实体框架在加入后返回不同的记录 [英] Entity Framework returning distinct records after join

问题描述

考虑到我们有这两个实体和一个自定义对象：

  public class Entiy1 
 {
 public int Id {get;组; } 
 public int DestinationId {get;组; } 
 public string Name {get;组; } 
 public string JobTitle {get;组; } 
} 
 
 public class Entiy2 
 {
 [Key] 
 public int DestinationId {get;组; } 
 public int DestinationName {get;组; } 
 
 
} 
 public class EntityDTO 
 {
 public int DestinationName {get;组; } 
 public int DestinationId {get;组; } 
 public string Name {get;组; } 
 public string JobTitle {get;组; 
} 
  

数据是这样的：

Entity1：
Id = 1，DestinationId = 1，Name = Name1，JobTitle = Job1

Id = 2，DestinationId = 1，Name = Name2，JobTitle = Job2
Id = 3，
DestinationId = 2，Name = Name3，JobTitle = Job3
Id = 4，
DestinationId = 2，Name = Name4，JobTitle = Job4 < br> Id = 5，
DestinationId = 2，Name = Name5，JobTitle = Job5

实体2：

DestinationId = 1，DestinationName = Destination1

DestinationId = 2，DestinationName = Destination2

如何选择不同的destinationId，然后从Entity1中选择Name和JobTitle，然后将它们与Entity2获取目标名称并将其作为EntityDTO返回？

解决方案

这是一种方法：

  var query = from e1 in 
（从e1 in entities1 
 group e1 by e1.DestinationId into grp 
 select grp.First（））
 join e2在e2上的entities2中.DestinationId等于e2.DestinationId 
选择新的EntityDTO 
 {
 DestinationId = e1.DestinationId，
 DestinationName = e2.DestinationName，
 Name = e1。名称，
 JobTitle = e1.JobTitle 
}; 
  

诀窍是的组，然后采取分组的第一个要素。像 MoreLinq 这样的图书馆也提供开箱即用的独特之处 。

Consider we have these two entities and one custom object :

    public class  Entiy1
{
    public int Id { get; set; }
    public int DestinationId { get; set; }
    public string Name { get; set; }
    public string JobTitle { get; set; }
}

public class Entiy2
{
    [Key]
    public int DestinationId { get; set; }
    public int DestinationName { get; set; }


}
public class EntityDTO
{
    public int DestinationName { get; set; }
    public int DestinationId { get; set; }
    public string Name { get; set; }
    public string JobTitle { get; set; }
}

Data is something like this :

Entity1 :
Id=1 , DestinationId=1,Name=Name1,JobTitle=Job1
Id=2 , DestinationId=1,Name=Name2,JobTitle=Job2
Id=3 , DestinationId=2,Name=Name3,JobTitle=Job3
Id=4 , DestinationId=2,Name=Name4,JobTitle=Job4
Id=5 , DestinationId=2,Name=Name5,JobTitle=Job5
Entity 2:
DestinationId=1 , DestinationName=Destination1
DestinationId=2 , DestinationName=Destination2

How can I select distinct destinationId and select Name and JobTitle from Entity1 then join them with Entity2 to fetch destination name and returning them as EntityDTO ?

解决方案

Here's a way to do it:

var query = from e1 in
    (from e1 in entities1
        group e1 by e1.DestinationId into grp
        select grp.First())
    join e2 in entities2 on e1.DestinationId equals e2.DestinationId
    select new EntityDTO 
            { 
                DestinationId = e1.DestinationId, 
                DestinationName = e2.DestinationName,
                Name = e1.Name,
                JobTitle = e1.JobTitle
            } ;

The trick is the group by and then taking the first element of the grouping. This is also referred to as "distinct by" that a library like MoreLinq provides out of the box.

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