MongoDB- Henter nøjagtigt array-element, undtagen andre

Dette er en standard og forståelig array-of-array misforståelse med MongoDB. Forespørgselskriterierne vil give det korrekte resultat i et dokument , ikke nødvendigvis bare elementerne i et array, du leder efter. Med andre ord, givet dit ønskede mål om at finde DATA NOT FOUND , vil de fleste simple forespørgsler finde ethvert dokument, hvor mindst ét ​​element i arrayet matcher -- men vil ikke bortfiltrere dem, der ikke gør det. Du skal blive lidt mere kompleks for at gøre dette på én gang:

db.foo.aggregate([
// Make sure at *least* one label has a remark of DATA NOT FOUND;
// otherwise, the $filter trick in the next stage yields a labels array
// of length 0 (which is not horrible).  Also, this is a good place to
// add other $match criteria, possibly index-optimized, to shrink down the
// size of response set:
{$match: {"divisionIn.sections.labels.remarks":"DATA NOT FOUND"}}

,{$project: {
        // Copy over the main doc level things we want:
        projectDR: "$projectDR",
        code: "$code",
        status: "$status"

        // divisionIn is a map, not an array, so we can dive down using dot notation
        // and make a new sections array called divSections that will ONLY have
        // DATA NOT FOUND: 
        divSections: {$map: {input: "$divisionIn.sections", as:"z", in:
            {
                // Again, copy over things we want to keep; may not need all of them
                "sectionNumber": "$$z.sectionNumber",
                "sectionName": "$$z.sectionName",

                // The Juice: Copy BUT FILTER the labels field conditionally based on
                // the value of labels.remarks:
                "labels": {$filter: {input: "$$z.labels",
                             as: "z2",
                             cond: {$eq: [ "$$z2.remarks", "DATA NOT FOUND"] }
                    }}
            }
            }}
    }}

                       ]);