/**
 * The `Model` class represents a local data resource. It provides methods to
 * persist changes via the API.
 *
 * @abstract
 */
export default class Model {
  /**
   * @param {Object} data A resource object from the API.
   * @param {Store} store The data store that this model should be persisted to.
   * @public
   */
  constructor(data = {}, store) {
    /**
     * The resource object from the API.
     *
     * @type {Object}
     * @public
     */
    this.data = data;

    /**
     * The time at which the model's data was last updated. Watching the value
     * of this property is a fast way to retain/cache a subtree if data hasn't
     * changed.
     *
     * @type {Date}
     * @public
     */
    this.freshness = new Date();

    /**
     * Whether or not the resource exists on the server.
     *
     * @type {Boolean}
     * @public
     */
    this.exists = false;

    /**
     * The data store that this resource should be persisted to.
     *
     * @type {Store}
     * @protected
     */
    this.store = store;
  }

  /**
   * Get the model's ID.
   *
   * @return {Integer}
   * @public
   * @final
   */
  id() {
    return this.data.id;
  }

  /**
   * Get one of the model's attributes.
   *
   * @param {String} attribute
   * @return {*}
   * @public
   * @final
   */
  attribute(attribute) {
    return this.data.attributes[attribute];
  }

  /**
   * Merge new data into this model locally.
   *
   * @param {Object} data A resource object to merge into this model
   * @public
   */
  pushData(data) {
    // Since most of the top-level items in a resource object are objects
    // (e.g. relationships, attributes), we'll need to check and perform the
    // merge at the second level if that's the case.
    for (const key in data) {
      if (typeof data[key] === 'object') {
        this.data[key] = this.data[key] || {};

        // For every item in a second-level object, we want to check if we've
        // been handed a Model instance. If so, we will convert it to a
        // relationship data object.
        for (const innerKey in data[key]) {
          if (data[key][innerKey] instanceof Model) {
            data[key][innerKey] = {data: Model.getRelationshipData(data[key][innerKey])};
          }
          this.data[key][innerKey] = data[key][innerKey];
        }
      } else {
        this.data[key] = data[key];
      }
    }

    // Now that we've updated the data, we can say that the model is fresh.
    // This is an easy way to invalidate retained subtrees etc.
    this.freshness = new Date();
  }

  /**
   * Merge new attributes into this model locally.
   *
   * @param {Object} attributes The attributes to merge.
   * @public
   */
  pushAttributes(attributes) {
    this.pushData({attributes});
  }

  /**
   * Merge new attributes into this model, both locally and with persistence.
   *
   * @param {Object} attributes The attributes to save. If a 'relationships' key
   *     exists, it will be extracted and relationships will also be saved.
   * @return {Promise}
   * @public
   */
  save(attributes) {
    const data = {
      type: this.data.type,
      id: this.data.id,
      attributes
    };

    // If a 'relationships' key exists, extract it from the attributes hash and
    // set it on the top-level data object instead. We will be sending this data
    // object to the API for persistence.
    if (attributes.relationships) {
      data.relationships = {};

      for (const key in attributes.relationships) {
        const model = attributes.relationships[key];

        data.relationships[key] = {
          data: model instanceof Array
            ? model.map(Model.getRelationshipData)
            : Model.getRelationshipData(model)
        };
      }

      delete attributes.relationships;
    }

    // Before we update the model's data, we should make a copy of the model's
    // old data so that we can revert back to it if something goes awry during
    // persistence.
    const oldData = JSON.parse(JSON.stringify(this.data));

    this.pushData(data);

    return app.request({
      method: this.exists ? 'PATCH' : 'POST',
      url: app.forum.attribute('apiUrl') + this.apiEndpoint(),
      data: {data}
    }).then(
      // If everything went well, we'll make sure the store knows that this
      // model exists now (if it didn't already), and we'll push the data that
      // the API returned into the store.
      payload => {
        this.store.data[payload.data.type] = this.store.data[payload.data.type] || {};
        this.store.data[payload.data.type][payload.data.id] = this;
        return this.store.pushPayload(payload);
      },

      // If something went wrong, though... good thing we backed up our model's
      // old data! We'll revert to that and let others handle the error.
      response => {
        this.pushData(oldData);
        throw response;
      }
    );
  }

  /**
   * Send a request to delete the resource.
   *
   * @param {Object} data Data to send along with the DELETE request.
   * @return {Promise}
   * @public
   */
  delete(data) {
    if (!this.exists) return m.deferred.resolve().promise;

    return app.request({
      method: 'DELETE',
      url: app.forum.attribute('apiUrl') + this.apiEndpoint(),
      data
    }).then(() => {
      this.exists = false;
      this.store.remove(this);
    });
  }

  /**
   * Construct a path to the API endpoint for this resource.
   *
   * @return {String}
   * @protected
   */
  apiEndpoint() {
    return '/' + this.data.type + (this.exists ? '/' + this.data.id : '');
  }

  /**
   * Generate a function which returns the value of the given attribute.
   *
   * @param {String} name
   * @param {function} [transform] A function to transform the attribute value
   * @return {*}
   * @public
   */
  static attribute(name, transform) {
    return function() {
      const value = this.data.attributes && this.data.attributes[name];

      return transform ? transform(value) : value;
    };
  }

  /**
   * Generate a function which returns the value of the given has-one
   * relationship.
   *
   * @param {String} name
   * @return {Model|Boolean|undefined} false if no information about the
   *     relationship exists; undefined if the relationship exists but the model
   *     has not been loaded; or the model if it has been loaded.
   * @public
   */
  static hasOne(name) {
    return function() {
      if (this.data.relationships) {
        const relationship = this.data.relationships[name];

        if (relationship) {
          return app.store.getById(relationship.data.type, relationship.data.id);
        }
      }

      return false;
    };
  }

  /**
   * Generate a function which returns the value of the given has-many
   * relationship.
   *
   * @param {String} name
   * @return {Array|Boolean} false if no information about the relationship
   *     exists; an array if it does, containing models if they have been
   *     loaded, and undefined for those that have not.
   * @public
   */
  static hasMany(name) {
    return function() {
      if (this.data.relationships) {
        const relationship = this.data.relationships[name];

        if (relationship) {
          return relationship.data.map(data => app.store.getById(data.type, data.id));
        }
      }

      return false;
    };
  }

  /**
   * Transform the given value into a Date object.
   *
   * @param {String} value
   * @return {Date|null}
   * @public
   */
  static transformDate(value) {
    return value ? new Date(value) : null;
  }

  /**
   * Get a relationship data object for the given model.
   *
   * @param {Model} model
   * @return {Object}
   * @protected
   */
  static getRelationshipData(model) {
    return {
      type: model.data.type,
      id: model.data.id
    };
  }
}