[{"_id":1,entity1:1,entity2:6},{"_id":2,entity1:0,entity2:3}]

db.collection.aggregate([{$group:{_id:"",data:{$mergeObjects:{$arrayToObject:[[{k:{$toString:"$_id"},v:{"entity1":"$entity1","entity2":"$entity2"}}]]},}}},{$replaceRoot:{newRoot:"$data"}}])