[{code:1,pw:"___"},{code:2,pw:"___"},{code:3,pw:"___"},{code:4,pw:"___"}]

db.collection.update({code:{$in:[1,2,3,4]}},[{$set:{pw:{"$getField":{"field":"pw","input":{$first:{$filter:{input:[{code:1,pw:"abc"},{code:2,pw:"grt"},{code:3,pw:"tpo"},{code:4,pw:"xyz"}],cond:{$eq:["$$this.code","$code"]}}}}}}}}],{multi:true})