[{"username":"joy","size_info":[{"data1":"apple bear cat","data2":100},{"data1":"dog eat fog good ","data2":100},{"data1":"hug ill jump","data2":100}]}]

db.collection.update({},[{$set:{size_info:{$map:{input:"$size_info",in:{$mergeObjects:["$$this",{data3:{$trim:{input:{$reduce:{input:{$firstN:{input:{$split:["$$this.data1"," "]},n:2}},initialValue:"",in:{$concat:["$$value"," ","$$this"]}}}}}}]}}}}}])