• It is quite common to have nested array in documents in MongoDB.
• It is also quite common to have to "flatify" those arrays in queries.

Here is an example where we have a collection with consultants and their professional experiences.

consultant [{
   name:"Toto",
   age: 25,
   experiences: [{
      role: "Sofware Engineer",
      from: "2010-01-01",
      to: "2012-01-01",
      company: "CompanyA"
   }, {
      role: "Data Analyst",
      from: "2018-01-01",
      to: "2020-01-01",
      company: "CompanyB"
   }]
}]

If we would like to list all experiences at a company during a certain period of time, we would have to do an aggregate query with 3 steps that are sometimes error prone:
- $match with $elemMatch
- $unwind
- same $match (without $elemMatch)

"views" exists and make those queries much easier to create, with a simple find
db.experiences.find({company:"CompanyA", from: {$gte: ... $lte...}})

Unfortunately when using "views" the engine simply adds the match constraint at the end of the aggregation which turns out to be not always performant see https://support.mongodb.com/case/00767327.

Would it be possible to create an option when creating a view that would be: Create a "view from nestedArray". It is quite simple to create an aggregation query where the query is better optimised because the match is done in the beginning.

    /** 1st match if elemMatch */
    stages.add(Aggregation.match(Criteria.where(nestedCollection).elemMatch(matchElemCriteria)));
    stages.add(Aggregation.unwind(nestedCollection));
    /** 2nd match without elemMatch */
    stages.add(Aggregation.match(matchCriteria));
    stages.add(new GenericAggregationOperation("$project", "{\"" + nestedCollection +"\": \"$" + nestedCollection + "\"}"));
    stages.add(new GenericAggregationOperation("$lookup", "{from: \"" + parentCollection + "\",\n" + "localField: \"_id\", foreignField: \"_id\", as: \"" + nestedCollection + "." + patientFieldName + "\"}"));
    stages.add(new GenericAggregationOperation("$unwind", "{ path : \"$"+ nestedCollection +"." + patientFieldName + "\"}"));
    stages.add(new GenericAggregationOperation("$replaceRoot", "{newRoot: \"$" + nestedCollection + "\"}"));