Mongodb aggregation convert array of pairs to key and list of values - mongodb

Question

Trying to condense an array with key value pairs into an array of objects with the key and all the unique values for that key.
I have a structure like:
{
fruits: [
{fruit: apple, type: gaja},
{fruit: apple, type: honey-crisp},
{fruit: apple, type: fuji},
{fruit: cherry, type: black},
{fruit: cherry, type: red},
{fruit: cherry, type: red},
]
}
How can I convert it to:
{
fruits: [
{fruit: apple, types: [gaja, honey-crisp, fuji]},
{fruit: cherry, types: [black, red]}
]
}
Using mongo aggregations I managed to get the first structure from my data using $group and $addToSet. Not sure how to map the array to new object with a key and list of values

Answer 1

Here's another way to do it by using "$reduce". Comments are in the aggregation pipeline.
db.collection.aggregate([
{
"$set": {
// rewrite fruits
"fruits": {
"$reduce": {
"input": "$fruits",
"initialValue": [],
"in": {
"$let": {
"vars": {
// get fruit index in $$value : will be -1 if not there
"idx": {"$indexOfArray": ["$$value.fruit", "$$this.fruit"]}
},
"in": {
"$cond": [
// is fruit not in $$value yet
{"$eq": ["$$idx", -1]},
// new fruit so put in $$value and make "type" an array
{
"$concatArrays": [
"$$value",
[{"$mergeObjects": ["$$this", {"type": ["$$this.type"]}]}]
]
},
// fruit already in $$value, so map $$value with "type" update
{
"$map": {
"input": "$$value",
"as": "val",
"in": {
"$cond": [
// is this array element not the right fruit?
{"$ne": ["$$val.fruit", "$$this.fruit"]},
// nope, leave the element as-is
"$$val",
// this element needs to be updated
{
"$mergeObjects": [
"$$val",
{
"type": {
"$cond": [
// is this "type" already in array?
{"$in": ["$$this.type", "$$val.type"]},
// yes, so leave it as-is
"$$val.type",
// this is a new "type", so add it to array
{"$concatArrays": ["$$val.type", ["$$this.type"]]}
]
}
}
]
}
]
}
}
}
]
}
}
}
}
}
}
}
])
Try it on mongoplayground.net.

Answer 2

Maybe something like this:
db.collection.aggregate([
{
$unwind: "$fruits"
},
{
$group: {
_id: "$fruits.fruit",
type: {
$push: "$fruits.type"
}
}
},
{
$project: {
fruit: "$_id",
type: 1,
_id: 0
}
},
{
$group: {
_id: "",
fruits: {
$push: "$$ROOT"
}
}
}
])
Explained:
Unwind the array
Group to form the type array ( you can use $push or $addToSet in case you need only unique )
Project the necessary fields
Group all documents inside single final one
Playground

Answer 3

Here's another, another way using a multiple "$map" and "$setUnion" to get unique array members.
db.collection.aggregate([
{
"$set": {
// rewrite fruits
"fruits": {
"$map": {
// map over unique fruits
"input": {"$setUnion": "$fruits.fruit"},
"as": "theFruit",
"in": {
// set fruit
"fruit": "$$theFruit",
// "type" are unique elements of fruits.type
// where fruits.fruit == theFruit
"type": {
"$setUnion": {
"$map": {
"input": {
"$filter": {
"input": "$fruits",
"as": "obj",
"cond": {"$eq": ["$$obj.fruit", "$$theFruit"]}
}
},
"in": "$$this.type"
}
}
}
}
}
}
}
}
])
Try it on mongoplayground.net.