From SQL Server to MONGODB: design problem

Question

For the first time I want to see other database than SQL Server for my development.

MongoDB is a good alternative for my project.

What is difficult to understand is "the design" of MONGODB for me.

User (table)

Id
Name
Address

Friends (table)

IdUser
IdFriends

With this SQL table, I want to allow an user to became a friend of other user like Facebook.

Which is the best design for MongoDB?

marc_s · Accepted Answer · 2011-08-26 07:44:31Z

3

Consider nesting friends ids inside User document:

{
    _id: 7,
    name: "Johnny",
    address: "Acme St.",
    friends: [8, 9, 15]
}

On one hand you don't need joins to fetch friends (at least ids), because everything is nested within a single document and the schema looks more pleasant. On the other - to fetch the actual friends, you hit N+1 problem. However fetching by id is rather cheap, also consider caching.

edited Aug 26, 2011 at 7:44

marc_s

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answered Aug 26, 2011 at 7:24

Tomasz Nurkiewicz

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2 Comments

Daniele Tassone Over a year ago

Ok, suppose that i have 1.000 or 10.000 friends?

Tomasz Nurkiewicz Over a year ago

MongoDB document size limit is 16 MiB (SERVER-431), also there are operations to work with nested arrays without fetching whole array itself.

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