Fix reading of entities containing "length" property.

Ext.bugfix.Reader.js

/**
 * Fix reading of entities containing "length" property
 *
 * @link http://www.sencha.com/forum/showthread.php?292395-data.Reader-throwing-exception-when-model-contains-quot-length-quot-field.&p=1067807
 * @link https://fiddle.sencha.com/#fiddle/at6
 */
Ext.define('Ext.bugfix.Reader', {
  override: 'Ext.data.reader.Reader',

  extractData: function(root, readOptions) {
    var me = this,
        entityType = readOptions && readOptions.model ? Ext.data.schema.Schema.lookupEntity(readOptions.model) : me.getModel(),
        schema = entityType.schema,
        includes = schema.hasAssociations(entityType) && me.getImplicitIncludes(),
        fieldExtractorInfo = me.getFieldExtractorInfo(entityType.fieldExtractors),
        length = root.length,
        records = new Array(length),
        typeProperty = me.getTypeProperty(),
        reader, node, nodeType, record, i;

    if (Ext.isObject(root)) {
      root = [root];
      length = 1;
    }

    for (i = 0; i < length; i++) {
      record = root[i];
      if (!record.isModel) {
        // If we're given a model instance in the data, just push it on
        // without doing any conversion. Otherwise, create a record.
        node = record;

        // This Reader may be configured to produce different model types based on
        // a differentiator field in the incoming data:
        // typeProperty name be a string, a function which yields the child type, or an object: {
        //     name: 'mtype',
        //     namespace: 'MyApp'
        // }
        if (typeProperty && (nodeType = me.getChildType(schema, node, typeProperty))) {

          reader = nodeType.getProxy().getReader();

          record = reader.extractRecord(node, readOptions, nodeType,
            schema.hasAssociations(nodeType) && reader.getImplicitIncludes(),
            reader.getFieldExtractorInfo(nodeType.fieldExtractors));
        } else {
          record = me.extractRecord(node, readOptions, entityType, includes,
            fieldExtractorInfo);
        }

        if (record.isModel) {
          // Trees need to be able to access the raw data (the XML node) in order to process its children.
          record.raw = node;
        }
      }
      if (record.onLoad) {
        record.onLoad();
      }
      records[i] = record;
    }

    return records;
  },

  deprecate: {
    '5.0.2' : {
      methods: {
        extractData: function() {
          //<debug>
          Ext.log.warn("This bugfix is probably deprecated!");
          //</debug>

          return Ext.data.reader.Reader.prototype.extractData.apply(this, arguments);
        }
      }
    }
  }
});

http://www.sencha.com/forum/showthread.php?292395-data.Reader-throwing-exception-when-model-contains-quot-length-quot-field.&p=1067807#post1067807