string
+from django.utils.translation import ugettext_lazy as _
 
 
 class GenericTeamTokenManager(models.Manager):
@@ -15,3 +19,37 @@ class GenericTeamToken(models.Model):
     token = models.CharField(max_length=50, unique=True)
 
     objects = GenericTeamTokenManager()
+
+
+class Group(DjangoGroup):
+
+    def give_admin_access(self):
+        perms = Permission.kfetcore.all()
+        self.permissions.add(*perms)
+
+    def give_staff_access(self):
+        perms = Permission.kfetcore.filter(
+            codename__in=['is_team', 'perform_deposit'])
+        self.permissions.add(*perms)
+
+    class Meta:
+        verbose_name = _("Groupe")
+        verbose_name_plural = _("Groupes")
+
+
+KFET_CORE_APP_LABELS = ['kfet', 'kfetauth']
+
+
+class CorePermissionManager(models.Manager):
+    def get_queryset(self):
+        return super().get_queryset().filter(
+            content_type__app_label__in=KFET_CORE_APP_LABELS)
+
+
+class Permission(DjangoPermission):
+    kfetcore = CorePermissionManager()
+
+    class Meta:
+        proxy = True
+        verbose_name = _("Permission")
+        verbose_name_plural = _("Permissions")
diff --git a/kfet/auth/tests.py b/kfet/auth/tests.py
index c2f183cd..d207ced6 100644
--- a/kfet/auth/tests.py
+++ b/kfet/auth/tests.py
@@ -1,76 +1,194 @@
 # -*- coding: utf-8 -*-
 from unittest import mock
 
+from django.contrib.auth.models import (
+    AnonymousUser, Group as DjangoGroup, User,
+)
+from django.contrib.contenttypes.models import ContentType
 from django.core import signing
 from django.core.urlresolvers import reverse
-from django.contrib.auth.models import AnonymousUser, Group, Permission, User
 from django.test import RequestFactory, TestCase
 
-from kfet.forms import UserGroupForm
 from kfet.models import Account
 
 from . import KFET_GENERIC_TRIGRAMME, KFET_GENERIC_USERNAME
 from .backends import AccountBackend, GenericBackend
+from .fields import GroupsField, CorePermissionsField
+from .forms import GroupForm, UserGroupForm
 from .middleware import TemporaryAuthMiddleware
-from .models import GenericTeamToken
+from .models import GenericTeamToken, Group, Permission
 from .utils import get_kfet_generic_user
 from .views import GenericLoginView
 
 
 ##
-# Forms
+# Forms and form fields
 ##
 
+
+class CorePermissionsFieldTests(TestCase):
+
+    def setUp(self):
+        kf_ct = ContentType.objects.get_for_model(GenericTeamToken)
+        self.kf_perm1 = Permission.objects.create(
+            content_type=kf_ct, codename='code1')
+        self.kf_perm2 = Permission.objects.create(
+            content_type=kf_ct, codename='code2')
+        self.kf_perm3 = Permission.objects.create(
+            content_type=kf_ct, codename='code3')
+
+        ot_ct = ContentType.objects.get_for_model(Permission)
+        self.ot_perm = Permission.objects.create(
+            content_type=ot_ct, codename='code')
+
+    def test_choices(self):
+        """
+        Only K-Fêt permissions are selectable.
+        """
+        field = CorePermissionsField()
+        for p in [self.kf_perm1, self.kf_perm2, self.kf_perm3]:
+            self.assertIn(p, field.queryset)
+        self.assertNotIn(self.ot_perm, field.queryset)
+
+
+class GroupsFieldTests(TestCase):
+
+    def setUp(self):
+        self.kf_group1 = Group.objects.create(name='KF Group1')
+        self.kf_group2 = Group.objects.create(name='KF Group2')
+        self.kf_group3 = Group.objects.create(name='KF Group3')
+        self.ot_group = DjangoGroup.objects.create(name='OT Group')
+
+    def test_choices(self):
+        """
+        Only K-Fêt groups are selectable.
+        """
+        field = GroupsField()
+        self.assertQuerysetEqual(
+            field.queryset,
+            map(repr, [self.kf_group1, self.kf_group2, self.kf_group3]),
+            ordered=False,
+        )
+
+
+class GroupFormTests(TestCase):
+
+    def setUp(self):
+        self.user = User.objects.create(username='user')
+        self.kf_group = Group.objects.create(name='A Group')
+
+        self.kf_perm1 = Permission.kfetcore.get(codename='is_team')
+        self.kf_perm2 = Permission.kfetcore.get(codename='add_checkout')
+
+        ot_ct = ContentType.objects.get_for_model(Permission)
+        self.ot_perm = Permission.objects.create(
+            content_type=ot_ct, codename='cool')
+
+    def test_creation(self):
+        data = {
+            'name': 'Another Group',
+            'permissions': [self.kf_perm1.pk],
+        }
+        form = GroupForm(data)
+        instance = form.save()
+
+        self.assertEqual(instance.name, 'Another Group')
+        self.assertQuerysetEqual(
+            instance.permissions.all(), map(repr, [self.kf_perm1]))
+
+    def test_keep_others(self):
+        """
+        Non-kfet permissions of Group are kept when the form is submitted.
+        Regression test for #168.
+        """
+        self.kf_group.permissions.add(self.ot_perm)
+
+        selected = [self.kf_perm1, self.kf_perm2]
+
+        data = {
+            'name': 'A Group',
+            'permissions': [str(p.pk) for p in selected],
+        }
+
+        form = GroupForm(data, instance=self.kf_group)
+        form.save()
+
+        self.assertQuerysetEqual(
+            self.kf_group.permissions.all(),
+            map(repr, [self.ot_perm] + selected),
+            ordered=False,
+        )
+
+
 class UserGroupFormTests(TestCase):
-    """Test suite for UserGroupForm."""
 
     def setUp(self):
         # create user
         self.user = User.objects.create(username="foo", password="foo")
 
         # create some K-Fêt groups
-        prefix_name = "K-Fêt "
-        names = ["Group 1", "Group 2", "Group 3"]
-        self.kfet_groups = [
-            Group.objects.create(name=prefix_name+name)
-            for name in names
-        ]
+        self.kf_group1 = Group.objects.create(name='KF Group1')
+        self.kf_group2 = Group.objects.create(name='KF Group2')
+        self.kf_group3 = Group.objects.create(name='KF Group3')
 
         # create a non-K-Fêt group
-        self.other_group = Group.objects.create(name="Other group")
-
-    def test_choices(self):
-        """Only K-Fêt groups are selectable."""
-        form = UserGroupForm(instance=self.user)
-        groups_field = form.fields['groups']
-        self.assertQuerysetEqual(
-            groups_field.queryset,
-            [repr(g) for g in self.kfet_groups],
-            ordered=False,
-        )
+        self.ot_group = DjangoGroup.objects.create(name="OT Group")
 
     def test_keep_others(self):
-        """User stays in its non-K-Fêt groups."""
-        user = self.user
-
+        """
+        User stays in its non-K-Fêt groups.
+        Regression test for #161.
+        """
         # add user to a non-K-Fêt group
-        user.groups.add(self.other_group)
+        self.user.groups.add(self.ot_group)
 
         # add user to some K-Fêt groups through UserGroupForm
-        data = {
-            'groups': [group.pk for group in self.kfet_groups],
-        }
-        form = UserGroupForm(data, instance=user)
-
-        form.is_valid()
+        selected = [self.kf_group1, self.kf_group2]
+        data = {'groups': [str(g.pk) for g in selected]}
+        form = UserGroupForm(data, instance=self.user)
         form.save()
+
+        transform = lambda g: g.pk
         self.assertQuerysetEqual(
-            user.groups.all(),
-            [repr(g) for g in [self.other_group] + self.kfet_groups],
-            ordered=False,
+            self.user.groups.all(),
+            map(transform, [self.ot_group] + selected),
+            ordered=False, transform=transform,
         )
 
 
+##
+# Models
+##
+
+class PermissionTests(TestCase):
+
+    def test_manager_kfet(self):
+        """
+        'kfet' manager only returns K-Fêt permissions.
+        """
+        kf_ct = ContentType.objects.get_for_model(GenericTeamToken)
+        kf_perm1 = Permission.objects.create(
+            content_type=kf_ct, codename='code1')
+        kf_perm2 = Permission.objects.create(
+            content_type=kf_ct, codename='code2')
+        kf_perm3 = Permission.objects.create(
+            content_type=kf_ct, codename='code3')
+
+        self.assertEqual(Permission.kfetcore.get(codename='code1'), kf_perm1)
+        self.assertEqual(Permission.kfetcore.get(codename='code2'), kf_perm2)
+        self.assertEqual(Permission.kfetcore.get(codename='code3'), kf_perm3)
+
+        ot_ct = ContentType.objects.get_for_model(Permission)
+        Permission.objects.create(content_type=ot_ct, codename='code')
+
+        with self.assertRaises(Permission.DoesNotExist):
+            Permission.kfetcore.get(codename='code')
+
+
+##
+# K-Fêt generic user object
+##
+
 class KFetGenericUserTests(TestCase):
 
     def test_exists(self):
diff --git a/kfet/auth/views.py b/kfet/auth/views.py
index 7b9f4099..073c558f 100644
--- a/kfet/auth/views.py
+++ b/kfet/auth/views.py
@@ -2,7 +2,7 @@ from django.contrib import messages
 from django.contrib.messages.views import SuccessMessageMixin
 from django.contrib.auth import authenticate, login
 from django.contrib.auth.decorators import permission_required
-from django.contrib.auth.models import Group, User
+from django.contrib.auth.models import User
 from django.contrib.auth.views import redirect_to_login
 from django.core.urlresolvers import reverse, reverse_lazy
 from django.db.models import Prefetch
@@ -15,7 +15,7 @@ from django.views.decorators.http import require_http_methods
 from django.views.generic.edit import CreateView, UpdateView
 
 from .forms import GroupForm
-from .models import GenericTeamToken
+from .models import GenericTeamToken, Group
 
 
 class GenericLoginView(View):
@@ -112,7 +112,6 @@ def account_group(request):
     )
     groups = (
         Group.objects
-        .filter(name__icontains='K-Fêt')
         .prefetch_related('permissions', user_pre)
     )
     return render(request, 'kfet/account_group.html', {
@@ -120,17 +119,23 @@ def account_group(request):
     })
 
 
-class AccountGroupCreate(SuccessMessageMixin, CreateView):
+class BaseAccountGroupFormViewMixin:
     model = Group
-    template_name = 'kfet/account_group_form.html'
     form_class = GroupForm
+    template_name = 'kfet/account_group_form.html'
+    success_url = reverse_lazy('kfet.account.group')
+
+
+class AccountGroupFormViewMixin(
+    SuccessMessageMixin,
+    BaseAccountGroupFormViewMixin,
+):
+    pass
+
+
+class AccountGroupCreate(AccountGroupFormViewMixin, CreateView):
     success_message = 'Nouveau groupe : %(name)s'
-    success_url = reverse_lazy('kfet.account.group')
 
 
-class AccountGroupUpdate(SuccessMessageMixin, UpdateView):
-    queryset = Group.objects.filter(name__icontains='K-Fêt')
-    template_name = 'kfet/account_group_form.html'
-    form_class = GroupForm
+class AccountGroupUpdate(AccountGroupFormViewMixin, UpdateView):
     success_message = 'Groupe modifié : %(name)s'
-    success_url = reverse_lazy('kfet.account.group')
diff --git a/kfet/management/commands/loadkfetdevdata.py b/kfet/management/commands/loadkfetdevdata.py
index 6dd25f29..332e7dfb 100644
--- a/kfet/management/commands/loadkfetdevdata.py
+++ b/kfet/management/commands/loadkfetdevdata.py
@@ -7,13 +7,15 @@ import random
 from datetime import timedelta
 
 from django.utils import timezone
-from django.contrib.auth.models import User, Group, Permission, ContentType
+from django.contrib.auth.models import User
 from django.core.management import call_command
 
 from gestioncof.management.base import MyBaseCommand
 from gestioncof.models import CofProfile
-from kfet.models import (Account, Checkout, CheckoutStatement, Supplier,
-                         SupplierArticle, Article)
+from kfet.models import (
+    Account, Article, Checkout, CheckoutStatement, Group, Supplier,
+    SupplierArticle, Article,
+)
 
 # Où sont stockés les fichiers json
 DATA_DIR = os.path.join(os.path.dirname(os.path.dirname(__file__)),
@@ -28,22 +30,13 @@ class Command(MyBaseCommand):
         # Groupes
         # ---
 
-        Group.objects.filter(name__icontains='K-Fêt').delete()
+        group_chef, _ = Group.objects.get_or_create(
+            name="K-Fêt César")
+        group_boy, _ = Group.objects.get_or_create(
+            name="K-Fêt Légionnaire")
 
-        group_chef = Group(name="K-Fêt César")
-        group_boy = Group(name="K-Fêt Légionnaire")
-
-        group_chef.save()
-        group_boy.save()
-
-        permissions_chef = Permission.objects.filter(
-                    content_type__in=ContentType.objects.filter(
-                        app_label='kfet'))
-        permissions_boy = Permission.objects.filter(
-                    codename__in=['is_team', 'perform_deposit'])
-
-        group_chef.permissions.add(*permissions_chef)
-        group_boy.permissions.add(*permissions_boy)
+        group_chef.give_admin_access()
+        group_boy.give_staff_access()
 
         # ---
         # Comptes
diff --git a/kfet/models.py b/kfet/models.py
index e547d248..b255a2e2 100644
--- a/kfet/models.py
+++ b/kfet/models.py
@@ -14,7 +14,7 @@ from datetime import date
 import re
 
 from .auth import KFET_GENERIC_TRIGRAMME
-from .auth.models import GenericTeamToken  # noqa
+from .auth.models import GenericTeamToken, Group, Permission  # noqa
 
 from .config import kfet_config
 from .utils import to_ukf
diff --git a/kfet/templates/kfet/account_group_form.html b/kfet/templates/kfet/account_group_form.html
index 90e3aa36..f0581b42 100644
--- a/kfet/templates/kfet/account_group_form.html
+++ b/kfet/templates/kfet/account_group_form.html
@@ -1,49 +1,10 @@
 {% extends 'kfet/base_form.html' %}
-{% load staticfiles %}
-{% load widget_tweaks %}
-
-{% block extra_head %}
-<link rel="stylesheet" text="text/css" href="{% static 'kfet/css/multiple-select.css' %}">
-<script src="{% static 'kfet/js/multiple-select.js' %}"></script>
-{% endblock %}
 
 {% block title %}Permissions - Édition{% endblock %}
 {% block header-title %}Modification des permissions{% endblock %}
 
 {% block main %}
 
-<form action="" method="post" class="form-horizontal">
-  {% csrf_token %}
-  <div class="form-group">
-    <label for="{{ form.name.id_for_label }}" class="col-sm-2 control-label">{{ form.name.label }}</label>
-    <div class="col-sm-10">
-      <div class="input-group">
-        <span class="input-group-addon">K-Fêt</span>
-        {{ form.name|add_class:"form-control" }}
-      </div>
-      {% if form.name.errors %}
-        <span class="help-block">{{ form.name.errors }}</span>
-      {% endif %}
-      {% if form.name.help_text %}
-        <span class="help-block">{{ form.name.help_text }}</span>
-      {% endif %}
-    </div>
-  </div>
-  {% include "kfet/form_field_snippet.html" with field=form.permissions %}
-  {% if not perms.kfet.manage_perms %}
-    {% include "kfet/form_authentication_snippet.html" %}
-  {% endif %}
-  {% include "kfet/form_submit_snippet.html" with value="Enregistrer" %}
-</form>
-
-<script type="text/javascript">
-$(document).ready(function() {
-    let $name_input = $("#id_name");
-    let raw_name = $name_input.val();
-    let prefix = "K-Fêt ";
-    if (raw_name.startsWith(prefix))
-        $name_input.val(raw_name.substring(prefix.length));
-});
-</script>
+{% include "kfet/form_full_snippet.html" with authz=perms.kfet.manage_perms submit_text="Enregistrer" %}
 
 {% endblock %}
diff --git a/kfet/templates/kfet/form_field_snippet.html b/kfet/templates/kfet/form_field_snippet.html
index a2aa087f..6a4efa16 100644
--- a/kfet/templates/kfet/form_field_snippet.html
+++ b/kfet/templates/kfet/form_field_snippet.html
@@ -5,7 +5,7 @@
   <div class="col-sm-10">
     {% if field|widget_type == "checkboxselectmultiple" %}
       <ul class="list-unstyled checkbox-select-multiple">
-        {% for choice in form.permissions %}
+        {% for choice in field %}
         <li class="col-sm-6 col-lg-4">
           <label for="{{ choice.id_for_label }}">
             {{ choice.tag }} {{ choice.choice_label }}
@@ -17,10 +17,10 @@
       {{ field|add_class:'form-control' }}
     {% endif %}
     {% if field.errors %}
-    <span class="help-block">{{field.errors}}</span>
+    <span class="help-block">{{ field.errors }}</span>
     {% endif %}
     {% if field.help_text %}
-    <span class="help-block">{{field.help_text}}</span>
+    <span class="help-block">{{ field.help_text }}</span>
     {% endif %}
   </div>
 </div>
diff --git a/setup.cfg b/setup.cfg
new file mode 100644
index 00000000..29425ce2
--- /dev/null
+++ b/setup.cfg
@@ -0,0 +1,3 @@
+[flake8]
+# E731: lambda expressions
+ignore = E731
diff --git a/utils/__init__.py b/utils/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/utils/forms.py b/utils/forms.py
new file mode 100644
index 00000000..367a6921
--- /dev/null
+++ b/utils/forms.py
@@ -0,0 +1,61 @@
+class KeepUnselectableModelFormMixin:
+    """
+    Keep unselectable items of queryset-based fields.
+
+    Mixin for 'ModelForm'.
+
+    Attribute
+        keep_unselectable_fields (list of field names)
+
+    This adding is performed in 'save' method. Specifically, if 'commit' arg
+    is False, it's done in 'save_m2m' method.
+
+    These fields must have a 'queryset' attribute (the selectable items), like
+    'ModelMultipleChoiceField' (default field for ManyToMany model fields).
+    """
+    keep_unselectable_fields = []
+
+    def get_unselectable(self, field_name):
+        """
+        Returns 'field_name' model field items of instance which can't be
+        selected with the corresponding form field.
+
+        Should be used before 'form.save' call, or 'form.save_m2m' call if
+        'commit=False' was passed as argument to 'form.save'.
+        """
+        if self.instance.pk:
+            previous = getattr(self.instance, field_name).all()
+            selectable = self.fields[field_name].queryset
+            return previous.exclude(pk__in=[o.pk for o in selectable])
+        else:
+            # Instance is being created, there is no previous item.
+            return []
+
+    def save(self, commit=True):
+        # Use 'commit=False' to get the 'save_m2m' method.
+        instance = super().save(commit=False)
+
+        _save_m2m = self.save_m2m
+
+        def save_m2m():
+            # Get the unselectable items.
+            # Force evaluate because those items are going to change.
+            unselectable_f = {
+                f_name: list(self.get_unselectable(f_name))
+                for f_name in self.keep_unselectable_fields
+            }
+            # Default 'save_m2m' use 'set' method of m2m relationships with
+            # fields' cleaned data.
+            _save_m2m()
+            # Add the unselectable elements.
+            for f_name, unselectable in unselectable_f.items():
+                getattr(instance, f_name).add(*unselectable)
+
+        # Implement the default behavior of 'save' method, with our own
+        # 'save_m2m'.
+        if commit:
+            instance.save()
+            save_m2m()
+        else:
+            self.save_m2m = save_m2m
+        return instance
diff --git a/utils/tests/__init__.py b/utils/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/utils/tests/test_forms.py b/utils/tests/test_forms.py
new file mode 100644
index 00000000..07fc951e
--- /dev/null
+++ b/utils/tests/test_forms.py
@@ -0,0 +1,131 @@
+from django import forms
+from django.contrib.auth import get_user_model
+from django.contrib.auth.models import Permission
+from django.contrib.contenttypes.models import ContentType
+from django.test import TestCase
+
+from utils.forms import KeepUnselectableModelFormMixin
+
+User = get_user_model()
+
+
+class KeepUnselectableModelFormMixinTests(TestCase):
+
+    class ExampleForm(KeepUnselectableModelFormMixin, forms.ModelForm):
+        user_permissions = forms.ModelMultipleChoiceField(
+            queryset=Permission.objects.filter(
+                codename__startswith='selec'),
+        )
+
+        keep_unselectable_fields = ['user_permissions']
+
+        class Meta:
+            model = User
+            fields = ('username', 'user_permissions')
+
+    def setUp(self):
+        ct = ContentType.objects.get_for_model(Permission)
+
+        self.unselec_perm1 = Permission.objects.create(
+            content_type=ct, codename='unselec_perm1')
+        self.unselec_perm2 = Permission.objects.create(
+            content_type=ct, codename='unselec_perm2')
+
+        # These two perms are the only selectable permissions from
+        # 'permissions' field of ExampleForm.
+        self.selec_perm1 = Permission.objects.create(
+            content_type=ct, codename='selec_perm1')
+        self.selec_perm2 = Permission.objects.create(
+            content_type=ct, codename='selec_perm2')
+
+    def test_creation(self):
+        """
+        The mixin functions properly when instance is being created.
+        """
+        data = {
+            'username': 'user',
+            'user_permissions': [self.selec_perm1.pk],
+        }
+        form = self.ExampleForm(data)
+
+        instance = form.save()
+
+        self.assertQuerysetEqual(
+            instance.user_permissions.all(),
+            map(repr, [self.selec_perm1]),
+        )
+
+    def test_creation_commit_false(self):
+        """
+        When instance is being created and 'save' method is called with
+        'commit=False', 'save_m2m' method functions properly.
+
+        https://docs.djangoproject.com/en/1.11/topics/forms/modelforms/#the-save-method
+        """
+        data = {
+            'username': 'user',
+            'user_permissions': [self.selec_perm1.pk],
+        }
+        form = self.ExampleForm(data)
+
+        instance = form.save(commit=False)
+
+        with self.assertRaises(User.DoesNotExist):
+            User.objects.get(username='user')
+
+        instance.save()
+        form.save_m2m()
+
+        self.assertQuerysetEqual(
+            instance.user_permissions.all(),
+            map(repr, [self.selec_perm1]),
+        )
+
+    def test_existing(self):
+        """
+        Unselectable items of an instance are kept.
+        """
+        instance = User.objects.create(username='user')
+        # Link instance with an unselectable and a selectable permissions.
+        instance.user_permissions.add(self.unselec_perm1, self.selec_perm2)
+
+        data = {
+            'username': 'user',
+            'user_permissions': [self.selec_perm1.pk],
+        }
+        form = self.ExampleForm(data, instance=instance)
+
+        instance = form.save()
+
+        self.assertQuerysetEqual(
+            instance.user_permissions.all(),
+            map(repr, [self.selec_perm1, self.unselec_perm1]),
+        )
+
+    def test_existing_commit_false(self):
+        """
+        When 'save' is called with 'commit=False', unselectable items of an
+        instance are kept by 'save_m2m'.
+        """
+        instance = User.objects.create(username='user')
+        # Link instance with an unselectable and a selectable permissions.
+        instance.user_permissions.add(self.unselec_perm1, self.selec_perm2)
+
+        data = {
+            'username': 'changed',
+            'user_permissions': [self.selec_perm1.pk],
+        }
+        form = self.ExampleForm(data, instance=instance)
+
+        instance = form.save(commit=False)
+
+        with self.assertRaises(User.DoesNotExist):
+            User.objects.get(username='changed')
+
+        instance.save()
+        form.save_m2m()
+
+        self.assertQuerysetEqual(
+            instance.user_permissions.all(),
+            map(repr, [self.selec_perm1, self.unselec_perm1]),
+        )