Как сделать запрос в Mongo?

var ArticleSchema = new Schema({
  created: {
    type: Date,
    default: Date.now
  },
  title: {
      en : {
          type: String,
          default: '',
          trim: true,
          required: 'Title cannot be blank'
            },
      ru : {
          type: String,
          default: '',
          trim: true,
          required: 'Title cannot be blank'
            },
      fr : {
          type: String,
          default: '',
          trim: true,
          required: 'Title cannot be blank'
          },
      es : {
          type: String,
          default: '',
          trim: true,
          required: 'Title cannot be blank'
          },
      it : {
          type: String,
          default: '',
          trim: true,
          required: 'Title cannot be blank'
          }

  },
...

var locale = i18n.getLocale();//get locale  en
  console.log('locale',locale); //выдаёт en

var query = {
    locale:{ $exists : true, $ne : null }
  };
  Article.find(query).sort('-created').populate('user', 'displayName').exec(function (err, articles) {
      if (err) {
        return res.status(400).send({
          message: errorHandler.getErrorMessage(err)
        });
      } else {
        console.log('articles',articles);
        res.json(articles);
      }
    });