@megawubs/avid/source/relations/belongsTo.js

API consumption on fire, inspired by Laravel's Eloquent

import {Api} from "../api";
import {map} from "../map";

export class BelongsTo {
    constructor(parent, child) {
        this.child = child;

        /**
         * create a new instance of the parent.
         * When defining a BelongsTo relation the constructor
         * for the parent relation is passed, here it's newed up
         * and available for us to use.
         */
        this.parent = new parent;

        /**
         * create an api wrapper with the parent as resource uri.
         * this way we can do a get with an id on the parents
         * api endpoint.
         */
        this.api = new Api(this.parent._resource);

        /**
         * If the child has the parent eager loaded we can resolve it without accessing
         * the api.
         */
        if (this.child.properties.hasOwnProperty(this.parent._name)) {
            this.relationValue = this.child.properties[this.parent._name];
        }
    }

    then(callback) {
        let self = this;

        /**
         * when the relation is eager loaded, return the promise that
         * maps it to a proxy.
         */
        if (typeof this.relationValue !== 'undefined') {
            return map(self.parent, self.relationValue).then(callback);
        }

        /**
         * when there is no relation eager loaded, build up an
         * accessor for the relation id, by default Avid assumes it is
         * `modelname_id`.
         *  Example:
         *  When a Home belongs to a User with id 3, the key to access the id of the user
         *  is assumed to be 'user_id'. With this key we can access the
         *  value of Home.user_id, giving us the id of the user this home belongs to.
         */
        let relation = [self.parent._name, "id"].join('_');

        /**
         * Fetch the relation from the api, using it's own api resource endpoint.
         */
        return self.api.find(self.child[relation]).then(user => {
            return map(this.parent, user);
        }).then(callback);
    }
}