Question

Good day dear colleagues, I decided to move some projects from MySQL to MongoDB and faced several difficulties:

For example there are two tables in MySQL:

Users:

CREATE TABLE `testdb`.`users` (
`id` INT( 11 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`name` VARCHAR( 55 ) NOT NULL ,
`password` VARCHAR( 32 ) NOT NULL
) ENGINE = MYISAM

Rules:

CREATE TABLE `testdb`.`rules` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`uid` INT NOT NULL ,
`title` VARCHAR( 155 ) NOT NULL ,
`points` INT NOT NULL
) ENGINE = MYISAM 

Now to chose all “rules”, which belong to a paticular user I can make SQL request:

SELECT r.`title`, r.`points` FROM `rules` r, `users` u WHERE r.`uid` = u.`id` AND u.`id` = '123'

By now, I can’t figure out how to do the same in MongoDB, can you please explain and provide an example.

P.S. I make implementation in Python with the help of pymongo
P.P.S. I also wanted to see the alternative ways of solving this problem with the help of ORM mongoengine or mongokit.

Thank you in advance:)

Answer 1

MongoDB does not support joins, unlike RDBMS’s like mysql. And that’s because MongoDB is not a relational database. Modelling data in MongoDB in the same way as you do in an RDBMS is therefore generally a bad idea – you have to design your schemas in a whole different mindset.

In this case for example, in MongoDB you could have 1 document per User, with the Rules belonging each user nested inside.

e.g.

{
    "ID" : 1,
    "name" : "John",
    "password" : "eek hope this is secure",
    "rules": [
        {
            "ID" : 1,
            "Title" : "Rule 1",
            "Points" : 100   
        },
        {
            "ID" : 2,
            "Title" : "Rule 2",
            "Points" : 200
        }
    ]
}

This means, you only need a single read to pull back a user and all their rules.

A good starting point is the Mongodb.org reference on Schema Design – what I’m talking about above is embedding objects.