found mergeObjects and replaceRoot aggregation methods
in order to merge collections.
examples of official documents are copied down and modified and found that there are no errors available, but there are a few areas that I do not understand. Please refer to you
.related codes
replaceRoot1 = {"$replaceRoot": {"newRoot": {"$mergeObjects": [{"$arrayElemAt": ["$recharge", 0]}, "$$ROOT"]}}}
replaceRoot2 = {"$replaceRoot": {"newRoot": {"$mergeObjects": [{"$arrayElemAt": ["$consume", 0]}, "$$ROOT"]}}}
-sharp replaceRoot
-sharp mergeObjects
-sharp arrayElemAt $replaceRoot , and what do newRoot and $ROOT mean?