在实体框架中选择不同的行

Question

我正在尝试使用 Entity Framework 查询 SQL Server 视图，并且仅返回基于多列不同的行。

我尝试了基于this answer（GroupBy 然后Select(g => g.FirstOrDefault()) 的解决方案，但我仍然得到重复的行。

表结构（这是真实数据库中相当复杂的视图，但最终输出的结构与本示例相似）：

CREATE TABLE Example (
    ID_A BIGINT,
    ID_B BIGINT,
    ID_C BIGINT,

    Type_A NVARCHAR(50),
    Type_B NVARCHAR(50),

    ID_Z BIGINT,

    Foo NVARCHAR(200),
    Bar NVARCHAR(200)
)

示例数据：

INSERT INTO Example (ID_A, ID_B, ID_C, Type_A, Type_B,  ID_Z,  Foo, Bar)
VALUES (1, 1, 1, 'TypeA1', 'TypeB1',  1,  'foo1', 'bar1'), -- This row and the next one represent the same main record (1) joined to different "Z" records (1 and 2)
       (1, 1, 1, 'TypeA1', 'TypeB1',  2,  'foo1', 'bar1'),
       (2, 1, 2, 'TypeA2', 'TypeA2',  1,  'foo2', 'bar2'), -- This row and the next two represent the same main record (2) joined to different "Z" records (1, 2 and 3)
       (2, 1, 2, 'TypeA2', 'TypeA2',  2,  'foo2', 'bar2'),
       (2, 1, 2, 'TypeA2', 'TypeA2',  3,  'foo2', 'bar2')

实体类：

public class ExampleEntity
{
    [Key]
    public long ID_A { get; set; }
    public long ID_B { get; set; }
    public long ID_C { get; set; }

    public string Type_A { get; set; }
    public string Type_B { get; set; }

    public long? ID_Z { get; set; }

    public string Foo { get; set; }
    public string Bar { get; set; }

实体配置：

public class ExampleEntityConfiguration : EntityTypeConfiguration<ExampleEntity>
{
    public ExampleEntityConfiguration()
    {
        // Properties
        this.Property(t => t.ID_A)
            .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);

        // Table & Column Mappings
        this.ToTable("Example");
        this.Property(t => t.ID_A).HasColumnName("ID_A");
        this.Property(t => t.ID_B).HasColumnName("ID_B");
        this.Property(t => t.ID_C).HasColumnName("ID_C");

        this.Property(t => t.Type_A).HasColumnName("Type_A");
        this.Property(t => t.Type_B).HasColumnName("Type_B");

        this.Property(t => t.ID_Z).HasColumnName("ID_Z");

        this.Property(t => t.Foo).HasColumnName("Foo");
        this.Property(t => t.Bar).HasColumnName("Bar");
    }
}

ID_A、ID_B、ID_C、Type_A 和 Type_B 标识“主”实体，ID_Z 标识连接的“Z”实体。 Foo 和 Bar 是非唯一的数据列，需要包含在最终结果中。

对于主要 ID/Type 值的每个组合，可以有多个 ID_Z 值。我需要按 ID_Z 值进行过滤，然后返回不同的主要实体值（基于 ID/Type 值）。

我尝试过如下查询：

// The `ID_Z` values to filter on
var zIDs = new List<long> { 1, 2 };

var result = context.Set<ExampleEntity>()
       .Where(e => zIDs.Contains(e.ID_Z))
       .GroupBy(e => new { e.ID_A, e.ID_B, e.ID_C, e.Type_A, e.Type_B })
       .Select(g => g.FirstOrDefault())
       .ToList();

这会导致这个 SQL：

SELECT 
    [Limit1].[ID_A] AS [ID_A], 
    [Limit1].[ID_B] AS [ID_B], 
    [Limit1].[ID_C] AS [ID_C], 
    [Limit1].[Type_A] AS [Type_A], 
    [Limit1].[Type_B] AS [Type_B], 
    [Limit1].[ID_Z] AS [ID_Z], 
    [Limit1].[Foo] AS [Foo], 
    [Limit1].[Bar] AS [Bar]
FROM   (
    SELECT [Extent1].[ID_A] AS [ID_A], [Extent1].[ID_B] AS [ID_B], [Extent1].[ID_C] AS [ID_C], [Extent1].[Type_A] AS [Type_A], [Extent1].[Type_B] AS [Type_B]
    FROM [dbo].[Example] AS [Extent1] WITH (NOLOCK)
    WHERE 
        ([Extent1].[ID_Z] IN (cast(1 as bigint), cast(2 as bigint))) AND ([Extent1].[ID_Z] IS NOT NULL)
) AS [Filter1]
OUTER APPLY  (
    SELECT TOP (1) 
        [Extent2].[ID_A] AS [ID_A], 
        [Extent2].[ID_B] AS [ID_B], 
        [Extent2].[ID_C] AS [ID_C], 
        [Extent2].[Type_A] AS [Type_A], 
        [Extent2].[Type_B] AS [Type_B], 
        [Extent2].[ID_Z] AS [ID_Z], 
        [Extent2].[Foo] AS [Foo], 
        [Extent2].[Bar] AS [Bar]
     FROM [dbo].[Example] AS [Extent2] WITH (NOLOCK)
     WHERE 
        ([Extent2].[ID_Z] IN (cast(1 as bigint), cast(2 as bigint))) AND ([Extent2].[ID_Z] IS NOT NULL) AND 
        ([Filter1].[ID_A] = [Extent2].[ID_A]) AND 
        ([Filter1].[ID_B] = [Extent2].[ID_B]) AND 
        ([Filter1].[ID_C] = [Extent2].[ID_C]) AND 
        (([Filter1].[Type_A] = [Extent2].[Type_A]) OR (([Filter1].[Type_A] IS NULL) AND ([Extent2].[Type_A] IS NULL))) AND 
        (([Filter1].[Type_B] = [Extent2].[Type_B]) OR (([Filter1].[Type_B] IS NULL) AND ([Extent2].[Type_B] IS NULL))) 
) AS [Limit1]

但这似乎返回了与Z_ID 过滤器匹配的所有行（导致重复的“主”值），而不是只返回每组“主”ID/类型值的第一行。

如果我在GroupBy 之后直接实现（ToList）查询，我似乎得到了正确的分组；但我想在数据库中运行这一切，避免使用 LINQ to Objects 查询。

如何创建此查询？

Answer 1

（因为显然这从未完成，我将下面的 Ivan Stoev's 评论复制为“答案”）

正如我所料，问题是由实体的 PK 定义引起的。通过将 ID_A 标记为 PK 本质上是在告诉 EF 该字段是唯一的，因此它将假定包括该字段在内的任何字段组合都是唯一的，并将删除正常的 GroupBy / Distinct 运算符。您确实需要为该实体定义一个真正的唯一键作为 PK。最终包括所有字段，但这不适用于可为空的字段。更好地扩展数据库视图以包含基于 ROW_NUMBER 或 NEWID 的计算列并将其映射为 PK。 – Ivan Stoev 10 月 10 日 15:58