node.js - Counting nested arrays with mongoose -

i'm using mongodb through mongoose in node.js application.

in db have general format:

books:

bookschema = { variousdata, //books contain pages pages: [{type: pageschema}] } 

pages:

pageschema = { variousdata, //pages contain frames frames: [frameschema] } 

frames:

frameschema = { variousdata } 

i want write count query count total number of frames in pages in books.

so if have content in db:

    book1 = {         data:data,         pages: [             {data:data,              frames: [frame1,frame2,frame3]             },             {data:data,                 frames: [frame1,frame2,frame3]             },             {data:data,                 frames: [frame1,frame2,frame3]             }         ]     } 

and

    book2 = {         data:data,         pages: [             {data:data,             frames: [frame1,frame2]             },             {data:data,             frames: [frame1,frame2]             },             {data:data,             frames: [frame1,frame2]             }         ]     } 

i'm expecting end result of 15 (9 frames in book 1 , 6 frames in book 2)

all attempts ended errors.

i can write loop, feel mongoose have better alternative.

any suggestions?

thanks all!

the efficient way this:

  book.aggregate([     { "$project": {       "pages": {          "$map": {           "input": "$pages",           "as": "page",           "in": { "$size": "$$page.frames" }         }}     }},     { "$unwind": "$pages" },     { "$group": {       "_id": null,       "pages": { "$sum": "$pages" }     }}   ],function(err,results) {    }) 

basically there $map operator allows traverse elements of "pages" array, take @ each "frames" array , return $size. content of "pages" array of sizes each "frames", , therefore count.

then need "add" together. $unwind array , $sum page counts each frame $group.