Read database path from config in alembic env.py

Slightly related to #135

alembic/env.py

@@ -1,4 +1,3 @@
-from __future__ import with_statement
 from alembic import context
 from sqlalchemy import engine_from_config, pool
 from logging.config import fileConfig
@@ -8,12 +7,19 @@ from os.path import abspath, dirname
 sys.path.insert(0, dirname(dirname(abspath(__file__))))
 
 from mautrix_telegram.base import Base
+from mautrix_telegram.config import Config
 import mautrix_telegram.db
 
 # this is the Alembic Config object, which provides
 # access to the values within the .ini file in use.
 config = context.config
 
+mxtg_config_path = context.get_x_argument(as_dictionary=True).get("config", "config.yaml")
+mxtg_config = Config(mxtg_config_path, None, None)
+mxtg_config.load()
+config.set_main_option("sqlalchemy.url",
+                       mxtg_config.get("appservice.database", "sqlite:///mautrix-telegram.db"))
+
 # Interpret the config file for Python logging.
 # This line sets up loggers basically.
 fileConfig(config.config_file_name)