Unable to determine the principal end of an association between the types '***. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

MVC中数据库表如果是一对一的主键关系时要加[Required]不然会出错Unable to determine the principal end of an association between the types

Unable to determine the principal end of an association between the types '***. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

 [Required]  
 public virtual User User { get; set; }

解决的是如下情况：

Unable to determine the principal end of an association between the types '***. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.