Question

我是Entity Framework的新手，所以我不知道我做得对，或者这是新Entity Framework Core中的错误。我需要为相关页面做一个类，例如1页与另一页相关。我上课a <- ifelse(data$category == 0, "red", "blue") ggplot(data, aes(x = x, y = y)) + geom_bar(stat = "identity", aes(fill = category)) + theme(axis.text.x = element_text(angle = 45, hjust = 1, colour = a))

Page

这是我public class Page { public int ID { get; set; } public string UrlName { get; set; } [Required] public string Title { get; set; } [Required] public string Description { get; set; } public string Content { get; set; } public List<RelatedPages> RelPages1 { get; set; } public List<RelatedPages> RelPages2 { get; set; } }的课程：

RelatedPages

我希望从public class RelatedPages { public int ID { get; set; } public int? Page1ID { get; set; } public Page Page1 { get; set; } public int? Page2ID { get; set; } public Page Page2 { get; set; } }获取Page实例，反之，与RelatedPages类似，但我有NullReferenceException。这是context.RelatedPages.Single(r => r.ID==1).Page2.UrlName OnModelCreating中的DbContext：

 modelBuilder.Entity<RelatedPages>()
                    .HasOne(x => x.Page1)
                    .WithMany(x => x.RelPages1)
                    .HasForeignKey(x => x.Page1ID);

 modelBuilder.Entity<RelatedPages>()
                    .HasOne(x => x.Page2)
                    .WithMany(x => x.RelPages2)
                    .HasForeignKey(x => x.Page2ID);

Answer 1

尝试加入Page2。

将context.RelatedPages.Single(r => r.ID==1).Page2.UrlName更改为context.RelatedPages.Include(x=>x.Page2).Single(r => r.ID==1).Page2.UrlName

实体框架核心中的外键NullReferenceExcepetion

1 个答案: