[英]Using LINQ-To-Entities to Generate Information
我正在一个网站上工作,在该网站上,用户可以将标签添加到已发布的书本上,就像目前针对Stack Overflow的问题所做的一样。
课程 :
Books
{
bookId,
Title
}
Tags
{
Id
Tag
}
BooksTags
{
Id
BookId
TagId
}
以下是一些示例记录。
Books
BookId Title
113421 A
113422 B
Tags
Id Tag
1 ASP
2 C#
3 CSS
4 VB
5 VB.NET
6 PHP
7 java
8 pascal
BooksTags
Id BookId TagId
1 113421 1
2 113421 2
3 113421 3
4 113421 4
5 113422 1
6 113422 4
7 113422 8
问题
我需要在LINQ中为实体查询写一些东西,它根据标签为我提供数据:
查询 : bookIds where tagid = 1
返回 : bookid: 113421, 113422
查询2 : tags 1 and 2
回报 : 113421
我需要标签及其计数才能显示在相关标签中,因此在第一种情况下,我的相关标签类应具有以下结果。
相关标签标签计数2 1 3 1 4 2 8 1
第二种情况 :
RelatedTags
Tag Count
3 1
4 1
如何在LINQ中做到这一点?
只需执行以1:N或1:1关系映射表的外键,然后让设计者为您创建导航属性。 (Books:BooksTags将1:N从Books.BookID映射到BooksTags.BookID,而BooksTags.TagID则将1:1映射到Tag.TagID)。 这实际上是伪装的N:M关系。 我不知道设计者是否直接选择了它,但是通过一些摆弄可以使导航属性正确。
现在开始提问:
model.Tags.Where(t => t.ID == 1).Books.Select(b => b.ID)
获取一本书的所有标签,然后将该表连接到BooksTags上,这样您就可以简单地使用Count()来获取计数。
在第一部分,有趣的限制是,该书必须匹配输入的每个标签,因此“ where tagid == someId”的where子句实际上不会起作用。 我设想这样的事情(LINQ到对象的示例)
List<int> selectedTagIds = new List<int>() { 1, 2 };
var query = from book in books
join booktag in booktags
on book.Id equals booktag.BookId
join selectedId in selectedTagIds
on booktag.TagId equals selectedId
group book by book into bookgroup
where bookgroup.Count() == selectedTagIds.Count
select bookgroup.Key;
它基本上执行从书籍到书签的连接,也执行到选定标签ID列表的连接,并将选择范围限制为book-> tag匹配的数目等于选定标签ID的数目。
要拉相关标签,也许像这样
var relatedTags = from book in query // use original query as base
join booktag in booktags
on book.Id equals booktag.BookId
join tag in tags
on booktag.TagId equals tag.Id
where !selectedTagIds.Contains(tag.Id) // exclude selected tags from related tags
group tag by tag into taggroup
select new
{
Tag = taggroup.Key,
Count = taggroup.Count()
};
快速示例的完整代码。 并不是完全面向对象的,但是您知道了。
using System;
using System.Collections.Generic;
using System.Linq;
namespace StackOverflow
{
class Program
{
static void Main()
{
List<Book> books = new List<Book>()
{
new Book() { Id = 113421, Title = "A" },
new Book() { Id = 113422, Title = "B" }
};
List<Tag> tags = new List<Tag>()
{
new Tag() { Id = 1, Name = "ASP" },
new Tag() { Id = 2, Name = "C#" },
new Tag() { Id = 3, Name = "CSS" },
new Tag() { Id = 4, Name = "VB" },
new Tag() { Id = 5, Name = "VB.NET" },
new Tag() { Id = 6, Name = "PHP" },
new Tag() { Id = 7, Name = "Java" },
new Tag() { Id = 8, Name = "Pascal" }
};
List<BookTag> booktags = new List<BookTag>()
{
new BookTag() { Id = 1, BookId = 113421, TagId = 1 },
new BookTag() { Id = 2, BookId = 113421, TagId = 2 },
new BookTag() { Id = 3, BookId = 113421, TagId = 3 },
new BookTag() { Id = 4, BookId = 113421, TagId = 4 },
new BookTag() { Id = 5, BookId = 113422, TagId = 1 },
new BookTag() { Id = 6, BookId = 113422, TagId = 4 },
new BookTag() { Id = 7, BookId = 113422, TagId = 8 }
};
List<int> selectedTagIds = new List<int>() { 1,2 };
// get applicable books based on selected tags
var query = from book in books
join booktag in booktags
on book.Id equals booktag.BookId
join selectedId in selectedTagIds
on booktag.TagId equals selectedId
group book by book into bookgroup
where bookgroup.Count() == selectedTagIds.Count
select bookgroup.Key;
foreach (Book book in query)
{
Console.WriteLine("{0}\t{1}",
book.Id,
book.Title);
}
// get related tags for selected tags
var relatedTags = from book in query // use original query as base
join booktag in booktags
on book.Id equals booktag.BookId
join tag in tags
on booktag.TagId equals tag.Id
where !selectedTagIds.Contains(tag.Id) // exclude selected tags from related tags
group tag by tag into taggroup
select new
{
Tag = taggroup.Key,
Count = taggroup.Count()
};
foreach (var relatedTag in relatedTags)
{
Console.WriteLine("{0}\t{1}\t{2}",
relatedTag.Tag.Id,
relatedTag.Tag.Name,
relatedTag.Count);
}
Console.Read();
}
}
class Book
{
public int Id { get; set; }
public string Title { get; set; }
}
class Tag
{
public int Id { get; set; }
public string Name { get; set; }
}
class BookTag
{
public int Id { get; set; }
public int BookId { get; set; }
public int TagId { get; set; }
}
}
因此,对于选定的标签1和2,您将获得书籍A,而相关的标签将是3(CSS)和4(VB)。
这与答案本身没有直接关系,但是您可能想看看linqpad,因为它可以帮助您直接从数据库中构建L2S语句。
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