Warn on potential configuration errors with multiple databases

* DEFAULT_DATABASE_URL as well as DATABASE_URL
* not_all_uppercase_DATABASE_URL

django12factor/__init__.py

@@ -74,12 +74,33 @@ def factorise(custom_settings=None):
         'default': dj_database_url.config(default='sqlite://:memory:')
     }
 
-    for (potential_database_url, value) in six.iteritems(os.environ):
+    for (key, value) in six.iteritems(os.environ):
         _SUFFIX = "_DATABASE_URL"
         _OFFSET = len(_SUFFIX)
-        if potential_database_url.endswith(_SUFFIX):
-            dbname = potential_database_url[:-_OFFSET].lower()
-            db = dj_database_url.parse(os.environ[potential_database_url])
+
+        if key.endswith(_SUFFIX):
+            prefix = key[:-_OFFSET]
+
+            if prefix != prefix.upper():
+                # i.e. it was not already all upper-cased
+                logger.warn(
+                    "Not parsing %s as a database url because the "
+                    "prefix (%s) was not all upper-case - django12factor "
+                    "will convert prefixes to lower-case for use as database "
+                    "names" % (key, prefix))
+                continue
+
+            dbname = key[:-_OFFSET].lower()
+
+            if dbname == "default" and 'DATABASE_URL' in os.environ:
+                logger.warn(
+                    "You have set the environment variables DATABASE_URL "
+                    "_and_ {key}, both of which would configure "
+                    "`DATABASES['default']`. {key} is being "
+                    "ignored.".format(key=key))
+                continue
+
+            db = dj_database_url.parse(os.environ[key])
             settings['DATABASES'][dbname] = db
 
     settings['DEBUG'] = getenv_bool('DEBUG')

tests/test_d12f.py

@@ -85,3 +85,42 @@ def test_multiple_db_support(self):
                 "variable %s" % (DBNAME, DB_URL_NAME)
             )
             self.assertIn('postgres', dbs[DBNAME]['ENGINE'])
+
+    def test_multiple_default_databases(self):
+        """
+        Ensure if DEFAULT_DATABASE_URL and DATABASE_URL are set, latter wins.
+        """
+
+        IGNORED_DB_NAME = "should_be_ignored"
+        DATABASE_URL = "postgres://username:password@host:1234/dbname"
+        IGNORED = "postgres://username:password@host:1234/%s" % IGNORED_DB_NAME
+
+        with debugenv(DATABASE_URL=DATABASE_URL, DEFAULT_DATABASE_URL=IGNORED):
+            default_db = d12f()['DATABASES']['default']
+            self.assertNotEquals(
+                default_db['NAME'],
+                IGNORED_DB_NAME,
+                "Parsed the contents of DEFAULT_DATABASE_URL instead of "
+                "ignoring it in favour of DATABASE_URL"
+            )
+
+    def test_non_capitalised_database_ignored(self):
+        """
+        Ensure "malformed" X_DATABASE_URLs aren't parsed.
+        """
+
+        e = {
+            'invalid_DATABASE_URL': "",
+            'AlsoInValid_DATABASE_URL': "",
+            'ALMOST_CORRECt_DATABASE_URL': "",
+        }
+
+        with debugenv(**e):
+            dbs = d12f()['DATABASES']
+
+            self.assertEquals(
+                len(dbs),
+                1,
+                "Loaded %d databases instead of just 1 (default) - got %s "
+                "from environment %s" % (len(dbs), dbs.keys(), e)
+            )