[{"_id":ObjectId("60c07a1f9853c900c2026440"),"ProductID":"A","Colours":[{"ID":"01","Name":"BLACK","Sizes":[{"Code":"S1","Stock":2},{"Code":"S2","Stock":3}]},{"ID":"02","Name":"WHITE","Sizes":[{"Code":"S1","Stock":5},{"Code":"S2","Stock":2}]}]},{"_id":ObjectId("6119802b108d5aa5760c59cf"),"ProductID":"B","Colours":[{"ID":"03","Name":"RED","Sizes":[{"Code":"S1","Stock":3},{"Code":"S2","Stock":5}]},{"ID":"04","Name":"BLUE","Sizes":[{"Code":"S1","Stock":2},{"Code":"S2","Stock":9}]}]}]

db.collection.aggregate([{"$unwind":"$Colours"},{"$addFields":{"Colours.TotalStock":{"$sum":"$Colours.Sizes.Stock"}}},{"$project":{"_id":0,"ProductID":1,"Colours.ID":1,"Colours.Name":1,"Colours.TotalStock":1}}])