Mongodb unwind array nested in array - mongodb

Question

This is my collection:
db.questions.insertMany([
{
"content": [
{
"languageId": "en",
"text": "What are you planning to buy today at the supermarket?"
},
{
"languageId": "nl",
"text": "Wat ben je van plan om vandaag in de supermarkt te kopen?"
},
],
"type": "multipleChoice",
"multipleChoice": {
"numAnswers": { "min": 1, "max": 1 },
"possibleAnswers": [
{
"sequence": 1,
"content": [
{
"languageId": "en",
"text": "apples"
},
{
"languageId": "nl",
"text": "appels"
},
],
},
{
"sequence": 2,
"content": [
{
"languageId": "en",
"text": "peers"
},
{
"languageId": "nl",
"text": "peren"
},
],
},
],
}
},
{
"content": [
{
"languageId": "en",
"text": "How do you feel?"
},
{
"languageId": "nl",
"text": "Hoe voel je je?"
},
],
"type": "ranking1to5",
}
]);
I want to transform into one language that are in two content arrays. So I want to have the output:
{
"_id" : ObjectId("5abe4c09d3831890de28ec8f"),
"content" :
{
"languageId" : "en",
"text" : "What are you planning to buy today at the supermarket?"
},
"type" : "multipleChoice",
"multipleChoice" : {
"numAnswers" : {
"min" : 1.0,
"max" : 1.0
},
"possibleAnswers" : [
{
"sequence" : 1.0,
"content" :
{
"languageId" : "en",
"text" : "apples"
}
},
{
"sequence" : 2.0,
"content" :
{
"languageId" : "en",
"text" : "peers"
}
}
]
}
}
/* 2 */
{
"_id" : ObjectId("5abe4c09d3831890de28ec90"),
"content" :
{
"languageId" : "en",
"text" : "How do you feel?"
},
"type" : "ranking1to5"
}
I tried using $unwind, $match and $group to tackle this problem. I have come pretty far only the last piece does not work:
db.getCollection('questions').aggregate([
{ $unwind: "$content" },
{ $match: { "content.languageId": "en" } },
{ $unwind: { path: '$multipleChoice.possibleAnswers', preserveNullAndEmptyArrays: true } },
{ $unwind: { path: '$multipleChoice.possibleAnswers.content', preserveNullAndEmptyArrays: true } },
{ $match: { "multipleChoice.possibleAnswers.content.languageId": "en" } },
{ $group: { _id: "$_id", content: { $first: "$content" }, type: { $first: "$type" }, multipleChoice: { $addToSet: "$multipleChoice" } } }
])
The problem is multipleChoice gets repeated while this should be possibleAnswers. Also the question that has no multipleChoice object should be included.
Any help is particularly appreciated !!

Answer 1

It looks like a good fit for the $redact. Please try the following aggregation:
db.questions.aggregate(
[
{
$redact: {
$cond: {
if: {
$eq: [
{ $ifNull: [ "$languageId", "en" ] },
"en"
]
},
then: "$$DESCEND",
else: "$$PRUNE"
}
}
},
{
$addFields: {
"content": { $arrayElemAt: [ "$content", 0 ] },
"multipleChoice.possibleAnswers": {
$map: {
input: "$multipleChoice.possibleAnswers",
as: "possibleAnswer",
in: {
"sequence": "$$possibleAnswer.sequence",
"content": { $arrayElemAt: [ "$$possibleAnswer.content", 0 ] }
}
}
}
}
},
{
$redact: {
$cond: {
if: { $eq: [ "$possibleAnswers", null ] },
then: "$$PRUNE",
else: "$$DESCEND"
}
}
}
]
);