What is the equivalent of findOne using .aggregate in Mongodb? - mongodb

Question

Basically with this I manage to return all my objects from a collection. How can I return a single element, for example
   in the style of findOne({_ id:" 5e82d378527bb420a4001aaf ")?
I know how to use $match, but this returns various results.
let _id="5e82d378527bb420a4001aaf"
Noticia.aggregate([
{
$addFields: {
like: {
$cond: [{ $in: [_id, "$likes"] }, true, false]
},
dislike: {
$cond: [{ $in: [_id, "$dislikes"] }, true, false]
}
}
}
], (err, noticia) => {
// console.log(trans);
if (err) {
return res.status(400).json({
ok: false,
err
});
}
return res.status(200).json({
ok: true,
data: noticia
});
})

Answer 1

Consider a sample names collection:
{ _id: 1, name: "Jack", favoriteColor: "blue" },
{ _id: 2, name: "James", favoriteColor: "red" },
{ _id: 3, name: "John", favoriteColor: "blue" }
and run the following three queries using findOne:
db.names.findOne( { _id: 1 } )
db.names.findOne()
db.names.findOne( { favoriteColor : "blue" } )
the result is same for the three queries:
{ "_id" : 1, "name" : "Jack", "favoriteColor" : "blue" }
The equivalent queries respectively using aggregation are the following, with the same result:
db.names.aggregate( [
{ $match: { _id: 1 } },
] )
db.names.aggregate( [
{ $limit: 1 }
] )
db.names.aggregate( [
{ $match: { "favoriteColor" : "blue" } },
{ $limit: 1 }
] )
db.collection.findOne definition says -
Returns one document that satisfies the specified query criteria on
the collection or view. If multiple documents satisfy the query, this
method returns the first document according to the natural order which
reflects the order of documents on the disk.
With findOne if no document is found it returns a null. But an aggregation returns a cursor, and you can apply the cursor methods on the result.

Answer 2

Just use $limit to limit no.of docs to be returned from aggregation pipeline :
Noticia.aggregate([
{
$addFields: {
like: {
$cond: [{ $in: [_id, "$likes"] }, true, false]
},
dislike: {
$cond: [{ $in: [_id, "$dislikes"] }, true, false]
}
}
} , {$limit :1} // will return first doc from aggregation result of '$addFields' stage
], (err, noticia) => {
// console.log(trans);
if (err) {
return res.status(400).json({
ok: false,
err
});
}
return res.status(200).json({
ok: true,
data: noticia
});
})
Or if you wanted to return random single doc try $sample.

Answer 3

//Express and mongoose
exports.getData = (req, res) => {
Noticia.aggregate([
{
$addFields: {
like: {
$cond: [{ $in: [_id, "$likes"] }, true, false]
},
dislike: {
$cond: [{ $in: [_id, "$dislikes"] }, true, false]
}
}
}
]
).then( data=>{
res.send(data[0]); //return one document in format { ..... }
}).catch(err => {
res.status(500).send({
message: err.message
});
});
}