MongoDB insert document "or" increment field if exists in array - mongodb

Question

What I try to do is fairly simple, I have an array inside a document ;
"tags": [
{
"t" : "architecture",
"n" : 12
},
{
"t" : "contemporary",
"n" : 2
},
{
"t" : "creative",
"n" : 1
},
{
"t" : "concrete",
"n" : 3
}
]
I want to push an array of items to array like
["architecture","blabladontexist"]
If item exists, I want to increment object's n value (in this case its architecture),
and if don't, add it as a new Item (with value of n=0) { "t": "blabladontexist", "n":0}
I have tried $addToSet, $set, $inc, $upsert: true with so many combinations and couldn't do it.
How can we do this in MongoDB?

Answer 1

With MongoDB 4.2 and newer, the update method can now take a document or an aggregate pipeline where the following stages can be used:
$addFields and its alias $set
$project and its alias $unset
$replaceRoot and its alias $replaceWith.
Armed with the above, your update operation with the aggregate pipeline will be to override the tags field by concatenating a filtered tags array and a mapped array of the input list with some data lookup in the map:
To start with, the aggregate expression that filters the tags array uses the $filter and it follows:
const myTags = ["architecture", "blabladontexist"];
{
"$filter": {
"input": "$tags",
"cond": {
"$not": [
{ "$in": ["$$this.t", myTags] }
]
}
}
}
which produces the filtered array of documents
[
{ "t" : "contemporary", "n" : 2 },
{ "t" : "creative", "n" : 1 },
{ "t" : "concrete", "n" : 3 }
]
Now the second part will be to derive the other array that will be concatenated to the above. This array requires a $map over the myTags input array as
{
"$map": {
"input": myTags,
"in": {
"$cond": {
"if": { "$in": ["$$this", "$tags.t"] },
"then": {
"t": "$$this",
"n": {
"$sum": [
{
"$arrayElemAt": [
"$tags.n",
{ "$indexOfArray": [ "$tags.t", "$$this" ] }
]
},
1
]
}
},
"else": { "t": "$$this", "n": 0 }
}
}
}
}
The above $map essentially loops over the input array and checks with each element whether it's in the tags array comparing the t property, if it exists then the value of the n field of the subdocument becomes its current n value
expressed with
{
"$arrayElemAt": [
"$tags.n",
{ "$indexOfArray": [ "$tags.t", "$$this" ] }
]
}
else add the default document with an n value of 0.
Overall, your update operation will be as follows
Your final update operation becomes:
const myTags = ["architecture", "blabladontexist"];
db.getCollection('coll').update(
{ "_id": "1234" },
[
{ "$set": {
"tags": {
"$concatArrays": [
{ "$filter": {
"input": "$tags",
"cond": { "$not": [ { "$in": ["$$this.t", myTags] } ] }
} },
{ "$map": {
"input": myTags,
"in": {
"$cond": [
{ "$in": ["$$this", "$tags.t"] },
{ "t": "$$this", "n": {
"$sum": [
{ "$arrayElemAt": [
"$tags.n",
{ "$indexOfArray": [ "$tags.t", "$$this" ] }
] },
1
]
} },
{ "t": "$$this", "n": 0 }
]
}
} }
]
}
} }
],
{ "upsert": true }
);

Answer 2

I don't believe this is possible to do in a single command.
MongoDB doesn't allow a $set (or $setOnInsert) and $inc to affect the same field in a single command.
You'll have to do one update command to attempt to $inc the field, and if that doesn't change any documents (n = 0), do the update to $set the field to it's default value.