我正在使用batch_id来访问CoursePurchaseModel，并使用user_id在startdate和endDate的条件下填充到userModel中，但每次都得到user_id为空。我是mongoose的新手。我错过了什么吗？
第一次

getUsersByBatchIdAccordingDate: async (startDate, endDate, batch_id) => {

    let findifUsers = await module.exports.getUsersAccordingDate(startDate, endDate);
    if (findifUsers.length == 0) {
        return Error("No users found");
    }

    let userData = await CoursePurchasedModel.find({ batch_id: batch_id })
        .populate({
            path:'user_id',
            model:'User',
            match: { createdAt: { $gte: startDate, $lte: endDate } },
            select: 'name email gender number address state pincode unique_code referedBy creditEarned refer_id LastAppOpenedTime createdAt -_id'
        }).exec();

    let userArray = [];
    for (let data of userData) {
        let newVideoObject = { ...data.user_id.toObject() }
        userArray.push(newVideoObject);
    }
    return userArray;
}