Class for management of APDB schema.
Attributes
----------
objects : `sqlalchemy.Table`
DiaObject table instance
objects_last : `sqlalchemy.Table`
DiaObjectLast table instance, may be None
sources : `sqlalchemy.Table`
DiaSource table instance
forcedSources : `sqlalchemy.Table`
DiaForcedSource table instance
Parameters
----------
engine : `sqlalchemy.engine.Engine`
SQLAlchemy engine instance
dia_object_index : `str`
Indexing mode for DiaObject table, see `ApdbConfig.dia_object_index`
for details.
schema_file : `str`
Name of the YAML schema file.
extra_schema_file : `str`, optional
Name of the YAML schema file with extra column definitions.
prefix : `str`, optional
Prefix to add to all scheam elements.