title: MongoDB query IN array of object
body: <p>I have problem to retrieve information between two collections.
  The first collection stores employees information:</p>
  
  <pre><code>{
          "_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",
  }
  </code></pre>
  
  <p>the second one the departments information</p>
  
  <pre><code>{
          "_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"
                  }
          ]
  }
  </code></pre>
  
  <p>I try to find: <strong>All departments for a given employee</strong>.</p>
  
  <p>I tried something like:</p>
  
  <pre><code>employees = db.employees.find({_id:ObjectId("some_id")});
  db.departments.find({_id:{$in:...}});
  </code></pre>
  
  <p>But I don't know how I can explain $in department_id of all departments from var employees.</p>