db={streets:[{"_id":{"$oid":"6452110c0881247e0a8d17d2"},"steertType":"CHILDREN","streetName":"street 1"},{"_id":{"$oid":"62392110c0881247e0a86729"},"steertType":"CHILDREN","streetName":"street 2"},{"_id":{"$oid":"3322110c0881247e0a867241"},"steertType":"ADULTS","streetName":"street 3"}],cities:[{"_id":{"$oid":"6571bcbd9be813d7440e8055"},"cityName":"Barcelona","linkedStreets":{"CHILDREN":{"streets":["6452110c0881247e0a8d17d2","6452110c0881247e0a86729"]},"ADULTS":{"streets":["332q2110c0881247e0a867241"]}}}]}

db.streets.aggregate([{$match:{steertType:"CHILDREN"}},{$lookup:{from:"cities",let:{steertType:"$steertType"},pipeline:[{$set:{linkedStreets:{$objectToArray:"$linkedStreets"}}},{"$project":{"fieldValue":{$arrayToObject:{$filter:{input:"$linkedStreets",cond:{$eq:["$$this.k","$$steertType"]}}}}}}],as:"result"}}])