[#2374] Add tests to ensure no duplicate employments

When an Employment with no group is created, the group is set to 'Users'
in the `post_save` method.  But, if there is already a similar
Employment for that user, the `post_save` fails because there's a
uniqueness constraint on (user, organisation, group)

akvo/rsr/tests/rest/test_user.py

@@ -0,0 +1,75 @@
+# -*- coding: utf-8 -*-
+
+"""
+Akvo RSR is covered by the GNU Affero General Public License.
+
+See more details in the license.txt file located at the root folder of the Akvo RSR module.
+For additional details on the GNU license please see < http://www.gnu.org/licenses/agpl.html >.
+"""
+
+from __future__ import print_function
+
+from django.conf import settings
+from django.test import TransactionTestCase, Client
+
+from akvo.rsr.models import Employment, Organisation, User
+from akvo.utils import check_auth_groups
+
+
+# NOTE: Since these tests actually trigger some integrity errors and we want to
+# see if they are handled correctly, we use a TransactionTestCase instead of
+# TestCase, since wrapping the tests in a transaction isn't desirable.
+
+class UserTestCase(TransactionTestCase):
+    """Tests REST endpoints in views/user.py."""
+
+    def setUp(self):
+        check_auth_groups(settings.REQUIRED_AUTH_GROUPS)
+        self.c = Client(HTTP_HOST=settings.RSR_DOMAIN)
+        self.org = Organisation.objects.create(name='akvo', long_name='akvo foundation')
+        self.user_password = 'password'
+        self.user = self._create_user('abc@example.com', self.user_password)
+        self.c.login(username=self.user.username,
+                     password=self.user_password)
+
+    def test_request_organisation_once(self):
+        # Given
+        self.c.login(username=self.user.username,
+                     password=self.user_password)
+        data = {'organisation': self.org.id}
+        pk = self.user.id
+
+        # When
+        response = self.c.post(
+            '/rest/v1/user/{}/request_organisation/?format=json'.format(pk), data
+        )
+
+        # Then
+        self.assertEqual(response.status_code, 200)
+        employment = Employment.objects.get(user=self.user, organisation_id=self.org.id)
+        self.assertEqual(employment.group.name, 'Users')
+
+    def test_request_organisation_twice(self):
+        # Given
+        data = {'organisation': self.org.id}
+        pk = self.user.id
+        self.c.post('/rest/v1/user/{}/request_organisation/?format=json'.format(pk), data)
+
+        # When
+        response = self.c.post(
+            '/rest/v1/user/{}/request_organisation/?format=json'.format(pk), data
+        )
+
+        # Then
+        self.assertEqual(response.status_code, 409)
+        employment = Employment.objects.get(user=self.user, organisation_id=self.org.id)
+        self.assertEqual(employment.group.name, 'Users')
+
+    def _create_user(self, email, password, is_active=True):
+        """Create a user with the given email and password."""
+
+        user = User.objects.create(email=email, username=email, is_active=is_active)
+        user.set_password(password)
+        user.save()
+
+        return user