alembic: Load configuration from ogn config

If environment variable OGN_CONFIG_MODULE is defined,
load configuration from this python module, otherwise load "config.default".

alembic.ini

@@ -1,37 +1,10 @@
-# A generic, single database configuration.
+# A single database configuration,
+# sqlalchemy.url defined in env.py
 
 [alembic]
 # path to migration scripts
 script_location = alembic
 
-# template used to generate migration files
-# file_template = %%(rev)s_%%(slug)s
-
-# max length of characters to apply to the
-# "slug" field
-#truncate_slug_length = 40
-
-# set to 'true' to run the environment during
-# the 'revision' command, regardless of autogenerate
-# revision_environment = false
-
-# set to 'true' to allow .pyc and .pyo files without
-# a source .py file to be detected as revisions in the
-# versions/ directory
-# sourceless = false
-
-# version location specification; this defaults
-# to alembic/versions.  When using multiple version
-# directories, initial revisions must be specified with --version-path
-# version_locations = %(here)s/bar %(here)s/bat alembic/versions
-
-# the output encoding used when revision files
-# are written from script.py.mako
-# output_encoding = utf-8
-
-sqlalchemy.url = sqlite:///beacons.db
-
-
 # Logging configuration
 [loggers]
 keys = root,sqlalchemy,alembic

alembic/env.py

@@ -1,4 +1,6 @@
-from __future__ import with_statement
+import os
+import importlib
+
 from alembic import context
 from sqlalchemy import engine_from_config, pool
 from logging.config import fileConfig
@@ -11,16 +13,17 @@ config = context.config
 # This line sets up loggers basically.
 fileConfig(config.config_file_name)
 
+# Get database path from ogn config
+os.environ.setdefault('OGN_CONFIG_MODULE', 'config.default')
+ogn_config = importlib.import_module(os.environ['OGN_CONFIG_MODULE'])
+
+alembic_config.set_main_option('sqlalchemy.url', ogn_config.SQLALCHEMY_DATABASE_URI)
 # add your model's MetaData object here
 # for 'autogenerate' support
 # from myapp import mymodel
 # target_metadata = mymodel.Base.metadata
 target_metadata = None
 
-# other values from the config, defined by the needs of env.py,
-# can be acquired:
-# my_important_option = config.get_main_option("my_important_option")
-# ... etc.
 
 
 def run_migrations_offline():