{
    _id: 10,
    type: 'library',
    address: 'aaaa'
},
{
    _id: 20,
    type: 'library',
    address: 'bbbb'
},
{
    _id: 30,
    type: 'shelf'
    library_id: 10
},
{
    _id: 40:
    type: 'book',
    shelf_id: 30
}

function (doc) {

    if (doc.type === "book") {
        channel("library-"+retrieve(doc.shelf_id).library_id)
    }

}