Simple remote method

gistfile1.txt

'use strict';

module.exports = function(Coupon) {
  Coupon.isGrabbedASIN = isGrabbedASIN;

  function isGrabbedASIN(productId, userId, cb) {
     Coupon.findOne(
      { where:
          {
            and: [
              { productId: productId },
              { isGrabbed: true },
              { userId: userId }
            ]
          }
      },
      function(err, coupon) {
        if(err || !coupon) {
          cb(null, false);
        }
        cb(null, true);
      });
  }

  Coupon.remoteMethod('isGrabbedASIN', {
    isStatic: true,
    accepts: [
      { arg: 'productId', type: 'string' },
      { arg: 'userId', type: 'string' }
    ],
    returns:{ arg: 'isGrabbed', type: 'boolean' }
  });

};

Using

    "loopback": "2.12.1",
    "loopback-connector-mongodb": "1.7.0",
    "loopback-datasource-juggler": "2.16.0",