Mongoose: Add data to returned result set
Once you convert the resulting docs to plain JS objects by using lean(), they won't have any of the Mongoose model instance methods available to them like set, so you need to directly manipulate them using normal JavaScript object techniques:
searchQuery.lean().exec(function (err, authors) { authors = authors.map(function(author) { author.thumbnail = 'test'; return author; }); res.json(authors);});If you want to maintain the results as mongoose docs, then you need to pass {strict: false} as a third parameter to set to allow arbitrary fields to be added:
searchQuery.exec(function (err, authors) { authors = authors.map(function(author) { author.set('thumbnail', 'test', {strict: false}); return author; }); res.json(authors);});