Entity Framework Core: foreign key and reference table Id column do not match

Question

I'm currently using EF Core 5 for developing. I have an existing database, so changing columns may not be a good option.

For example I got two tables:

public class CalssA
{
    public long AId { get; set; }
    public long TheBId { get; set; }
    [Foreign("TheBId")]
    public virtual ClassB B { get; set; }
}

public class ClassB
{
    public long BId { get; set; }

    [InverseProperty(nameof(CalssA.B))]
    public virtual ICollection<ClassA> Aas {get;set;}
}

When I try to get A data with B, it seems B info is not join to the A.

var a = _AContext.As.Include(a => a.B).FirstOrDefault();
var b = a.B // get null

Is it join issue? How should I specify the column to join with EF Core?

Thanks