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db.collection.aggregate([{"$addFields":{"Colours":{"$map":{"input":"$Colours","as":"color","in":{"$mergeObjects":["$$color",{"TotalStock":{"$reduce":{"input":"$$color.Sizes","initialValue":0,"in":{"$let":{"vars":{"total_stock":"$$value","csize":"$$this"},"in":{"$add":["$$total_stock","$$csize.Stock"]}}}}}}]}}}}}])