Database
db={"collection":[{"_id":ObjectId("5ce72e415267960532b8df09"),"name":"foo1","desc":"first foo"},{"_id":ObjectId("5ce72e4a5267960532b8df0a"),"name":"foo2","desc":"second foo"}],"other":[{"_id":ObjectId("5ce72e0c5267960532b8df06"),"name":"bar1","foo":"foo1"},{"_id":ObjectId("5ce72e165267960532b8df07"),"name":"bar2","foo":"foo1"},{"_id":ObjectId("5ce72e1d5267960532b8df08"),"name":"bar3","foo":"foo2"}]}
Query
db.collection.aggregate([{$lookup:{from:"other",localField:"name",foreignField:"foo",as:"bars"}},{$addFields:{bars:"$bars.name"}}])