Magento 按国家/地区id筛选客户集合

我正在按性别和姓名收集客户信息。我需要按国家id添加筛选器，但要按地址集合中每个客户的国家id添加筛选器

$collection = Mage::getModel('customer/customer')->getCollection()
        ->addAttributeToSelect('*');
        ->addAttributeToFilter('firstname', $post['firstname']);
        ->addAttributeToFilter('gender', $post['gender']);
        ->load();

所以我需要这样的东西：

$collection = Mage::getModel('customer/customer')->getCollection()
        ->addAttributeToSelect('*');
        ->addAttributeToFilter('firstname', $post['firstname']);
        ->addAttributeToFilter('gender', $post['gender']);

        ->addAttributeToFilter('country_id', $post['country_id']);//US
        ->load();

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如果您觉得有帮助，请接受答案

$collection = Mage::getResourceModel('customer/customer_collection')
    ->joinAttribute('billing_country_id', 'customer_address/country_id', 'default_billing', null, 'left')
    ->addFieldToFilter('billing_country_id','US')
    ;