gistfile1.js

//e.g.
var _ = require('underscore');
var common_model_extensions = require('lib/util/model_extensions').common;

// which would be:

module.exports = {
  common: {
    classMethods: {
      updateById: function(id, data, cb){
        logger.debug("Updating record with id", id);
        this.update(data, { where: { id: id } }).then(cb);
      },
      deleteById: function(id, cb){
        logger.debug("Deleting record with id", id);
        this.destroy(data, { where: { id: id } }).then(cb);
      }
    }
  }
}

// then define the model
var Metric = sequelize.define('metrics',{
  date: Sequelize.STRING,
  group: Sequelize.STRING,
  key: Sequelize.STRING,
  value: Sequelize.FLOAT
}, {
  timestamps: false,
  classMethods: _.extend(common_model_extensions,{
    customMethod: function(){
      
    }
  })
});

// then you can call Metric.updateById, essential were are extending Sequalize models rather than adding another layer in front of them