[英]Entity Framework returning distinct records after join
考慮我們有這兩個實體和一個自定義對象:
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; }
}
數據是這樣的:
實體1:
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
實體2:
DestinationId = 1,DestinationName = Destination1
DestinationId = 2,DestinationName = Destination2
如何選擇不同的destinationId並從Entity1中選擇Name和JobTitle,然后將它們與Entity2結合起來以獲取目標名稱並將其作為EntityDTO返回?
這是一種方法:
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
} ;
訣竅是group by
,然后取分組的第一元素。 這也稱為“與眾不同”,例如MoreLinq之類的庫提供了開箱即用的功能。
使用LINQ擴展,我更喜歡它們:
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
});
確實和Gert的答案相同。 您可以使用Distinct
但是必須從IEquatible<T>
繼承並實現Equals
方法,並重寫GetHashCode
方法以使其起作用。
您可以像這樣使用LINQ join運算符:
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
};
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