Database
[{price:1,title:"ball"},{price:2,title:"umbrella"},{price:3,title:"cactus"},{price:1,title:"penguin"},{price:2,title:"chair"},{price:5,title:"map"},{price:1,title:"mug"}]
Query
db.collection.update({},[{$replaceRoot:{newRoot:{$cond:[{$gt:["$price",4]},"$$ROOT",{a:12,b:15}]}}}],{multi:true,upsert:true})