node.js - Get result as an array instead of documents in mongodb for an attribute -

i have user collection schema

{      name: string,       books: [          id: { type: schema.types.objectid, ref: 'book' } ,          name: string      ]   } 

is possible array of book ids instead of object?

something like:

["53eb797a63ff0e8229b4aca1", "53eb797a63ff0e8229b4aca2", "53eb797a63ff0e8229b4aca3"] 

or

{ids: ["53eb797a63ff0e8229b4aca1", "53eb797a63ff0e8229b4aca2", "53eb797a63ff0e8229b4aca3"]} 

and not

{     _id: objectid("53eb79d863ff0e8229b97448"),     books:[         {"id" : objectid("53eb797a63ff0e8229b4aca1") },          { "id" : objectid("53eb797a63ff0e8229b4acac") },          { "id" : objectid("53eb797a63ff0e8229b4acad") }     ] } 

currently doing

user.findone({}, {"books.id":1} ,function(err, result){     var bookids = [];     result.books.foreach(function(book){         bookids.push(book.id);     });  }); 

is there better way?

it done aggregation pipeline, using $unwind , $group.

db.users.aggregate({   $unwind: '$books' }, {   $group: {     _id: 'books',     ids: { $addtoset: '$books.id' }   } }) 

the same operation using mongoose model.aggregate() method:

user.aggregate().unwind('$books').group(   _id: 'books',   ids: { $addtoset: '$books.id' } }).exec(function(err, res) {   // use res[0].ids }) 

note books here not mongoose document, plain js object.

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you can add $match select part of users collection run aggregation query on.

for example, may select 1 particular user:

user.aggregate().match({   _id: uid }).unwind('$books').group(   _id: 'books',   ids: { $addtoset: '$books.id' } }).exec(function(err, res) {   // use res[0].ids }) 

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but if you're not interested in aggregating books different users single array, it's best without using $group , $unwind:

user.aggregate().match({   _id: uid }).project({   _id: 0,   ids: '$books.id' }).exec(function(err, users) {   // use users[0].ids })