[{"_id":"65e6afb50f89ab48c027c54f","items":[{"A":{"id":"ci33985107542","sku":"2519438"}},{"A":{"id":"ci34045151326","sku":"2517213"}},{"B":{"id":"ci34045151457","sku":"2261539"}},{"B":{"id":"ci34045151509","sku":"2302283"}},{"B":{"id":"ci34045151436","sku":"2520098"}}]}]

db.collection.aggregate([{$unwind:"$items"},{$set:{items:{$objectToArray:"$items"}}},{$group:{_id:{_id:"$_id",key:{$first:"$items.k"}},items:{$push:{$first:"$items.v"}}}},{$group:{_id:"$_id._id",items:{$push:{$arrayToObject:[[{k:"$_id.key",v:"$items"}]]}}}}])