Foreign Keys in Mongo

Foreign keys in mongo?

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.

MongoDB normalization, foreign key and joining

MongoDB doesn't support server side foreign key relationships, normalization is also discouraged. You should embed your child object within parent objects if possible, this will increase performance and make foreign keys totally unnecessary. That said it is not always possible, so there is a special construct called DBRef which allows to reference objects in a different collection. This may be then not so speedy because DB has to make additional queries to read objects but allows for kind of foreign key reference.

Still you will have to handle your references manually. Only while looking up your DBRef you will see if it exists, the DB will not go through all the documents to look for the references and remove them if the target of the reference doesn't exist any more. But I think removing all the references after deleting the book would require a single query per collection, no more, so not that difficult really.

If your schema is more complex then probably you should choose a relational database and not nosql.

There is also a book about designing MongoDB databases: Document Design for MongoDB

UPDATE The book above is not available anymore, yet because of popularity of MongoDB there are quite a lot of others. I won't link them all, since such links are likely to change, a simple search on Amazon shows multiple pages so it shouldn't be a problem to find some.

See the MongoDB manual page for 'Manual references' and DBRefs for further specifics and examples

Foreign key like relationship in Mongo DB

hiya see this: MongoDB normalization, foreign key and joining && further http://shop.oreilly.com/product/0636920018391.do ===> http://books.google.com/books/about/Document_Design_for_MongoDB.html?id=TbIHkgEACAAJ&redir_esc=y

MongoDB doesn't support server side foreign key relationships,
normalization is also discouraged. You should embed your child object
within parent objects if possible, this will increase performance and
make foreign keys totally unnecessary. That said it is not always
possible, so there is a special construct called DBRef which allows to
reference objects in a different collection. This may be then not so
speedy because DB has to make additional queries to read objects but
allows for kind of foreign key reference.

Still you will have to handle your references manually. Only while
looking up your DBRef you will see if it exists, the DB will not go
through all the documents to look for the references and remove them
if the target of the reference doesn't exist any more. But I think
removing all the references after deleting the book would require a
single query per collection, no more, so not that difficult really.

Edit update

http://levycarneiro.com/tag/mongodb/

levycarneiro.com/tag/mongodb [quote] So you create 4 collections: Clients, Suppliers, Employees and Contacts. You connect them all together via a db reference. This acts like a foreign key. But, this is not the mongoDB way to do things. Performance will penalized. [unquote]

Mongo DB Join on Primary/Foreign Key

db.mp.aggregate([
  {
    $lookup: {
      from: "clib",
      let: {
        clibId: "$clibId"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $and: [
                {
                  $eq: [ "$_id", "$$clibId" ],
                }
              ]
            }
          }
        },
        {
          $project: { type: 1, _id: 0 }
        }
      ],
      as: "clib"
    }
  },
  {
    "$unwind": "$clib"
  },
  {
    "$match": {
      "clib.type": 0
    }
  }
])

Test Here

How to enforce foreign keys in NoSql databases (MongoDB)?

MongoDB doesn't have foreign keys (as you have presumably noticed). Fundamentally the answer is therefore, "Don't let users tamper with the requests. Only let the application insert data that follows your referential integrity rules."

MongoDB is great in lots of ways... but if you find that you need foreign keys, then it's probably not the correct solution to your problem.

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