db={content:[{_id:ObjectId("5a934e000102030405000000"),title:"a",source:"b",type:"tt"},{_id:ObjectId("5a934e000102030405000001"),title:"c",source:"d",type:"cc"},{_id:ObjectId("5a934e000102030405000002"),title:"e",source:"f",type:"pp"}],surey:[{"_id":ObjectId("5a934e000102030405000007"),"content":[{content_id:ObjectId("5a934e000102030405000000"),placebo:true},{content_id:ObjectId("5a934e000102030405000002"),placebo:false}]},{"_id":ObjectId("5a934e000102030405000008"),"content":[{content_id:ObjectId("5a934e000102030405000001"),placebo:false},{content_id:ObjectId("5a934e000102030405000000"),placebo:true},]},]}

db.surey.aggregate([{$lookup:{from:"content",localField:"content.content_id",foreignField:"_id",as:"someField"}},{$set:{someField:"$$REMOVE",content:{$map:{input:"$content",in:{$mergeObjects:["$$this",{type:{$arrayElemAt:["$someField",{$indexOfArray:["$someField._id","$$this.content_id"]}]}}]}}}}},{$set:{content:{$map:{input:"$content",in:{"$mergeObjects":["$$this",{type:"$$this.type.type"}]}}}}},{$merge:{into:"surey"}}])