How to create an AccessToken in LoopBack for a user

seller-model.js

/** A quick github search for sample code:
 *   > https://github.com/search?utf8=%E2%9C%93&q=-1+AccessToken.create+path%3A%2Fcommon%2Fmodels&type=Code&ref=searchresults
 *  Yields two similar result:
 *   > https://github.com/patriciamolina/test/blob/90a8829728e4a404109e1eafa0a1075687042792/common/models/customer.js#L421
 *   > https://github.com/stormpath/loopback-stormpath/blob/e1cb4d98eacfe36ade2d58e1a48e6cfd590308ff/common/models/stormpath-user.js#L458
 */

module.exports = function(SellerModel) {
  SellerModel.observe('after save', function(ctx, next) {
    console.log('`after save` supports isNewInstance?', ctx.isNewInstance !== undefined);
    if (ctx.isNewInstance !== undefined && ctx.isNewInstance) {
      if (ctx.instance) { // don't want/expect to work with an array of SellerModels being created
        console.log('Will create an additional AccessToken for user');
        ctx.instance.accessTokens.create({
          ttl:-1 // https://github.com/strongloop/loopback/pull/1797
        }, function(err, obj){
          if(err){
            next(err);
          } else {
            next();
          }
        });
      }
      else {
        next();
      }
    }
    else {
      next();
    }
  });
};