Foreign keys in mongo?

Question

How do I design a scheme such this in MongoDB? I think there are no foreign keys!

Answer 1

How to design table like this in mongodb?

First, to clarify some naming conventions. MongoDB uses collections instead of tables.

I think there are no foreign keys!

Take the following model:

student {    _id: ObjectId(...),   name: 'Jane',   courses: [     { course: 'bio101', mark: 85 },     { course: 'chem101', mark: 89 }   ] }  course {   _id: 'bio101',   name: 'Biology 101',   description: 'Introduction to biology' } 

Clearly Jane's course list points to some specific courses. The database does not apply any constraints to the system (i.e.: foreign key constraints), so there are no "cascading deletes" or "cascading updates". However, the database does contain the correct information.

In addition, MongoDB has a DBRef standard that helps standardize the creation of these references. In fact, if you take a look at that link, it has a similar example.

How can I solve this task?

To be clear, MongoDB is not relational. There is no standard "normal form". You should model your database appropriate to the data you store and the queries you intend to run.

Answer 2

You may be interested in using a ORM like Mongoid or MongoMapper.

http://mongoid.org/docs/relations/referenced/1-n.html

In a NoSQL database like MongoDB there are not 'tables' but collections. Documents are grouped inside Collections. You can have any kind of document – with any kind of data – in a single collection. Basically, in a NoSQL database it is up to you to decide how to organise the data and its relations, if there are any.

What Mongoid and MongoMapper do is to provide you with convenient methods to set up relations quite easily. Check out the link I gave you and ask any thing.

Edit:

In mongoid you will write your scheme like this:

class Student   include Mongoid::Document      field :name     embeds_many :addresses     embeds_many :scores     end  class Address   include Mongoid::Document      field :address     field :city     field :state     field :postalCode     embedded_in :student end  class Score   include Mongoid::Document      belongs_to :course     field :grade, type: Float     embedded_in :student end   class Course   include Mongoid::Document    field :name   has_many :scores   end 

Edit:

> db.foo.insert({group:"phones"}) > db.foo.find()                   { "_id" : ObjectId("4df6539ae90592692ccc9940"), "group" : "phones" } { "_id" : ObjectId("4df6540fe90592692ccc9941"), "group" : "phones" } >db.foo.find({'_id':ObjectId("4df6539ae90592692ccc9940")})  { "_id" : ObjectId("4df6539ae90592692ccc9940"), "group" : "phones" } 

You can use that ObjectId in order to do relations between documents.