表单的 .save()方法 为什么不能把表单填写的数据 成功保存到数据中？

def reporttable(request):
report_table = RepairReport.objects.all()
pre_report_entering_form = ReportEnteringForm()
if  request.method == 'POST':
    pre_report_entering_form = ReportEnteringForm( request.POST )
    if pre_report_entering_form.is_valid():
        report_entering_form = pre_report_entering_form.save( commit = False )
        report_entering_form.save()
        return render_to_response('reporttable.html', RequestContext(request,
                                                                     locals(),
                                                                     )
                                  )
    else:
        return HttpResponse('hello world')
else:
    return render_to_response('reporttable.html',RequestContext(request,
                                                                {'report_entering_form': pre_report_entering_form, 'report_table':report_table}
                                                                )
                              )