Project matching field value from array - mongodb

Question

I want to change the document structure and only display the definition in the definitionarray that matches the idLanguage in the document. How do I do that?
Example of a document with an definition array of 3 elements (three different id of idLanguage):
{
"_id" : ObjectId("59bc29897d7934a6a7577ee0"),
"reference" : "FIIG=A23900 INC=62356",
"idTerm" : "0161-1#TM-218801#1",
"idLanguage" : "0161-1#LG-000002#1",
"statusTerm" : 0,
"idOrganisation" : "0161-1#OG-000194#1",
"idConcept" : "0161-1#01-000001#1",
"term" : "ZRYCHLOVAC ZçVERU, KE KULOMETU ",
"definition" : [
{
"_id" : ObjectId("59bc0bd77d7934a6a7243f05"),
"reference" : "FIIG=A23900 INC=62356",
"idDefinition" : "0161-1#DF-000001#1",
"idLanguage" : "0161-1#LG-000001#1",
"statusDefinition" : 0,
"idOrganisation" : "0161-1#OG-002462#1",
"definition" : "A metallic claw shaped pivoting item, designed to accelerate the weapon's recovery from recoil by assisting in realigning the breech with the barrel.",
"idConcept" : "0161-1#01-000001#1"
},
{
"_id" : ObjectId("59bc29047d7934a6a7370782"),
"reference" : "FIIG=A23900 INC=62356",
"idDefinition" : "0161-1#DF-283090#1",
"idLanguage" : "0161-1#LG-000002#1",
"statusDefinition" : 0,
"idOrganisation" : "0161-1#OG-000194#1",
"definition" : "Kovov‡ otocn‡ p‡kov‡ polo_ka pro zrychlov‡n’ obnoven’ stavu zbrane pred zpetn_m r‡zem /v_strelem t’m, _e napom‡h‡ osov_mu ztoto_nen’ z‡vorn’ku /z‡veru s hlavn’.",
"idConcept" : "0161-1#01-000001#1"
},
{
"_id" : ObjectId("59bc290b7d7934a6a73ce124"),
"reference" : "FIIG=A23900 INC=62356",
"idDefinition" : "0161-1#DF-668740#1",
"idLanguage" : "0161-1#LG-000005#1",
"statusDefinition" : 0,
"idOrganisation" : "0161-1#OG-000200#1",
"definition" : "Metalowy element wahliwy w ksztalcie szpona, przeznaczony do przyspieszenia powrotu broni po odrzucie poprzez wspomaganie ponownego ustawienia w linii zamka i lufy.",
"idConcept" : "0161-1#01-000001#1"
}
]
}

Answer 1

You can use $indexOfArray and $arrayElemAt to match the values. Earlier questions suggest you are using MongoDB 3.4 at least, so that should not be a problem:
db.collection.aggregate([
{ "$addFields": {
"definition": {
"$arrayElemAt": [
"$definition.definition",
{ "$indexOfArray": [
"$definition.idLanguage",
"$idLanguage"
}}
]
}
}}
])
That extracts from the array by "definition" ( the field ) at the matched position of idLanguage. So you would be replacing the "array" with the singular value that matches between those properties.

Answer 2

You may want this if understood your problem correctly
db.collection.aggregate([
{
$project: {
reference: 1,
defination: {
$filter: {
input: "$definition",
as: "elem",
cond: {$eq: ["$$elem.idLanguage", "0161-1#LG-000001#1"]}
// instead of "0161-1#LG-000001#1" you can use your variable
}
}
}
}
])
OR to return only definition.definition
db.collection.aggregate([
{"$unwind": "$definition"},
{"$match": {"definition.idLanguage": "0161-1#LG-000001#1"}},
{
$group: {
_id: "$_id",
defination: {$push: "$definition.definition"}
}
}
])