禁止保存className：@Entity(noClassnameStored = true)
https://stackoverflow.com/questions/17719018/a-field-name-classname-is-being-inserted-into-mongodb-by-morphia
嵌套文档实体型识别：@Embedded("info")
https://www.ibm.com/developerworks/cn/java/j-morphia/index.html