add django check and dependency on modeladmin

The UI for wagtail-sharing depends on wagtail.contrib.modeladmin, but
this is an optional package that is not part of the standard Wagtail
installation. This commit adds a note to the README about this and also
adds a Django check that will fail if wagtail.contrib.modeladmin isn't
in INSTALLED_APPS.

As a side effect this change also adds a custom AppConfig for this app.

CHANGELOG.md

@@ -6,6 +6,7 @@ We follow the [Semantic Versioning 2.0.0](http://semver.org/) format.
 - Simplified CHANGELOG format.
 - Removed superfluous tests of `Page.get_admin_display_title`.
 - Add additional tests for the `before_serve_shared_page` and `after_serve_shared_page` hooks.
+- Added Django check and note in README to clarify dependency on `wagtail.contrib.modeladmin` app.
 
 
 ## 0.6 - 2017-11-27

README.rst

@@ -42,6 +42,8 @@ Add ``wagtailsharing`` as an installed app in your Django settings:
       ...
   )
 
+``wagtail.contrib.modeladmin`` is also required and must be included in your list of installed apps.
+
 Run migrations to create required database tables:
 
 .. code-block:: bash

wagtailsharing/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'wagtailsharing.apps.WagtailSharingAppConfig'

wagtailsharing/apps.py

@@ -0,0 +1,10 @@
+from django.apps import AppConfig
+from django.utils.translation import ugettext_lazy as _
+
+from . import checks  # noqa F401
+
+
+class WagtailSharingAppConfig(AppConfig):
+    name = 'wagtailsharing'
+    label = 'wagtailsharing'
+    verbose_name = _("Wagtail Sharing")

wagtailsharing/checks.py

@@ -0,0 +1,23 @@
+from django.apps import apps
+from django.core.checks import Error, register
+
+
+@register()
+def modeladmin_installed_check(app_configs, **kwargs):
+    errors = []
+
+    MODELADMIN_APP = 'wagtail.contrib.modeladmin'
+    if not apps.is_installed(MODELADMIN_APP):
+        error_hint = (
+            "Is '{}' in settings.INSTALLED_APPS?".format(MODELADMIN_APP)
+        )
+
+        errors.append(
+            Error(
+                "wagtail-sharing requires the Wagtail ModelAdmin app.",
+                hint=error_hint,
+                id='wagtailsharing.E001'
+            )
+        )
+
+    return errors

wagtailsharing/tests/test_checks.py

@@ -0,0 +1,21 @@
+from django.apps import apps
+from django.core.checks import Error
+from django.test import SimpleTestCase, override_settings
+
+from wagtailsharing.checks import modeladmin_installed_check
+
+
+class TestModelAdminInstalledCheck(SimpleTestCase):
+    @override_settings(INSTALLED_APPS=[
+        'wagtail.contrib.modeladmin',
+        'wagtailsharing',
+    ])
+    def test_check_passes_if_modeladmin_installed(self):
+        self.assertFalse(modeladmin_installed_check(apps.get_app_configs()))
+
+    @override_settings(INSTALLED_APPS=['wagtailsharing'])
+    def test_check_fails_if_modeladmin_not_installed(self):
+        errors = modeladmin_installed_check(apps.get_app_configs())
+        self.assertEqual(len(errors), 1)
+        self.assertIsInstance(errors[0], Error)
+        self.assertEqual(errors[0].id, 'wagtailsharing.E001')