Problemm using concat in array mongodb - arrays

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