Problemm using concat in array mongodb

Question

i have 3 tables

1) Actor: actor_id, first_name, last_name 
2) Film: film_id, title
3) Film_Actor: film_id, actor_id

Example document:

_id
:
60aedac769985522a024daca
actor_id
:
"1"
first_name
:
"Penelope"
last_name
:
"Guiness"

I wanto to result not only first_name, but last_name too. I'm facing problem with using concat in $group function. my FULLY codes:

    db.film.aggregate([
  {
    $lookup: {
      from: "film_actor",
      localField: "film_id",
      foreignField: "film_id",
      as: "film_actor"
    }
  },
  {
    $lookup: {
      from: "actor",
      localField: "film_actor.actor_id", 
      foreignField: "actor_id",
      as: "actor"
    }
  },
  {
    $group: {
      _id: "$film_id",
      title: {"$first":"$title"},
      name: {$push: "$actor.first_name"}
                }
  }
]);

Error report:

$concat only supports strings, not array

desired output:

id:"207"
title:"Dangerous Uptown"
name:Array
0:"Penelope Guiness"
1:"Mary Watson"
2:"Ralph Holts"
3:"Spencer Dani"

Answer 1

What you have done is appreciated, I've done some changes in your code

• $lookup to join collections. I have started form Flim collection
• $unwind to deconstruct the array
• $group to reconstruct the array that we already deconstructed, this will
• Since we have nested array we need to use $map to loop over them to collect the first name and lastname
• The above stage will end up with again nested array, so we use $reduce to loop again and remove inner arrays using $setUnion

remove some duplicate entries, depends on your requirements

Here is the code

db.Film.aggregate([
  {
    $lookup: {
      from: "Film_Actor",
      localField: "film_id",
      foreignField: "film_id",
      as: "join_flim"
    }
  },
  { "$unwind": "$join_flim" },
  {
    $lookup: {
      from: "Actor",
      localField: "join_flim.actor_id",
      foreignField: "actor_id",
      as: "join_flim.join_actor"
    }
  },
  {
    $group: {
      _id: "$_id",
      title: { $first: "$title" },
      join_flim: { $push: "$join_flim" }
    }
  },
  {
    "$project": {
      title: 1,
      actornames: {
        $map: {
          input: "$join_flim",
          as: "f",
          in: {
            $map: {
              input: "$$f.join_actor",
              as: "a",
              in: {
                $concat: [ "$$a.first_name", " ", "$$a.last_name" ]
              }
            }
          }
        }
      }
    }
  },
  {
    "$project": {
      title: 1,
      actornames: {
        "$reduce": {
          "input": "$actornames",
          "initialValue": [],
          "in": {
            "$setUnion": [ "$$this", "$$value" ]
          }
        }
      }
    }
  }
])

Working Mongo playground