Default MetaData kwarg naming_convention can create issues with certain databases

• this is a docs ticket; assign to @jvanasco
• reference group discussion: https://groups.google.com/forum/#!topic/sqlalchemy/8xOu-Mlcobo

The alembic docs reference this naming convention: https://alembic.sqlalchemy.org/en/latest/naming.html

convention = {
  "ix": "ix_%(column_0_label)s",
  "uq": "uq_%(table_name)s_%(column_0_name)s",
  "ck": "ck_%(table_name)s_%(constraint_name)s",
  "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
  "pk": "pk_%(table_name)s"
}

metadata = MetaData(naming_convention=convention)

a Boolean/Enum column will try to create a constraint on sqlite by default, but constraint_name is not defined. this will cause an error in Engine.create_all(

Tasks:

[ ] have the docs use custom callables to offer an automatic constraint name. [ ] long term: descope into another issue a fallback option for constraint name.

see referenced discussion for details of above.