Record.constructor.prototype.fromJS = function(values) {
  var nested = fromJS(values, function(key, value){
    //See https://facebook.github.io/immutable-js/docs/#/fromJS for docs on custom reviver functions
    if(this.prototype[key] && this.prototype[key].constructor.prototype instanceof Record){
      return this.prototype[key].constructor.fromJS(value.toJS()); //use toJS() here if nest more than once
    }
    else {
      return value;
    }
  }.bind(this));
  console.log()
  return this(nested);
};

import { fromJS, mergeDeep, OrderedMap, Record } from 'immutable'

// Putting the suggested function here doesn't seem to do anything
// I must be missing some steps to get fromJS() to work with this
// wrapper code.

const mergeEntities = (state, payload) => {
  return state.merge(payload.map( (id) => new Entity(id) ))
}

return mergeEntities(state, fromJS(action.payload.entities));