Does $limit stop fetching if one result matched?

Question

I do not clearly understand what does $limit in aggregate. I'm using aggregate because I need to populate my item, but I can't do it in a findOne (don't know why, but it always return nothing).

return Item.aggregate(
    [
        { $match: { _id: mongoose.Types.ObjectId(id) } },
        { $limit: 1 },
        {
            $lookup: {
                from: 'variations',
                localField: '_id',
                foreignField: 'item',
                as: 'variations'
            }
        }
    ]
).then(result => { console.log(result) });

In this case, is fetching stop when one result match, or is $limit just keep one item from the total result ? In the second case, how can I stop fetching when an item is matching ?

Answer 1

$limit is used to restrict the number of items passed through to the next stage in an aggregation pipeline. say your $match stage selected records by { status : 'active' } and it produced 1 million records and you only need the top 10 records passed to the next stage or to return those 10 records to the client then you'd add a { $limit : 10 } stage.

but in your case, since you're matching by objectId, you don't need the $limit stage because objectIds are unique and there will only ever be a single record with that Id. so you can remove it.

also you cannot use the find interface for retrieval because it doesn't allow you to do joins/lookups with another collection. find interface is for directly retrieving records from a single collection. so you need to use the aggregation framework for scenarios such as lookups.