MongoDB: query nested arrays with pagination

Question

Lets say I have a collection of the following documents:

{
  _id,
  profile: {...},
  arrayProp1: [....],
  arrayProp2: [....],
  arrayProp3: [....],
  ...
}

Number of objects in arrayPropX arrays may be become big, at least thousands. So I need to use pagination and the question is what is the best way to do this?

For example how to return:

Total number of elements in arrayProp1 for specified _id
X elements from position Y in arrayProp1 for specified _id ?

Same as above but with filter on some field on arrayProp1 elements?

mickl · Accepted Answer · 2020-03-23 14:56:21Z

1

You need two operators: $size and $slice:

db.collection.aggregate([
    {
        $match: { _id: queryId }
    },
    {
        $project: {
            _id: 1,
            size: { $size: "$arrayProp1" } 
        }
    }
])

db.collection.aggregate([
    {
        $match: { _id: queryId }
    },
    {
        $project: {
            _id: 1,
            sliced: { $slice: ["$arrayProp1", Yposition, Xelements] } 
        }
    }
])

answered Mar 23, 2020 at 14:56

mickl

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2 Comments

Дмитрий Алферьев Over a year ago

Thank you, it works fine, but what if I need to sort array before slice? Looks like it is not possible without $unwind first?

mickl Over a year ago

@ДмитрийАлферьев that is correct, you have to $unwind - sorting inner array is one of the features I would like to have in MongoDB

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