Include

// 示例1：简单投影 - 只取需要的字段
// 场景：显示订单列表，只需要订单基本信息和客户名称
var orders = dbContext.Orders
    .Select(o => new OrderListDto
    {
        OrderId = o.Id,
        OrderDate = o.OrderDate,
        TotalAmount = o.TotalAmount,
        CustomerName = o.Customer.Name  // 跨越关系取值
    })
    .ToList();

// 示例2：投影关联集合
// 场景：显示订单详情，包含该订单的所有项目
var orderDetails = dbContext.Orders
    .Where(o => o.Id == orderId)
    .Select(o => new OrderDetailDto
    {
        OrderId = o.Id,
        OrderDate = o.OrderDate,
        CustomerName = o.Customer.Name,
        Items = o.OrderItems.Select(oi => new OrderItemDto
        {
            ProductName = oi.Product.Name,
            Quantity = oi.Quantity,
            UnitPrice = oi.UnitPrice,
            Subtotal = oi.Quantity * oi.UnitPrice
        }).ToList()  // 嵌套投影
    })
    .FirstOrDefault();

// 示例3：匿名类型投影
// 场景：快速查询，不需要定义 DTO
var productSummary = dbContext.Products
    .Select(p => new
    {
        p.Id,
        p.Name,
        p.Price,
        CategoryName = p.Category.Name,
        OrderCount = p.OrderItems.Count()  // 聚合
    })
    .ToList();

// 示例4：条件投影
// 场景：根据条件返回不同的字段
var userInfo = dbContext.Users
    .Select(u => new UserDto
    {
        UserId = u.Id,
        Username = u.Name,
        Email = u.Email,
        IsAdmin = u.Roles.Any(r => r.Name == "Admin"),  // 条件判断
        RoleCount = u.Roles.Count()
    })
    .ToList();

// 示例5：与 Include 对比
// ❌ 不好的做法 - 加载完整对象图
var ordersWithInclude = dbContext.Orders
    .Include(o => o.Customer)
    .Include(o => o.OrderItems)
    .ThenInclude(oi => oi.Product)
    .ToList();  // 加载了大量不需要的数据

// ✅ 更好的做法 - 只投影需要的字段
var ordersWithProjection = dbContext.Orders
    .Select(o => new
    {
        o.Id,
        o.OrderDate,
        CustomerName = o.Customer.Name,
        ItemCount = o.OrderItems.Count,
        Items = o.OrderItems.Select(oi => new
        {
            oi.Product.Name,
            oi.Quantity,
            oi.UnitPrice
        }).ToList()
    })
    .ToList();

// 示例6：分页 + 投影
// 场景：列表显示通常需要分页，投影能保证高效
var page = dbContext.Products
    .Where(p => p.IsActive)
    .OrderByDescending(p => p.CreatedDate)
    .Skip((pageNumber - 1) * pageSize)
    .Take(pageSize)
    .Select(p => new ProductListItemDto
    {
        Id = p.Id,
        Name = p.Name,
        Price = p.Price,
        CategoryName = p.Category.Name,
        ReviewCount = p.Reviews.Count(),
        AverageRating = p.Reviews.Any() ? p.Reviews.Average(r => r.Rating) : 0
    })
    .ToList();