[{"id":"1234","name":"test","elements":[{"name":"element1","type":"exaple"},{"name":"element2","type":"important"},{"name":"element3","type":"exaple"},{"name":"element4","type":"imporant"},{"name":"element5","type":"exaple"}]}]

db.collection.aggregate([{"$project":{result:{"$reduce":{"input":"$elements","initialValue":[],"in":{"$cond":{"if":{$eq:["important","$$this.type"]},"then":{$setUnion:["$$value",["$$this.name"]]},"else":"$$value"}}}}}}])