mongodb value is set code example
Example 1: mongodb exists
db.records.find( { a: { $exists: true } } )
Example 2: $set
// https://mongodb.github.io/node-mongodb-native/markdown-docs/insert.html
//To update only selected fields, $set operator needs to be used. Following replacement object replaces author value but leaves everything else intact.
collection.update({_id:"123"}, {$set: {author:"Jessica"}});