我想编写一个优雅的linq查询来处理以下SAMPLE对象模型:
class Category
{
public string Name { get; set; }
public IList<Product> Products { get; set;}
}
class Product
{
public string Title { get; set; }
public IList<Photo> Photos { get; set; }
}
class Photo
{
public int Id { get; set; }
}
我构造了以下查询来获取Photo Id:
var query = from category in Factory.GetCategories()
where category.Name == "Cameras"
select (from product in category.Products
where product.Title == "Sony"
select (from photo in product.Photos
select photo.Id)
);
var v = query.ToList();
当查询没有正确投影时,我必须为每个子选择添加FirstOrDefault()!:
var query = from category in Factory.GetCategories()
where category.Name == "Cameras"
select (from product in category.Products
where product.Title == "Sony"
select (from photo in product.Photos
select photo.Id).FirstOrDefault()
).FirstOrDefault();
var v = query.ToList();
有更好的方法吗?忽略我们没有处理数据库和PK / FK的事实并没有发挥作用。
当我在linq查询中做同样的事情时,我真的想避免编写一个大的for循环:
foreach (var category in Factory.GetCategories())
{
if (category.Name == "Camera")
{
foreach (var product in category.Products)
{
if (product.Title == "Sony")
{
foreach (var photo in product.Photos)
{
//get data
int id = photo.Id;
}
}
}
}
}
实际实现比这个简单的对象模型更复杂。我想从这个简单的样本中得到基本的想法,所以我将它应用到我的真实对象模型中。
干杯!
答案 0 :(得分:5)
你只想要扁平的ids吗?
var query = from category in Factory.GetCategories()
where category.Name == "Cameras"
from product in category.Products
where product.Title == "Sony"
from photo in product.Photos
select photo.Id;
答案 1 :(得分:2)
是的 - 您必须添加FirstOrDefault,因为没有“照片ID”这样的东西。每张相机可能有很多照片 - 您想要哪种?
如果您只关心第一张照片,那很好 - 虽然多个“来自”条款会让您的生活更轻松:
var query = from category in Factory.GetCategories()
where category.Name == "Cameras"
from product in category.Products
where product.Title == "Sony"
select product.Photos.Select(photo => photo.Id)
.FirstOrDefault();
请注意,对于没有照片的产品,它将返回0。这就是你想要的吗?
如果您的要求更清楚,我们会更好地帮助您。
编辑:如果您只想要任何 Sony相机的第一个ID,请使用:
var query = from category in Factory.GetCategories()
where category.Name == "Cameras"
from product in category.Products
where product.Title == "Sony"
from photo in product.Photos
select photo.Id;
var firstId = query.FirstOrDefault();
答案 2 :(得分:1)
与其他人一样的答案,但使用扩展方法调用而不是查询理解。 SelectMany
允许您“解包”子集合并继续在该级别进行查询。
var query = Factory.GetCategories()
.Where(category => category.Name == "Cameras")
.SelectMany(category => category.Products)
.Where(product => product.Title == "Sony")
.SelectMany(product => product.Photos)
.Select(photo => photo.Id);
答案 3 :(得分:0)
var ids = from category in Factory.Categories
where category.Name == "Cameras"
from product in category.Products
where product.Title == "Sony"
from photo in product.Photos
select photo.Id;
返回照片的所有ID,无照片时可枚举为空。 如果你想要第一个,或默认为0:
var result = ids.FirstOrDefault();