db={"estates":[{"_id":ObjectId("65494baa21b99c8cff188a11"),"name":"Estate 1","buildings":[ObjectId("65494baa21b99c8cff188a14"),ObjectId("65494baa21b99c8cff188a15")]},{"_id":ObjectId("65494baa21b99c8cff188a12"),"name":"Estate 2","buildings":[ObjectId("65494baa21b99c8cff188a16"),ObjectId("65494baa21b99c8cff188a17")]}],"buildings":[{"_id":ObjectId("65494baa21b99c8cff188a14"),"estateId":ObjectId("65494baa21b99c8cff188a11"),"name":"Building 1"},{"_id":ObjectId("65494baa21b99c8cff188a15"),"estateId":ObjectId("65494baa21b99c8cff188a11"),"name":"Building 2"},{"_id":ObjectId("65494baa21b99c8cff188a16"),"estateId":ObjectId("65494baa21b99c8cff188a12"),"name":"Building 3"},{"_id":ObjectId("65494baa21b99c8cff188a17"),"estateId":ObjectId("65494baa21b99c8cff188a12"),"name":"Building 4"}]}

db.estates.aggregate([{"$lookup":{"from":"buildings","localField":"buildings","foreignField":"_id","as":"buildings"}}])