任何人都可以写迷你指南,解释如何使用EF中的集合？
例如我有以下型号：
public class BlogPost
{
public int Id { get; set; }
public string Title { get; set; }
public string Content { get; set; }
public DateTime DateTime { get; set; }
public List<PostComment> Comments { get; set; }
}
public class PostComment
{
public int Id { get; set; }
public BlogPost ParentPost { get; set; }
public string Content { get; set; }
public DateTime DateTime { get; set; }
}
和上下文类：
public class PostContext : DbContext
{
public DbSet<BlogPost> Posts { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UsesqlServer(@"Server=(localdb)\mssqllocaldb;Database=Posts;Trusted_Connection=True;MultipleActiveResultSets=true");
}
protected override void OnModelCreating(ModelBuilder builder)
{
base.OnModelCreating(builder);
}
}
解决方法
以下是我在实体框架核心中使用导航属性的提示.
提示1：初始化集合
class Post
{
public int Id { get; set; }
// Initialize to prevent NullReferenceException
public ICollection<Comment> Comments { get; } = new List<Comment>();
}
class Comment
{
public int Id { get; set; }
public string User { get; set; }
public int PostId { get; set; }
public Post Post { get; set; }
}
提示2：使用HasOne和WithMany或HasMany和WithOne方法构建
protected override void OnModelCreating(ModelBuilder model)
{
model.Entity<Post>()
.HasMany(p => p.Comments).WithOne(c => c.Post)
.HasForeignKey(c => c.PostId);
}
提示3：加载集合
var posts = db.Posts.Include(p => p.Comments);
提示4：如果您不热切地明确加载
db.Comments.Where(c => c.PostId == post.Id).Load();
