Nullable reverse navigation properties

[mjolnerbkl]

I have a table B with a primary key which is also a foreign key to table A.

When running my T4 file I get a class A with a reverse navigation property of type B. But it is not nullable, although B might not exist for all A's.

Is it possible to have these created as nullables?

Eg.

class A {
    public B? {get; set;)
}

Now it's created without the questionmark, so when I create an instance of A without a B I get a warning about the B property not being nullable.

[sjh37]

Hi @mjolnerbkl
Could you send me the SQL to create tables A and B so I can check this out?

[mjolnerbkl]

Hi Simon

Sorry for the late response.

Here is a dummy example with simple tables in a 1-1 relation. For b, id is the same as a.id, so id and foreign key to a is the same. So a may have a b referencing it and it may not.

CREATE TABLE core.a (
    Id uniqueidentifier NOT NULL,
    [Data] nvarchar(100) NULL,
    CONSTRAINT PK_a PRIMARY KEY (Id)
)

CREATE TABLE core.b (
    Id uniqueidentifier NOT NULL,
    [Data] nvarchar(100) NULL,
    CONSTRAINT PK_b PRIMARY KEY (Id),
    CONSTRAINT FK_b_a FOREIGN KEY(Id) REFERENCES core.a (Id)
)

When I generate code with the t4 template I get something like this:

public class A
{
      public Guid Id { get; set; } // Id (Primary key)
      public string? Data { get; set; } // Data (length: 100)

      // Reverse navigation

      /// <summary>
      /// Parent (One-to-One) A pointed by [b].[Id] (FK_b_a)
      /// </summary>
      public B B { get; set; } // b.FK_b_a
}

public class B
{
      public Guid Id { get; set; } // Id (Primary key)
      public string? Data { get; set; } // Data (length: 100)

      // Foreign keys

      /// <summary>
      /// Parent A pointed by [b].([Id]) (FK_b_a)
      /// </summary>
      public A A { get; set; } // FK_b_a
}

The Data property on both classes is correctly made nullable as the database columns allows null.

The problem is with the reverse navigation property B on class A. It is not nullable although there may not be a B referencing it. Should it not be as specified below instead?

      public B? B { get; set; } // b.FK_b_a

[sjh37]

I think you are correct, let me investigate

[innominate227]

This also causes this EF issue dotnet/efcore#30227 to occur where EF generates an inner join instead of a left join for the nullable properties.

[sjh37]

Add this to your <database>.tt file

Settings.NullableReverseNavigationProperties = true;  // If true, reverse navigation properties for one-to-one relationships will be nullable (e.g. MyEntity? MyEntity). The parent entity can exist without the child entity.

I recommend below the line

Settings.AllowNullStrings = false;

Next take the ttinclude file in the latest commit
And replace the one in your project.

Let me know how you get on.

I've still got some things to tidy up and won't close this just yet
One of the items to tidy up is a compiler warning

Non-nullable property 'Parent' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

[sjh37]

This is definitely the most significant code change I've ever had to make for an issue. It's been long overdue, too — thanks for the nudge. Still working on it btw, getting better with each commit...