[{"_id":"PRQA2311","name":"Binder","category":"Office Supplies","price":3.99,"...":"...","variants":[{"barcode":"2314399903212","attributes":{"color":"red","size":"Letter"}},{"barcode":"2314399903212","attributes":{"color":"red","size":"A4"}},{"barcode":"2314399903212","attributes":{"color":"blue","size":"Letter"}},{"barcode":"2314399903212","attributes":{"color":"blue","size":"A4"}}]}]

db.collection.aggregate([{$unwind:"$variants"},{$group:{_id:{$concat:[{"$toString":"$_id"},".",{"$toString":"$variants.attributes.color"}]},variants:{$push:"$variants"},_root:{$first:"$$ROOT"}}},{$replaceRoot:{newRoot:{$mergeObjects:["$_root",{_id:"$_id",variants:"$variants"}]}}}])