How can i display count of related documents on parent level? - mongodb

Question

I'm trying to build a voting system where you can have X num of options to vote for on an entry.
The query I'm building now is when retrieving an entry, I would like to get the numbers of votes per option on an entry.
I have a very clear understanding of how I would do this in SQL but grasping to understand the concepts of aggregation, lookup, and group in MongoDB
The model looks like this:
Entries
{
"_id": "5fc2765938401a2308e18ac5",
"options": [
{
"name": "First Option"
"_id": "5fc2765938401a2308e18ac6",
},
{
"name": "Second Option"
"_id": "5fc2765938401a2308e18are",
},
{
"name": "Third Option"
"_id": "5fc2765938401a2308e18aef",
},
],
},
{
"_id": "5fc2766438401a2308e18ac8",
"options": [
{
"name": "Some other option"
"_id": "5fc2766438401a2308e18ac9",
},
{
"_id": "5fc2766438401a2308e18aca",
"name": "This is also an option"
}
],
}
Votes
{
"_id": "5fc2765938401a2308e18ac5",
"entryId": "5fc2765938401a2308e18ac6"
},
{
"_id": "5fc2765938401a2308e18aer",
"entryId": "5fc2765938401a2308e18are"
},
{
"_id": "5fc2765938401a2308e18ek",
"entryId": "5fc2765938401a2308e18ac6"
}
...
And I want the results of Entry to look like this.
{
"_id": "5fc2765938401a2308e18ac5",
"options": [
{
"name": "First Option"
"_id": "5fc2765938401a2308e18ac6",
"votes": 1,
},
{
"name": "Second Option"
"_id": "5fc2765938401a2308e18are",
"votes": 0,
},
{
"name": "Third Option"
"_id": "5fc2765938401a2308e18aef",
"votes": 5,
},
],
},
{
"_id": "5fc2766438401a2308e18ac8",
"options": [
{
"name": "Some other option"
"_id": "5fc2766438401a2308e18ac9",
"votes": 3,
},
{
"_id": "5fc2766438401a2308e18aca",
"name": "This is also an option"
"votes": 10,
}
],
}

Answer 1

$lookup to join votes collection, pass local field optoins._id and foreign field entryId
$project get options votes, $map to iterate loop of options array, $filter to get matching entryId records and $size to get count of element in return array, merge votes field and current object using $mergeObjects
db.entries.aggregate([
{
$lookup: {
from: "votes",
localField: "options._id",
foreignField: "entryId",
as: "votes"
}
},
{
$project: {
options: {
$map: {
input: "$options",
as: "a",
in: {
$mergeObjects: [
"$$a",
{
votes: {
$size: {
$filter: {
input: "$votes",
cond: { $eq: ["$$this.entryId", "$$a._id"] }
}
}
}
}
]
}
}
}
}
}
])
Playground