如何扁平化多对多关系

Question

我有这个表和关系。

编辑

类

public class Person
{
  public Person()
  {
    Books = new HashSet<Book>();
  }

  public int PersonId { get; set; }
  public string fname { get; set; }
  public string lname { get; set; }
  public string Company { get; set; }

  public virtual ICollection<Book> Books { get; set; }
}

public class Book
{
  public Book()
  {
    Persons = new HashSet<Person>();
  }

  public int BookId { get; set; }
  public string Title { get; set; }

  public virtual ICollection<Person> Persons { get; set; }
}

类数据

{
  lname = "Bar", fname = "Foo", Company = "FooBar", Books = { Title = "Book1" }, { Title = "Book2" }
}

人员表

+-------+-------+---------+----------+
| fname | lname | Company | PersonId |
+-------+-------+---------+----------+
| Foo   | Bar   | FooBar  | 1        |
| Bob   | Joe   | Forest  | 2        |
| Temp  | Yoo   | Temp    | 3        |
+-------+-------+---------+----------+

书桌

+--------+-------+
| BookId | Title |
+--------+-------+
| 1      | Book1 |
| 2      | Book2 |
| 3      | Book3 |
+--------+-------+

PersonBook 表

+----------+--------+
| PersonId | BookId |
+----------+--------+
| 1        | 2      |
| 1        | 1      |
| 2        | 1      |
+----------+--------+

结果

+----------+---------+------+
| Name     | Company | Book |
+----------+--------+-------+
| Yoo, Temp| Temp   |       |
| Joe, Bob | Forest | Book1 |
| Bar, Foo | FooBar | Book1 |
| Bar, Foo | FooBar | Book2 |
+----------+--------+-------+

现在，Person 和 Book 基本上是多对多的关系。我已经有一个 Sql 查询，但如何在 Linq 查询版本中执行此操作？

嗯，是这样的：

select 
    (isnull(c.lname, '') + ', ' + isnull(c.fname, '')) as Name
    ,isnull(c.Company , '') as Company
    ,isnull(f.Book, '') as Book
from Person c
left join PersonBook cf
on c.PersonId = cf.PersonId
left join Book f
on f.BookId = cf.BookId
order by f.Title, c.lname

任何帮助将不胜感激！谢谢！

Answer 1

LINQ 中的左连接有点棘手，它需要join into 和DefaultIfEmpty：

from p in db.People
join bj in db.Books on p.PersonId equals bj.PersonID into j
from b in j.DefaultIfEmpty()
select new { p.fName, p.lName, p.Company, b == null ? null : b.Title }

Answer 2

查看您的类，您的模型中不应该有 PersonBook 类。使用 EF，您可以在没有联结类的情况下对多对多关联进行建模。 EF 会透明地查询数据库中的联结表。

所以你的查询应该是这样的

var query = from p in db.People
            from b in p.Books.DefaultIfEmpty() // For a left outer join
            select new { p.fName, p.lName, p.Company, b.Title };
var result = query.asEnumerable()
                  .Select(x => new 
                    {
                        Name = string.Format("{0}, {1}", x.lName, x.fName),
                        p.Company,
                        Book = b.Title
                    }

如果您监控生成的 SQL，您会看到 EF 加入了PersonBook 表。

Answer 3

这就是我所拥有的：-

var result = from x in (from p in persons
             join pb in personBooks
             on p.PersonID equals pb.PersonID into g
             from result1 in g.DefaultIfEmpty()
             select new
             {
                 Name = p.lName + "," + p.fName,
                 Company = p.Company,
                 BookID = result1 == null ? 0 : result1.BookID
              })
              join y in books 
              on x.BookID equals y.BookID into g2
              from result2 in g2.DefaultIfEmpty()
              select new 
              {
                  Name = x.Name,
                  Company = x.Company,
                  Book = result2 == null ? String.Empty : result2.Title
              };

我使用了一些乱七八糟的范围变量名称，请在您的实际查询中更改它们：)

得到以下结果：-

Answer 4

使用 Linq 方法，您可以这样使用 SelectMany：

var PersonBook = Persons
            .SelectMany(p => p.Books,
                (p, b) =>
                    new
                    {
                        Name = p.fname + ", " + p.lname,
                        Company = p.Company,
                        BookId = b == null ? -1 : b.BookId,
                    })
              .Join(Books, p => p.BookId, b => b.BookId,
                     (p, b) =>
                         new
                         { 
                             Name = p.Name,
                             Company = p.Company,
                             Book = p.BookId == -1 ? "" : b.Title
                          });