[{query_a:[{"ID":"c80ea2cb-3272-77ae-8f46-d95de600c5bf","date_a":"1"},{"ID":"c80ea2cb-3272-77ae-8f46-d95de600c5bf","date_a":"2"},{"ID":"cdbcc129-548a-9d51-895a-1538200664e6","date_a":"3"},{"ID":"cdbcc129-548a-9d51-895a-1538200664e6","date_a":"4"},{"ID":"cdbcc129-548a-9d51-895a-1538200664e6","date_a":"5"},{"ID":"a4ece1ba-42ae-e735-17b0-f619daa506f9","date_a":"6"},{"ID":"a4ece1ba-42ae-e735-17b0-f619daa506f9","date_a":"7"},{"ID":"a4ece1ba-42ae-e735-17b0-f619daa506f9","date_a":"8"},{"ID":"a4ece1ba-42ae-e735-17b0-f619daa506f9","date_a":"9"}],query_b:[{"ID":"c80ea2cb-3272-77ae-8f46-d95de600c5bf","date_b":"10"},{"ID":"cdbcc129-548a-9d51-895a-1538200664e6","date_b":"11"},{"ID":"a4ece1ba-42ae-e735-17b0-f619daa506f9","date_b":"12"}],}]

db.collection.aggregate([{"$unwind":"$query_a"},{"$unwind":"$query_b"},{"$match":{$expr:{"$eq":["$query_a.ID","$query_b.ID"]}}},{"$project":{union:{"$mergeObjects":["$query_a","$query_b"]}}}])