[{code:"0WE3A5CMY",name:"lorem",category:"voluptas",variants:[{color:"PapayaWhip",stock:17,barcode:4937310396997},{color:"RoyalBlue",stock:13,barcode:9787252504890},{color:"DodgerBlue",stock:10,barcode:9794456959791},{color:"DodgerBlue",stock:110,barcode:97194456959791}]},{code:"0WE3A5CMX",name:"ipsum",category:"temporibus",variants:[{color:"RoyalBlue",stock:113,barcode:23425202111840},{color:"RoyalBlue",stock:13,barcode:2342520211840},{color:"DodgerBlue",stock:10,barcode:2342520211841}]}]

db.collection.aggregate([{$unwind:"$variants"},{$group:{_id:{"category":"$category","color":"$variants.color"}}},{$replaceRoot:{newRoot:"$_id"}}])