在填充订单之前，您必须首先检索Customer实体。

var customer = await _context.Customers.FirstOrDefaultAsync(c => c.CustomerId == order.CustomerId);

    if(customer == null) {
        return BadRequest();
    }

    order.Customer = customer;

所以，你可以像这样编辑你的方法：

[HttpPost]
public async Task<ActionResult<Order>> PostOrder(Order order)
{
    if (_context.Orders == null)
    {
        return Problem("Entity set 'ecopowerdatabaseContext.Orders'  is null.");
    }
    
    var customer = await _context.Customers.FirstOrDefaultAsync(c => c.CustomerId == order.CustomerId);

    if(customer == null) {
        return BadRequest();
    }

    order.Customer = customer;
    
    _context.Orders.Add(order);
    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateException)
        {
        if (OrderExists(order.OrderId))
        {
            return Conflict();
        }
        else
        {
            throw;
        }
    }

    return CreatedAtAction("GetOrder", new { id = order.OrderId }, order);
}

我找到了解决办法。如果我将订单所需的字段添加到方法的输入参数中，以便在Swagger中将它们显式地输入到它们自己的单独文本框中，那么问题似乎就解决了。而不是将订单对象作为输入参数-因为对象还不存在。然后，我在方法中创建一个新的Order对象，并在将新的order对象传递给Add（）方法之前分配所有必需的属性。正确的代码如下：

[HttpPost]
public async Task<ActionResult<Order>> PostOrder(short OrderId, 
DateTime OrderDate, short CustomerId, string Address)
{
    if (_context.Orders == null)
    {
        return Problem("Entity set 'ecopowerdatabaseContext.Orders'  
        is null.");
    }

    Order order = new Order();
    order.OrderId = OrderId;
    order.OrderDate = OrderDate;
    order.CustomerId = CustomerId;
    order.DeliveryAddress = Address;

    _context.Orders.Add(order);
    try
    {
        await _context.SaveChangesAsync();          
    }
    catch (DbUpdateException)
    {
        if(OrderExists(order.OrderId))
        {
            return Conflict();
        }
        else
        {
            throw;
        }
    }
    return CreatedAtAction("GetOrder", new { id = order.OrderId }, 
    order);
}