    /**
     * @author Ed Spencer
     * @class Ext.data.association.BelongsTo
     *
     * Represents a many to one association with another model. The owner model is expected to have
     * a foreign key which references the primary key of the associated model:
     *
     *     Ext.define('Category', {
     *         extend: 'Ext.data.Model',
     *         fields: [
     *             { name: 'id',   type: 'int' },
     *             { name: 'name', type: 'string' }
     *         ]
     *     });
     *
     *     Ext.define('Product', {
     *         extend: 'Ext.data.Model',
     *         fields: [
     *             { name: 'id',          type: 'int' },
     *             { name: 'category_id', type: 'int' },
     *             { name: 'name',        type: 'string' }
     *         ],
     *         // we can use the belongsTo shortcut on the model to create a belongsTo association
     *         associations: [
     *             { type: 'belongsTo', model: 'Category' }
     *         ]
     *     });
     *
     * In the example above we have created models for Products and Categories, and linked them together
     * by saying that each Product belongs to a Category. This automatically links each Product to a Category
     * based on the Product's category_id, and provides new functions on the Product model:
     *
     * ## Generated getter function
     *
     * The first function that is added to the owner model is a getter function:
     *
     *     var product = new Product({
     *         id: 100,
     *         category_id: 20,
     *         name: 'Sneakers'
     *     });
     *
     *     product.getCategory(function(category, operation) {
     *         // do something with the category object
     *         alert(category.get('id')); // alerts 20
     *     }, this);
     *
     * The getCategory function was created on the Product model when we defined the association. This uses the
     * Category's configured {@link Ext.data.proxy.Proxy proxy} to load the Category asynchronously, calling the provided
     * callback when it has loaded.
     *
     * The new getCategory function will also accept an object containing success, failure and callback properties
     * - callback will always be called, success will only be called if the associated model was loaded successfully
     * and failure will only be called if the associatied model could not be loaded:
     *
     *     product.getCategory({
     *         reload: true, // force a reload if the owner model is already cached
     *         callback: function(category, operation) {}, // a function that will always be called
     *         success : function(category, operation) {}, // a function that will only be called if the load succeeded
     *         failure : function(category, operation) {}, // a function that will only be called if the load did not succeed
     *         scope   : this // optionally pass in a scope object to execute the callbacks in
     *     });
     *
     * In each case above the callbacks are called with two arguments - the associated model instance and the
     * {@link Ext.data.Operation operation} object that was executed to load that instance. The Operation object is
     * useful when the instance could not be loaded.
     *
     * Once the getter has been called on the model, it will be cached if the getter is called a second time. To
     * force the model to reload, specify reload: true in the options object.
     *
     * ## Generated setter function
     *
     * The second generated function sets the associated model instance - if only a single argument is passed to
     * the setter then the following two calls are identical:
     *
     *     // this call...
     *     product.setCategory(10);
     *
     *     // is equivalent to this call:
     *     product.set('category_id', 10);
     *
     * An instance of the owner model can also be passed as a parameter.
     *
     * If we pass in a second argument, the model will be automatically saved and the second argument passed to
     * the owner model's {@link Ext.data.Model#save save} method:
     *
     *     product.setCategory(10, function(product, operation) {
     *         // the product has been saved
     *         alert(product.get('category_id')); //now alerts 10
     *     });
     *
     *     //alternative syntax:
     *     product.setCategory(10, {
     *         callback: function(product, operation), // a function that will always be called
     *         success : function(product, operation), // a function that will only be called if the load succeeded
     *         failure : function(product, operation), // a function that will only be called if the load did not succeed
     *         scope   : this //optionally pass in a scope object to execute the callbacks in
     *     })
     *
     * ## Customisation
     *
     * Associations reflect on the models they are linking to automatically set up properties such as the
     * {@link #primaryKey} and {@link #foreignKey}. These can alternatively be specified:
     *
     *     Ext.define('Product', {
     *         fields: [...],
     *
     *         associations: [
     *             { type: 'belongsTo', model: 'Category', primaryKey: 'unique_id', foreignKey: 'cat_id' }
     *         ]
     *     });
     *
     * Here we replaced the default primary key (defaults to 'id') and foreign key (calculated as 'category_id')
     * with our own settings. Usually this will not be needed.
     */
    Ext.define('Ext.data.association.BelongsTo', {
        extend: 'Ext.data.association.Association',
        alternateClassName: 'Ext.data.BelongsToAssociation',
        alias: 'association.belongsto',

        /**
         * @cfg {String} foreignKey The name of the foreign key on the owner model that links it to the associated
         * model. Defaults to the lowercased name of the associated model plus "_id", e.g. an association with a
         * model called Product would set up a product_id foreign key.
         *
         *     Ext.define('Order', {
         *         extend: 'Ext.data.Model',
         *         fields: ['id', 'date'],
         *         hasMany: 'Product'
         *     });
         *
         *     Ext.define('Product', {
         *         extend: 'Ext.data.Model',
         *         fields: ['id', 'name', 'order_id'], // refers to the id of the order that this product belongs to
         *         belongsTo: 'Order'
         *     });
         *     var product = new Product({
         *         id: 1,
         *         name: 'Product 1',
         *         order_id: 22
         *     }, 1);
         *     product.getOrder(); // Will make a call to the server asking for order_id 22
         *
         */

        /**
         * @cfg {String} getterName The name of the getter function that will be added to the local model's prototype.
         * Defaults to 'get' + the name of the foreign model, e.g. getCategory
         */

        /**
         * @cfg {String} setterName The name of the setter function that will be added to the local model's prototype.
         * Defaults to 'set' + the name of the foreign model, e.g. setCategory
         */

        /**
         * @cfg {String} type The type configuration can be used when creating associations using a configuration object.
         * Use 'belongsTo' to create a BelongsTo association.
         *
         *     associations: [{
         *         type: 'belongsTo',
         *         model: 'User'
         *     }]
         */
        constructor: function(config) {
            this.callParent(arguments);

            var me             = this,
                ownerProto     = me.ownerModel.prototype,
                associatedName = me.associatedName,
                getterName     = me.getterName || 'get' + associatedName,
                setterName     = me.setterName || 'set' + associatedName;

            Ext.applyIf(me, {
                name        : associatedName,
                foreignKey  : associatedName.toLowerCase() + "_id",
                instanceName: associatedName + 'BelongsToInstance',
                associationKey: associatedName.toLowerCase()
            });

            ownerProto[getterName] = me.createGetter();
            ownerProto[setterName] = me.createSetter();
        },

        /**
         * @private
         * Returns a setter function to be placed on the owner model's prototype
         * @return {Function} The setter function
         */
        createSetter: function() {
            var me = this,
                foreignKey = me.foreignKey;

            //'this' refers to the Model instance inside this function
            return function(value, options, scope) {
                // If we pass in an instance, pull the id out
                if (value && value.isModel) {
                    value = value.getId();
                }
                this.set(foreignKey, value);

                if (Ext.isFunction(options)) {
                    options = {
                        callback: options,
                        scope: scope || this
                    };
                }

                if (Ext.isObject(options)) {
                    return this.save(options);
                }
            };
        },

        /**
         * @private
         * Returns a getter function to be placed on the owner model's prototype. We cache the loaded instance
         * the first time it is loaded so that subsequent calls to the getter always receive the same reference.
         * @return {Function} The getter function
         */
        createGetter: function() {
            var me              = this,
                associatedName  = me.associatedName,
                associatedModel = me.associatedModel,
                foreignKey      = me.foreignKey,
                primaryKey      = me.primaryKey,
                instanceName    = me.instanceName;

            //'this' refers to the Model instance inside this function
            return function(options, scope) {
                options = options || {};

                var model = this,
                    foreignKeyId = model.get(foreignKey),
                    success,
                    instance,
                    args;

                if (options.reload === true || model[instanceName] === undefined) {
                    instance = Ext.ModelManager.create({}, associatedName);
                    instance.set(primaryKey, foreignKeyId);

                    if (typeof options == 'function') {
                        options = {
                            callback: options,
                            scope: scope || model
                        };
                    }

                    // Overwrite the success handler so we can assign the current instance
                    success = options.success;
                    options.success = function(rec){
                        model[instanceName] = rec;
                        if (success) {
                            success.apply(this, arguments);
                        }
                    };

                    associatedModel.load(foreignKeyId, options);
                    // assign temporarily while we wait for data to return
                    model[instanceName] = instance;
                    return instance;
                } else {
                    instance = model[instanceName];
                    args = [instance];
                    scope = scope || options.scope || model;

                    //TODO: We're duplicating the callback invokation code that the instance.load() call above
                    //makes here - ought to be able to normalize this - perhaps by caching at the Model.load layer
                    //instead of the association layer.
                    Ext.callback(options, scope, args);
                    Ext.callback(options.success, scope, args);
                    Ext.callback(options.failure, scope, args);
                    Ext.callback(options.callback, scope, args);

                    return instance;
                }
            };
        },

        /**
         * Read associated data
         * @private
         * @param {Ext.data.Model} record The record we're writing to
         * @param {Ext.data.reader.Reader} reader The reader for the associated model
         * @param {Object} associationData The raw associated data
         */
        read: function(record, reader, associationData){
            record[this.instanceName] = reader.read([associationData]).records[0];
        }
    });