How to get entire list of nested inner list groups, and add sort & limit to them. MongoDB query

Question

I have the following model of an object called EvaluationsGroup in a MongoDB collection:

{
    "_id" : "5fecfb83d61ae459df1bceda",
    "Status" : "noDataFound",
    "Evaluations" : [ 
        {
            "EvaluatedAt" : ISODate("2020-12-30T22:13:21.168Z"),
            "ReferenceValue" : 2.0
        }
        {
            "EvaluatedAt" : ISODate("2020-12-30T22:13:15.168Z"),
            "ReferenceValue" : 3.0,
             (... several other properties)
        }
    ]
}

What I want to do is get the entire list of all inner Evaluations of all the 'EvaluationsGroups' indistinctively, sort them by EvaluatedAt date, and limit the results for paging.

How would I make such a query in mongo?

Answer 1

• $project to show required fields
• $unwind deconstruct Evaluations array
• $replaceRoot to replace object in root
• $sort by EvaluatedAt date in descending order
• $skip number of documents for pagination
• $limit number of documents

let page = 0;
let limit = 10;
let skip = page*limit;
db.collection.aggregate([
  {
    $project: {
      _id: 0,
      Evaluations: 1
    }
  },
  { $unwind: "$Evaluations" },
  { $replaceRoot: { newRoot: "$Evaluations" } },
  { $sort: { EvaluatedAt: -1 } },
  { $skip: skip },
  { $limit: limit }
])

Playground