在单个函数中包含sqlalchemy的filter和filter_by函数(Wrap sqlalchemy's filter and filter_by functions in a single function)

 把我的评论转化为答案...... 
 
 您应该查看浏览器看到的实际HTML（而不是ASP模板代码），因为我认为jQuery不会在HTML中看到ListItem。 HTML中的标准下拉列表是<option>标记。 你的ASP可能正在生成那个或其他东西。 
 
 在浏览器中执行View / Source并查看浏览器实际看到的HTML并将jQuery编码为该。 $('ListItem', this)可能找不到任何东西。 在编写jQuery来操作它时，你几乎不应该使用ASP代码。 在将ASP模板转换为常规HTML后，您需要知道浏览器实际看到了什么。 
 
 仅供参考，您的代码的简化版本也可以在下拉列表选择更改时运行，如下所示： 
 
$(document).ready(function () {

    function checkDropDownList() {
        if ($("#DropDownList1 option:selected").text() != "a") {
           $("#select").fadeOut();
        } else {
            $("#select").fadeIn();
        }
    }

    checkDropDownList();

    $("#DropDownList1").change(checkDropDownList);

});
 
 没有理由使用.each()如果只有一个选定的项目，我们已经知道只有一个DropDownList1 。 

Turning my comments into an answer...
 
You should look at the actual HTML that the browser sees (not the ASP template code) because I don't think jQuery sees a ListItem in the HTML. Standard dropdowns in HTML are <option> tags. Your ASP might be generating that or something else.
 
Do a View/Source in the browser and look at the HTML that the browser actually sees and code your jQuery to that. $('ListItem', this) probably isn't finding anything. You should pretty much never go by the ASP code when writing jQuery to operate on it. You need to know what the browser actually sees after the ASP template has been turned into regular HTML.
 
FYI, a simplified version of your code that also runs any time the drop down list selection is changed works like this:
 
$(document).ready(function () {

    function checkDropDownList() {
        if ($("#DropDownList1 option:selected").text() != "a") {
           $("#select").fadeOut();
        } else {
            $("#select").fadeIn();
        }
    }

    checkDropDownList();

    $("#DropDownList1").change(checkDropDownList);

});
 
There's no reason to use .each() if there can only be one selected item and we already know there's only one DropDownList1.