为什么在then块中返回缓存的jQuery promise无法解决它？

Question

我利用jQuery promises来缓存AJAX请求，当我尝试在then子句中返回缓存的promise时，这种方法似乎会中断;我不确定为什么会这样，并想知道是否有办法解决它。

如果我伪造密钥并将if (self.fGetSearchResults.oCache[...块放在then块之外的缓存定义下，则代码按预期工作。

MCG.searchControls.fGetSearchResults = function fGetSearchResults() {
    var self = this;

    var oDeferred = $$.Deferred(); // `new` keyword is optional and we don't like it
    var oPromise = oDeferred.promise();

    self.fGetSearchResults.oCache = self.fGetSearchResults.oCache || {}; // creates a persistent cache across function calls

    // perform the search
    $$.when(
        self.fPromiseOfFilterObject() 
    ).done(
        function fDoneCallback(oFilterObject) {
            /* create the RecordListService parameters which we will query with */
            ..
            var oRecordListServiceParametersAsKey = JSON.stringify(oRecordListServiceParameters);
            // leverage our cache here
            if (self.fGetSearchResults.oCache[oRecordListServiceParametersAsKey] !== undefined) {
                return self.fGetSearchResults.oCache[oRecordListServiceParametersAsKey];
            } else {
                self.fGetSearchResults.oCache[oRecordListServiceParametersAsKey] = oPromise; // cache the result for next time
            }

            // do the ajax call now
            $.orchestracmsRestProxy.doAjaxServiceRequest('ocms_socfp.RecordListService', oRecordListServiceParameters,
                function(status, json) {
                    ...
                    oDeferred.resolve(json)
                });
        });
    return oPromise; // return a our promise object which will resolve at some point
}