使BelongsTo下拉选项依赖于第一个选项

<select id="country">
    <option value="usa">USA</option>
    <option value="canada">Canada</option>
</select>

<select id="city">
    <!-- Cities will be loaded here -->
</select>

$(document).ready(function() {
    $('#country').change(function() {
        var country = $(this).val();
        $.ajax({
            url: '/get-cities', // 假设这是一个后端API
            method: 'GET',
            data: { country: country },
            success: function(response) {
                var citySelect = $('#city');
                citySelect.empty();
                $.each(response, function(index, city) {
                    citySelect.append($('<option>', { 
                        value: city.id,
                        text : city.name 
                    }));
                });
            }
        });
    });
});

const express = require('express');
const app = express();

app.get('/get-cities', (req, res) => {
    const country = req.query.country;
    let cities = [];

    if (country === 'usa') {
        cities = [
            { id: 1, name: 'New York' },
            { id: 2, name: 'Los Angeles' }
        ];
    } else if (country === 'canada') {
        cities = [
            { id: 3, name: 'Toronto' },
            { id: 4, name: 'Vancouver' }
        ];
    }

    res.json(cities);
});

app.listen(3000, () => {
    console.log('Server is running on port 3000');
});