VersionGroup.generation = relation(Generation, backref='version_groups')
VersionGroup.version_group_regions = relation(VersionGroupRegion, backref='version_group')
VersionGroup.regions = association_proxy('version_group_regions', 'region')
VersionGroup.generation = relation(Generation, backref='version_groups')
VersionGroup.version_group_regions = relation(VersionGroupRegion, backref='version_group')
VersionGroup.regions = association_proxy('version_group_regions', 'region')