[英]Cannot get correct results from Linq query
I am trying to query my database using Linq. 我正在尝试使用Linq查询数据库。 In short, my linq statement is not returning the data that I want and I am getting errors. 简而言之,我的linq语句未返回所需的数据,并且出现错误。
public class Product{
[Key]
public int id{get;set;}
[Required]
[MinLength(3)]
public string Name{get;set;}
[MinLength(10)]
public string Description{get;set;}
[Required]
[Range(0, double.MaxValue)]
public decimal Price{get;set;}
public DateTime CreatedAt{get;set;} = DateTime.Now;
public DateTime UpdatedAt{get;set;} = DateTime.Now;
public List<ProductCategory> ProductCategories{get;set;}
}
public class Category{
[Key]
public int id{get;set;}
[Required]
[MinLength(2)]
public string Name{get;set;}
public DateTime CreatedAt{get;set;} = DateTime.Now;
public DateTime UpdatedAt{get;set;} = DateTime.Now;
public List<ProductCategory> ProductCategories{get;set;}
}
public class ProductCategory{
[Key]
public int id{get;set;}
public int ProductId{get;set;}
public int CategoryId{get;set;}
public Product Product{get;set;}
public Category Category{get;set;}
}
#Variable used in troublesome code (in controller)
Product product = context.Products
.Include(p => p.ProductCategories)
.ThenInclude(pc => pc.Category)
.FirstOrDefault(p => p.id == id);
#Troublesome code (in controller)
List<Category> categories = context.Categories
.Include(c => c.ProductCategories)
.ThenInclude(pc => pc.Product)
.Where(c => c.ProductCategories.Select(pc => pc.Product) != product)
.ToList();
Products and Categories have a many to many relationship. 产品和类别具有多对多关系。 I want the categories variable to contain a list of all categories that are NOT in the retrieved product. 我希望category变量包含检索到的产品中没有的所有类别的列表。 Cannot somebody guide me in the right direction or tell me what I am doing wrong here? 有人不能指引我正确的方向或告诉我我在做什么错吗?
Error: 'System.Nullable 1[System.Int32]' cannot be used as the data type for a sequence with an ItemExpression of type 'System.Nullable
1[System.Int32]' 错误:“ System.Nullable 1[System.Int32]' cannot be used as the data type for a sequence with an ItemExpression of type 'System.Nullable
1 [System.Int32]” 1[System.Int32]' cannot be used as the data type for a sequence with an ItemExpression of type 'System.Nullable
As said in the comments, the direct error is in 如评论所述,直接错误在
c.ProductCategories.Select(pc => pc.Product) != product
Because c.ProductCategories.Select(pc => pc.Product)
is a sequence of Product
s, which can't be compared to one Product
. 因为c.ProductCategories.Select(pc => pc.Product)
是Product
的序列,所以不能与一个Product
进行比较。
Another issue is that you use product
in the second query. 另一个问题是您在第二个查询中使用product
。 Even when used correctly, for example... 即使正确使用,例如...
List<Category> categories = context.Categories
.Include(c => c.ProductCategories)
.ThenInclude(pc => pc.Product)
.Where(c => !c.ProductCategories.Select(pc => pc.Product)
.Any(p => p == product))
.ToList();
...the problem is that product
can't be translated into SQL and EF switches to client-side evaluation. ...问题是product
无法转换为SQL,EF不能转换为客户端评估。
(I assume you're working in EF-core. EF6 wouldn't allow it and throw an exception if you'd use product
like that in a subsequent query). (我假设您使用的是EF-core。如果您在后续查询中使用类似的product
,EF6将不允许它并引发异常)。
But there's a simple solution that even saves you one roundtrip. 但是有一个简单的解决方案,甚至可以节省一次往返。 Just use id
directly: 只需直接使用id
:
List<Category> categories = context.Categories
.Include(c => c.ProductCategories)
.ThenInclude(pc => pc.Product)
.Where(c => !c.ProductCategories.Any(pc => pc.ProductId == id))
.ToList();
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