这是错误的：

this.HasRequired(x => x.ContentType)
    .WithMany()
    .HasForeignKey(x => x.ContentTypeId);

你有反向导航属性，所以你必须在 WithMany 或EF可能会创建两个关系：

You have reverse navigation property so you must use int in WithMany or EF will probably create two relations:

this.HasRequired(x => x.ContentType)
    .WithMany(y => y.ContentTypeColumns)
    .HasForeignKey(x => x.ContentTypeId);

Btw。这种映射根本不需要，因为它是通过默认约定自动发现的。

Btw. this mapping should not be needed at all because it is discovered automatically through default conventions.