实体框架在加入后返回不同的记录

Question

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 ?

Answer 1

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.

Answer 2

Using LINQ extensions, I'm more of a fan of them:

var results = entityList1
            .GroupBy(e => e.DestinationId)
            .Select(e => e.First())
            .Join(entityList2, e1 => e1.DestinationId, e2 => e2.DestinationId, (e1, e2) => 
                new EntityDTO
                {
                    DestinationId = e1.DestinationId,
                    DestinationName = e2.DestinationName,
                    JobTitle = e1.JobTitle,
                    Name = e1.Name
                });

Same thing as Gert's anwser really. You can use Distinct but, you would have to inherit from IEquatible<T> and implement the Equals method and override the GetHashCode method to get it to work.

Answer 3

You can use the LINQ join operator like this:

var results = from e1 in context.Entity1s
              join e2 in context.Entity2s
              on e1.DestinationId equals e2.DestinationId
              select new EntityDTO
              {
                  DestinationId = e1.DestinationId,
                  Name = e1.Name,
                  JobTitle = e1.JobTitle,
                  DestinationName = e2.DestinationName
              };