// # Recline Backends
//
// Backends are connectors to backend data sources and stores
//
// This is just the base module containing a template Base class and convenience methods.
this.recline = this.recline || {};
this.recline.Backend = this.recline.Backend || {};

// ## recline.Backend.Base
//
// Exemplar 'class' for backends showing what a base class would look like.
this.recline.Backend.Base = function() {
  // ### __type__
  //
  // 'type' of this backend. This should be either the class path for this
  // object as a string (e.g. recline.Backend.Memory) or for Backends within
  // recline.Backend module it may be their class name.
  //
  // This value is used as an identifier for this backend when initializing
  // backends (see recline.Model.Dataset.initialize).
  this.__type__ = 'base';

  // ### readonly
  //
  // Class level attribute indicating that this backend is read-only (that
  // is, cannot be written to).
  this.readonly = true;

  // ### sync
  //
  // An implementation of Backbone.sync that will be used to override
  // Backbone.sync on operations for Datasets and Records which are using this backend.
  //
  // For read-only implementations you will need only to implement read method
  // for Dataset models (and even this can be a null operation). The read method
  // should return relevant metadata for the Dataset. We do not require read support
  // for Records because they are loaded in bulk by the query method.
  //
  // For backends supporting write operations you must implement update and delete support for Record objects.
  //
  // All code paths should return an object conforming to the jquery promise API.
  this.sync = function(method, model, options) {
  },
  
  // ### query
  //
  // Query the backend for records returning them in bulk. This method will
  // be used by the Dataset.query method to search the backend for records,
  // retrieving the results in bulk.
  //
  // @param {recline.model.Dataset} model: Dataset model.
  //
  // @param {Object} queryObj: object describing a query (usually produced by
  // using recline.Model.Query and calling toJSON on it).
  //
  // The structure of data in the Query object or
  // Hash should follow that defined in <a
  // href="http://github.com/okfn/recline/issues/34">issue 34</a>.
  // (Of course, if you are writing your own backend, and hence
  // have control over the interpretation of the query object, you
  // can use whatever structure you like).
  //
  // @returns {Promise} promise API object. The promise resolve method will
  // be called on query completion with a QueryResult object.
  // 
  // A QueryResult has the following structure (modelled closely on
  // ElasticSearch - see <a
  // href="https://github.com/okfn/recline/issues/57">this issue for more
  // details</a>):
  //
  // <pre>
  // {
  //   total: // (required) total number of results (can be null)
  //   hits: [ // (required) one entry for each result record
  //     {
  //        _score:   // (optional) match score for record
  //        _type: // (optional) record type
  //        _source: // (required) record/row object
  //     } 
  //   ],
  //   facets: { // (optional) 
  //     // facet results (as per <http://www.elasticsearch.org/guide/reference/api/search/facets/>)
  //   }
  // }
  // </pre>
  this.query = function(model, queryObj) {}
};

// ### makeRequest
// 
// Just $.ajax but in any headers in the 'headers' attribute of this
// Backend instance. Example:
//
// <pre>
// var jqxhr = this._makeRequest({
//   url: the-url
// });
// </pre>
this.recline.Backend.makeRequest = function(data, headers) {
  var extras = {};
  if (headers) {
    extras = {
      beforeSend: function(req) {
        _.each(headers, function(value, key) {
          req.setRequestHeader(key, value);
        });
      }
    };
  }
  var data = _.extend(extras, data);
  return $.ajax(data);
};