I have problem to retrieve information between two collections. The first collection stores employees information:

{
        "_id" : ObjectId("4f9643967f8b9a3f0a00005a"),
        "birth_date" : "1963-09-09",
        "departments" : [
                {
                        "departments_id" : ObjectId("4f9643957f8b9a3f0a000007"),
                        "from_date" : "1990-01-03",
                        "to_date" : "1990-01-15"
                }
        ],
        "first_name" : "Parviz",
        "gender" : "M",
        "hire_date" : "1990-01-03",
        "last_name" : "Lortz",
}

the second one the departments information

{
        "_id" : ObjectId("4f9643957f8b9a3f0a000004"),
        "dept_name" : "Marketing",
}

I try to find: All departments for a given employee.

I tried something like:

employees = db.employees.find({_id:ObjectId("some_id")});
db.departments.find({_id:{$in:...}});

But I don't know how I can explain $in department_id of all departments from var employees.

I have problem to retrieve information between two collections. The first collection stores employees information:

{
        "_id" : ObjectId("4f9643967f8b9a3f0a00005a"),
        "birth_date" : "1963-09-09",
        "departments" : [
                {
                        "departments_id" : ObjectId("4f9643957f8b9a3f0a000007"),
                        "from_date" : "1990-01-03",
                        "to_date" : "1990-01-15"
                }
        ],
        "first_name" : "Parviz",
        "gender" : "M",
        "hire_date" : "1990-01-03",
        "last_name" : "Lortz",
}

the second one the departments information

{
        "_id" : ObjectId("4f9643957f8b9a3f0a000004"),
        "dept_name" : "Marketing",
        "managers" : [
                {
                        "employees_id" : ObjectId("4f96439b7f8b9a3f0a0186a9"),
                        "from_date" : "1985-01-01",
                        "to_date" : "1991-10-01"
                },
                {
                        "employees_id" : ObjectId("4f96439b7f8b9a3f0a0186aa"),
                        "from_date" : "1991-10-01",
                        "to_date" : "9999-01-01"
                }
        ]
}

I try to find: All departments for a given employee.

I tried something like:

employees = db.employees.find({_id:ObjectId("some_id")});
db.departments.find({_id:{$in:...}});

But I don't know how I can explain $in department_id of all departments from var employees.