[{materials:{"m1":{inventory:[{price:100,amount:65}]}}}]

db.collection.update({},[{$set:{"materials.m1.inventory":{$map:{input:{$range:[0,{$size:"$materials.m1.inventory"}]},in:{$cond:[{$eq:["$$this",0]},{$mergeObjects:[{$arrayElemAt:["$materials.m1.inventory","$$this"]},{amount:100}]},{$arrayElemAt:["$materials.m1.inventory","$$this"]}]}}}}}])