index.js
function modelInstance(data){
    var self = this;
    // Add any user supplied schema instance methods.
    if(schema){
      _.assign(self, schema.methods);
    }
    Model.call(self, data);
  }
function modelInstance(data){
    var self = this;
    // Add any user supplied schema instance methods.
    if(schema){
      _.assign(self, schema.methods);
    }
    Model.call(self, data);
  }
create a neweable function object.