How can I query matching MongoDB documents and return a subset of fields from an array of embedded documents?

Question

I have documents similar to the following:

db.etudiants.insert(
 {
 nom:'emile',
 prenom:'zola',
 cours_reussis:[
 { 
 intitule: ['PAI'],
 Periodes: NumberInt(60),
 note: 60,
 date_reussite: NumberInt(2014),
 dispense: true,
 nom_ecole:'EPFC'
 },
 {
 intitule:'TGP',
 Periodes: NumberInt(40),
 note:75,
 date_reussite: NumberInt(2013),
 dispense: true,
 nom_ecole: 'HEB'
 },
 {
 intitule: 'SO2',
 Periodes: NumberInt(60),
 note: 70,
 date_reussite: NumberInt(2015),
 dispense: true,
 nom_ecole: 'ISFCE'
 },
 {
 intitule: 'IBD',
 Periodes: NumberInt(60),
 note: 80,
 date_reussite: NumberInt(2015),
 dispense:true,
 nom_ecole:'ISFCE'
 }
 ]
 }
)

How can query matching documents and return nom_ecole: 'ISFCE' and year (date_reussite) from the cours_reussis array?

Answer 1

If you want to return a subset of matching array elements in a MongoDB document, the best approach to do so is using the Aggregation Framework. The Aggregation Framework processes data similar to a Unix pipeline: the output of each aggregation stage is passed as input to the next:

db.etudiants.aggregate([
 // Find relevant documents: this can take advantage of an index, if applicable
 { $match: {
 'cours_reussis.nom_ecole': 'ISFCE',
 }},
 // Expand cours_reussis array into a stream of documents
 { $unwind: '$cours_reussis' },
 // Filter based on matching array elements
 { $match: {
 'cours_reussis.nom_ecole': 'ISFCE',
 }},
 // Project desired subset of fields
 { $project: {
 '_id': "$_id",
 'nom': '$nom',
 'prenom': '$prenom',
 'cours_reussis' : { 'nom_ecole': 1, date_reussite: 1 }
 }},
 // Regroup data as per the original document _id
 { $group: {
 _id: "$_id",
 nom: { $first: "$nom" },
 prenom: { $first: "$prenom" },
 cours_reussis: { $push: "$cours_reussis" }
 }}
])

You can debug the aggregation pipeline by starting with a single stage and then adding on additional stages one at a time assuming you are seeing the expected output.

Sample output from MongoDB 3.2:

{
 "_id": ObjectId("58462f40f8475aba9f391216"),
 "nom": "emile",
 "prenom": "zola",
 "cours_reussis": [
 {
 "date_reussite": 2015,
 "nom_ecole": "ISFCE"
 },
 {
 "date_reussite": 2015,
 "nom_ecole": "ISFCE"
 }
 ]
}

Answer 2

You can use Aggregation Pipeline Stages Operators, In the db.collection.aggregate method, pipeline stages appear in an array. Documents pass through the stages in sequence.

Note :

For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.

For Syntax

db.collection.aggregate( [ { <stage> }, ... ] )

$match (aggregation)

Filters the documents to pass only the documents that match the specified condition(s) to the next pipeline stage.

The $match stage has the following prototype form:

{ $match: { <query> } }

As here mention below there is your documents details

enter image description here

After using the $match (aggregation) operator to filters the documents to pass only the documents that match the specified condition(s) to the next pipeline stage.

enter image description here

Behavior

Pipeline Optimization

Place the $match as early in the aggregation pipeline as possible. Because $match limits the total number of documents in the aggregation pipeline, earlier $match operations minimize the amount of processing down the pipe.

If you place a $match at the very beginning of a pipeline, the query can take advantage of indexes like any other db.collection.find() or db.collection.findOne().