new generic `get` allow to retrieve an ORM object with given column and value, and
optionally join load attributes.
In `delete`, it is now possible to specify instances to add to session, this allow, to
update them in the same transaction. It is also now possible to specify a list of object
to delete them all in the same transaction.
rel 367
a first Alembic revision file is joined to migrate existing databases to new mapping.
This patch:
- set `nullable=False` in various places
- drop legacy `message_types` table, and use `Enum` instead
- use naming convention, and get rid of anonymous constraint (this is the main reason
why the revision file is so long)
Alembic database migration tool, which is the recommended one for SQLAlchemy has been
integrated. When a database is created, it will be used to stamp to current (head)
revision, otherwise, DB will be checked to see if it needs to be updated, and upgrade will
be triggered if necessary.