TODO

# TODO
#   callback before/after
#   validations
#   associations
#x   default value
#x   property list
#   property Key
#   property others
#   nil=>false
#x   index=>false setUnindexedProperty
#   version
#   schema_version
#x   new_record?
#   find_by_xxx
#x   get_by_id(id)         get_by_name(name)         : root entity only!!
#x   get_by_id(parent, id) get_by_name(parent, name) : ancestor+Kind => key get
#x   Foo.query.filter(:key=>123)
#   Foo.filter(:key=>123)
#   logger
#   logging low level API calls.
#   create({...}, {:name_prefix=>"u"}) => prefix+allocate_id
#   __key__ range query
#   support reload! on script/console
#   keysonly query returns array of key. not instance of TinyDS::Base