// Aggregate from user to user's settings
var custUsers = db.customers.aggregate([
    {  $match: { active: true } },
//     {  $project: { lang: { $ifNull: [ "$lang", "en" ] } } },
    {
        $lookup:
        {
            from: "users",
            localField: "user",
            foreignField: "_id",
            as: "c_users"
        }
    },
    { $unwind: { path: "$c_users", "preserveNullAndEmptyArrays": true } },
    {
        $project: {
            '_id': 0,         
            'id': '$c_users._id',
            'name': 1,
//             'contact': 1,            
            'username':'$c_users.username'
        }
    }
]).toArray();
// print (custUsers)
var phead = true, hline = '', line = '';
custUsers.forEach(cu => {
    for (var p in cu) {
        
        if (phead) {
            if (p == 'id')
                hline = 'userId,' + hline;
            else 
                hline += p + ','
        }
        if (p == 'id')
            line = cu[p] + ',' + line;
        else { 
//             if (cu[p].indexOf(',') != -1)
//                 print (cu[p]) 
            cu[p] = (cu[p] || '').replace(/,/g, '');
            line = line + cu[p] + ',';
        }
    }
    if (phead) {
        print (hline.slice(0, -1))
        phead = false;
    }
    print (line.slice(0, -1))
    line = '';
});