"invoiceItems" : [
    {
        "$ref" : "JobsPostings",
        "$id" : ObjectId("5aad26b6f78a341ef831b4f7"),
        "price" : 15,
        "publishDate" : ISODate("2018-03-17T00:00:00.000Z"),
        "_id" : ObjectId("5aaf4133b72ed853c09d60db")
    },
    {
        "$ref" : "JobsPostings",
        "$id" : ObjectId("5aad27e4f78a341ef831b513"),
        "price" : 20,
        "publishDate" : ISODate("2018-03-17T00:00:00.000Z"),
        "_id" : ObjectId("5aaf4133b72ed853c09d60da")
    }
]

PostPaidInvoice.find({_id: { $in : [ObjectId("5aaf4133b72ed853c09d60d9")]}}).then(result => {
      console.log(JSON.stringify(result[0].invoiceItems));
    }).catch(err => {
      console.log(err);
    });

[{"_bsontype":"DBRef","namespace":"JobsPostings","oid":"5aad26b6f78a341ef831b4f7"},{"_bsontype":"DBRef","namespace":"JobsPostings","oid":"5aad27e4f78a341ef831b513"}]