better configuration handling for mongodb

Author: galacticglum

config/app_settings.py

@@ -1,17 +1,41 @@
 """Application settings."""
 import os
+from typing import Optional, Union
 
-SECRET_KEY = os.getenv('SECRET_KEY')
-
+# Server configuration
+APP_NAME = os.getenv('COMPOSE_PROJECT_NAME', 'gator')
 SERVER_NAME = os.getenv('SERVER_NAME')
+SECRET_KEY = os.getenv('SECRET_KEY')
+GATOR_HOST = os.getenv('GATOR_HOST')
 
 # MongoDB configuration
+
+
+def _get_mongodb_credential(credential_type: str,
+                            default: Optional[str] = None) -> Union[str, None]:
+    """Return a credential for the MongoDB database.
+
+    Will first check the environment variable MONGODB_{credential_type},
+    and if that is not set, will check
+    MONGODB_INITDB_ROOT_{credential_type}.
+    """
+    assert credential_type in {'username', 'password'},\
+        'credential_type must be either "username" or "password"'
+
+    credential_type = credential_type.upper()
+    credential = os.getenv(f'MONGODB_{credential_type}')
+    if not credential:
+        credential = os.getenv(f'MONGO_INITDB_ROOT_{credential_type}', default)
+    return credential
+
+
 MONGODB_SETTINGS = {
-    'db': os.getenv('MONGODB_DB', 'sqrl'),
-    'host': os.getenv('MONGODB_HOST', 'mongodb'),
+    'db': os.getenv('MONGODB_DB', APP_NAME),
+    'host': os.getenv('MONGODB_HOST', 'localhost'),
     'port': int(os.getenv('MONGODB_PORT', 27017)),
-    'username': os.getenv('MONGODB_USERNAME'),
-    'password': os.getenv('MONGODB_PASSWORD'),
+    # If no username or password is given, use the root credentials used to
+    # init the Docker service.
+    'username': _get_mongodb_credential('username', default='username'),
+    'password': _get_mongodb_credential('password', default='password'),
+    'authentication_source': os.getenv('MONGODB_AUTH_SOURCE', None),
 }
-
-GATOR_HOST = os.getenv('GATOR_HOST')