关系数据库和nosql设计schema时的差别的例子
在关系数据库中和nosql的设计时,对于schema的设计是不同的.比如MYSQL中,设计如下一个例子:

mysql> select * from people;+----+------------+| id | name       |+----+------------+|  1 | Stephane   ||  2 | John       ||  3 | Michael    ||  4 | Cinderella |+----+------------+mysql> select * from passports;+----+-----------+---------+-------------+| id | people_id | country | valid_until |+----+-----------+---------+-------------+|  4 |         1 | FR      | 2020-01-01  ||  5 |         2 | US      | 2020-01-01  ||  6 |         3 | RU      | 2020-01-01  |+----+-----------+---------+-------------+

这个如果要转换为mongodb要如何设计呢,首先一个方法是全部放在一个collection中,如

> db.people_all.find().pretty(){"_id" : ObjectId("51f7be1cd6189a56c399d3bf"),"name" : "Stephane","country" : "FR","valid_until" : ISODate("2019-12-31T23:00:00Z")}{"_id" : ObjectId("51f7be3fd6189a56c399d3c0"),"name" : "John","country" : "US","valid_until" : ISODate("2019-12-31T23:00:00Z")}{"_id" : ObjectId("51f7be4dd6189a56c399d3c1"),"name" : "Michael","country" : "RU","valid_until" : ISODate("2019-12-31T23:00:00Z")}{ "_id" : ObjectId("51f7be5cd6189a56c399d3c2"), "name" : "Cinderella" }

这样的话,如果要查询people的所有信息,放在同一个collection中是很快的,当然也可以分开两个collection进行嵌套,如:

> db.people_embed.find().pretty(){"_id" : ObjectId("51f7c0048ded44d5ebb83774"),"name" : "Stephane","passport" : {"country" : "FR","valid_until" : ISODate("2019-12-31T23:00:00Z")}}{"_id" : ObjectId("51f7c70e8ded44d5ebb83775"),"name" : "John","passport" : {"country" : "US","valid_until" : ISODate("2019-12-31T23:00:00Z")}}{"_id" : ObjectId("51f7c71b8ded44d5ebb83776"),"name" : "Michael","passport" : {"country" : "RU","valid_until" : ISODate("2019-12-31T23:00:00Z")}}{ "_id" : ObjectId("51f7c7258ded44d5ebb83777"), "name" : "Cinderella" }

甚至可以这样:

> db.passports_embed.find().pretty(){"_id" : ObjectId("51f7c7e58ded44d5ebb8377b"),"country" : "FR","valid_until" : ISODate("2019-12-31T23:00:00Z"),"person" : {"name" : "Stephane"}}{"_id" : ObjectId("51f7c7ec8ded44d5ebb8377c"),"country" : "US","valid_until" : ISODate("2019-12-31T23:00:00Z"),"person" : {"name" : "John"}}{"_id" : ObjectId("51f7c7fa8ded44d5ebb8377d"),"country" : "RU","valid_until" : ISODate("2019-12-31T23:00:00Z"),"person" : {"name" : "Michael"}}{"_id" : ObjectId("51f7c8058ded44d5ebb8377e"),"person" : {"name" : "Cinderella"}}

如果是大部分只查询people的本身信息的话,那么建议分开两个collection存放,避免每次把不需要加载的信息加载进去了.