MongoDB Aggregate lookup in Go (mgo.v2) -

i'm trying implement $lookup functionality in 1 of mongodb queries in go (golang) using mgo package.

below collections:

folders:

"_id"    : objectid("22222222222222"), "name"   : "media", "level"  : 1, "userids": [objectid("4444444444444")] 

documents:

"_id"      : objectid("11111111111111"), "title"    : "media management", "body"     : bindata(0,"pvq6z2nbm4265duo/e2xsyxa5bxko="), "level"    : 1, "folderid" : objectid("22222222222222"), // foreign key/field "userids"  : [objectid("44444444444444")] 

below query i've written runs on shell:

var query = [ {    "$lookup": {     "from":         "documents",     "localfield":   "_id",     "foreignfield": "folderid",     "as":           "documents",   } }  ,{    "$match": {       "userids": objectid("useridhere"), // filder userid       "level": {$gte: 0},                // filter folder level     },   } ];  db.folders.aggregate(query).pretty().shellprint(); 

if run script on shell, desired result. basically, folder collection returned me containing full relevant documents linked through $lookup. i'm not including here because question seems long.

i've tried translate query mgo able parse , execute. here below in go code:

query := bson.m{   "$lookup": bson.m{ // lookup documents table here   "from":         "documents",   "localfield":   "_id",   "foreignfield": "folderid",   "as":           "documents", },   "$match": bson.m{     "level":   bson.m{"$gte": user.level}, // filter level     "userids": user.id,                    // filter user   }, }  pipe := collection.pipe(query) // querying "folders" collection err := pipe.all(&result) 

i same error: wrong type field (pipeline) 3 != 4

if understand correctly, it's because can't parse result $result object. i've done can ensure struct has exact structure required. i've tried pass in genereric []interface{} , empty bson.m{} objects. still receive same error.

below folders struct:

type folder struct {   id        bson.objectid   `json:"id" bson:"_id"`   name      string          `json:"name"`   level     int             `json:"level"`   userids   []bson.objectid `json:"userids" bson:"userids"`   users     []user          `json:"-" bson:"-"` // doesn't stored in database   documents []document      `json:"-" bson:"-"` // doesn't stored in database } 

i've removed $match clause see if @ $lookup query. still same error.

perhaps mgo package doesn't support $lookup? if so, there way? perhaps send raw query text mongo , receive raw response , parse myself?

found solution!

the trick create query in slice ([]bson.m) , change structure of query bit:

query := []bson.m{{   "$lookup": bson.m{ // lookup documents table here     "from":         "documents",     "localfield":   "_id",     "foreignfield": "folderid",     "as":           "documents",   }},   {"$match": bson.m{     "level": bson.m{"$lte": user.level},     "userids": user.id, }}}  pipe := collection.pipe(query) err := pipe.all(&folders) 

i found clue in mgo's pipe function docs. also, had change tags documents field in folders struct mgo pupolate field:

type folder struct {   id        bson.objectid   `json:"id" bson:"_id"`   name      string          `json:"name"`   level     int             `json:"level"`   userids   []bson.objectid `json:"userids" bson:"userids"`   users     []user          `json:"-" bson:"-"` // doesn't stored in database   documents []document      // `json:"-" bson:"-" removed mgo can unmarshal                             // documents correctly } 

now have figure out way not store documents field in database when save folder.