MongoDb with Java foreign key

Question

I need to save two collections in my MongoDB using Java. Where one collection is Department and other collection is Employee. Where one Departm

Answer 1

If $id didn't work try just id like this

"employees" : [ { "$ref" : "employee", "id" : 1 }, { "$ref" : "employee", "id" : 2 } ]

Answer 2

By the provided document and tags I assume you are using spring data to deal with mongodb. So here you may want to use DBRefs to bind employees into departments. Luckily Spring Data gives you @DBRef annotation.

Employee class:

import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.Document;

@Document
public class Employee {

    @Id
    private Integer id;
    ...

}

Department class:

import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.DBRef;
import org.springframework.data.mongodb.core.mapping.Document;

@Document
public class Department {

    @Id
    private String id;

    @DBRef
    private Collection<Employee> employees;
    ...
}

MongoDB document:

{
    "_id" : ObjectId("598dc04ac4fdd0e29867ccbb"),
    "_class" : "foo.bar.Department",
    "employees" : [ 
        {
            "$ref" : "employee",
            "$id" : 1
        }, 
        {
            "$ref" : "employee",
            "$id" : 2
        }
    ]
}

Note: Employee instance must already exist in MongoDB. DBRef will not save Employees in cascade style. Look at this article about cascading.