js/libs/ember-data.js
function(key, kind) {
    key = Ember.String.decamelize(key);
    if (kind === "belongsTo") {
      return key + "_id";
    } else if (kind === "hasMany") {
      return Ember.String.singularize(key) + "_ids";
    } else {
      return key;
    }
  }
function(key, kind) {
    key = Ember.String.decamelize(key);
    if (kind === "belongsTo") {
      return key + "_id";
    } else if (kind === "hasMany") {
      return Ember.String.singularize(key) + "_ids";
    } else {
      return key;
    }
  }
Underscores relationship names and appends "_id" or "_ids" when serializing
relationship keys.

@method keyForRelationship
@param {String} key
@param {String} kind
@returns String