How to join collection in in mongodb? -

i have 3 collections:

collection1 like:

{ "_id" : objectid("5716617f4af77ca97a9614bd"), "count" : 1, "author" : "tony" } { "_id" : objectid("5716617f4af77ca97a9614be"), "count" : 2, "author" : "joe"} { "_id" : objectid("5716617f4af77ca97a9614bf"), "count" : 3, "author" : "mary" }  { "_id" : objectid("5716617f4af77ca97a9314bf"), "count" : 2, "author" : "lee" } 

means author tony writes 1 book, author joe writes 2 books , author mary writes write 3 books.

collection2 like:

{ "_id" : objectid("5716617f4af77ca97a9614bd"), "count" : 2, "author" : "tony" } { "_id" : objectid("5716617f4af77ca97a9614be"), "count" : 2, "author" : "joe"} 

means author tony writes 2 papers, author joe writes 2 papers collection3 like:

{ "_id" : objectid("5716617f4af77ca97a9614bd"),  "author" : "tony" } { "_id" : objectid("5716617f4af77ca97a9614be"),  "author" : "joe"} { "_id" : objectid("5716617f4af77ca97a9624be"),  "author" : "mary"} 

i hope author appear in collection3 , sort these authors number of works： result like:

author    number: joe        2 + 2 = 4 tony       1 + 2 = 3 mary       3 + 0 = 3 

how can write single query finish in mongodb?

this work in 3.2 , above.

db.books.aggregate([{     $lookup: {         from: "papers",         localfield: "author",         foreignfield: "author",         as: "papers"     } }, {     $unwind: "$papers"   }, {     $group: {         _id: "$author",         bookcount: {$sum: "$count"},         paperscount: {$sum: "$papers.count"},     } }, {     $project: {         _id: 0,         author: "$_id",         total: { $add: [ "$bookcount", "$paperscount"] }     } }]) 

outputs:

/* 1 */ {     "author" : "tony",     "total" : 3 }  /* 2 */ {     "author" : "joe",     "total" : 4 }