Mongo Query - $group

Question

Is it possible with Mongo to make this transformation (with a $group I think) ? or should it be done with JavaScript on the client side ?

[
  {
    id: 1,
    lib: 'x'
  },
  {
    id: 2,
    lib: 'a'
  },
  {
    id: 1,
    lib: 'b'
  },
  {
    id: 1,
    lib: 'v'
  }
]

to

[
  {
    id: 1,
    lib_1: 'x',
    lib_2: 'b',
    lib_3: 'v'
  },
  {
    id: 2,
    lib_1: 'a'
  }
]

Answer 1

Query

• with $group you can easily put them in an array, and i think its best to just do this
• but if you want the exact output like in your expected output its more complicated, because you need to convert array to object, and add numbers in keys etc, in general in mongodb fields are not made for data, because queries become harder, fields are for the schema

Playmongo

aggregate(
[{"$group": {"_id": "$id", "libs": {"$push": "$lib"}}},
 {"$set": 
   {"libs": 
     {"$map": 
       {"input": {"$range": [0, {"$size": "$libs"}]},
        "in": 
         {"k": {"$concat": ["lib_", {"$toString": {"$add": ["$$this", 1]}}]},
          "v": {"$arrayElemAt": ["$libs", "$$this"]}}}}}},
 {"$set": {"libs": {"$arrayToObject": ["$libs"]}}},
 {"$replaceRoot": {"newRoot": {"$mergeObjects": ["$libs", "$$ROOT"]}}},
 {"$project": {"libs": 0}}])