EF6-使用可为空的属性（外键，TPH）时，SQL查询无效

Question

Entity Framework generates invalid SQL query for the following LINQ query:

Car[] results = this.DbContext.Cars
    .Where(x => !x.ParentId.HasValue) // also for x.ParentId == null
    .ToArray();

My ParentId property is nullable int, foreign key to the same table (Id property). My results is an empty array, but it shouldn't. I used similar query (checking that nullable property has no value) using other tables and it worked just fine. In this case, the difference is that ParentId is foreign key and database table uses TPH. Is this is a bug or I made some mistakes? Why EF ignores nullable property at all? My configuration and generated SQL by EF (naming is only for example, eg "Car"):

Context configuration:

// TPH (Table per Hierarchy)
modelBuilder.Entity<Car>()
    .Map<CarA>(x => x.Requires("type").HasValue(1))
    .Map<CarB>(x => x.Requires("type").HasValue(2))
    .Map<CarC>(x => x.Requires("type").HasValue(3))
    .Map<CarD>(x => x.Requires("type").HasValue(4));

// parent child relationship
modelBuilder.Entity<Car>()
    .HasMany(x => x.Children)
    .WithRequired()
    .HasForeignKey(child => child.ParentId);

My class properties:

[Column("parent_id")]
public int? ParentId { get; set; }

public virtual List<Car> Children { get; set; }

for:

Car[] results = this.DbContext.Cars
    .Where(x => !x.ParentId.HasValue)
    .ToArray();

I get empty result with generated SQL:

SELECT 
    CAST(NULL AS int) AS [C1], 
    CAST(NULL AS int) AS [C2], 
    ...
    ...
    ...
    CAST(NULL AS decimal(18,2)) AS [C20], 
    CAST(NULL AS datetime2) AS [C21]
    FROM  ( SELECT 1 AS X ) AS [SingleRowTable1]
    WHERE 1 = 0

But it should be:

...
...
WHERE [Extent1].[car] IS NULL

for:

var results = this.DbContext.Cars
    .Where(x => x.ParentId.HasValue)
    .ToArray();

I get all entities (ParentId is ignored) with generated SQL:

SELECT 
    [Extent1].[type] AS [type], 
    [Extent1].[id] AS [id], 
    [Extent1].[parent_id] AS [parent_id], 
    [Extent1].[name] AS [name], 
    ...
    ...
    ...
    FROM [dbo].[car] AS [Extent1]
    WHERE [Extent1].[type] IN (1,2,3,4)

Tested on EF6 6.0.2 / 6.1.1 and MS SQL Server.

Answer 1

Wow… that is a funny behavior, but the solution is pretty simple. You have a nullable foreign key property but you defined the relationship as required. Just change

modelBuilder.Entity<Car>()
    .HasMany(x => x.Children)
    .WithRequired()
    .HasForeignKey(child => child.ParentId);

to

modelBuilder.Entity<Car>()
    .HasMany(x => x.Children)
    .WithOptional()
    .HasForeignKey(child => child.ParentId);

And It will work.