如何从IQueryable<T>获得IQueryable<IQueryable<T>>？

Question

我正试着从一家供应商那里得到一台PagedList of Orders。它有ICollection of Orders。

CreateAsync方法需要IQueryable of Orders，但我有IQueryable of Venders？

public async Task<PagedList<Order>> GetVenderOrders(int venderId, OrderParams orderParams)
{
    var vender = _dataContext.Venders.Include(v => v.Orders)
        .ThenInclude(o => o.RentedCar).Where(v => v.VenderId == venderId);

    var orders = vender.Select(x => x.Orders.AsQueryable());

    return await PagedList<Order>.CreateAsync(orders, orderParams.PageNumber, orderParams.PageSize);
}

public static async Task<PagedList<T>> CreateAsync(IQueryable<T> source, int pageNumber, int pageSize)
{
    var count = await source.CountAsync();
    var items = await source.Skip((pageNumber - 1) * pageSize).Take(pageSize).ToListAsync();
    return new PagedList<T>(items, count, pageNumber, pageSize);
}

Answer 1

解决此问题的方法是使用SelectMany而不是Select.

选择：返回一个IQueryable，其元素是对源的每个元素调用投影函数的结果。

SelectMany:是一个IQueryable，它的元素是在源的每个元素上调用一对多的投影函数collectionSelector的结果，然后将每个序列元素及其对应的源元素映射到结果元素。

public async Task<PagedList<Order>> GetVenderOrders(int venderId, OrderParams orderParams)
        {
            var orders = _dataContext.Venders.Include(v => v.Orders)
                .ThenInclude(o => o.RentedCar).Where(v => v.VenderId == venderId)
                .SelectMany(x => x.Orders);


            return await PagedList<Order>.CreateAsync(orders ,orderParams.PageNumber, orderParams.PageSize);
            
        }