Mongoose fulltext search - unexpected results

I am using Mongoose to provide a search facility within a web app.

I am scraping some text elsewhere, and placing it in a SearchDoc collection. I am then wishing to do a fulltext search on these documents.

Suppose the user is looking for 'biomarkers' in the text.

The issue is that partial queries don't seem to return results. For example:

• User searches for 'bio' : no results returned
• User searches for 'biomar' : no results returned
• User searches for 'biomarker' : results are found

I would expect all the above to return results for 'biomarker' in the text.

models/search-doc.js:

let mongoose = require('mongoose'),
  Schema = mongoose.Schema;

mongoose.Promise = require('bluebird');

let SearchDoc = new Schema({
  title : {type : String},
  description : {type : String},
  tags : {type : Array},
  body : {type : String},
  thumb :  { data: Buffer, contentType: String },
  docType : {type : String},
  docId : {type : String}
}, {strict : true});


let fields = { title: 'text', description: 'text', tags: 'text', body: 'text' };
let opts = {
  name : 'textIndex',
  weights : {title: 10, description: 5, tags : 3, body : 1},
  default_language: 'en'
};


SearchDoc.index(fields, opts);


module.exports = mongoose.model('SearchDoc', SearchDoc);

And here is the query:

SearchDoc
.find({
  $text: {
    $search: q
  }
}, {
  score: {
    $meta: 'textScore'
  }
})
.sort({
  score: {
    $meta: 'textScore'
  }
})
//.lean()
.exec((err, data) => {
  if (err) {
    return reject(err.toString());
  }

  // Convert data
  let highlightedResults = this.highlightResults(q, data);
  resolve(highlightedResults);
});

via bristol.james