谁能写迷你指南来解释如何在EF中使用集合?
例如,我有以下模型:
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);
}
}
我需要用OnModelCreating方法编写什么内容,以便可以在代码中的任何地方使用Posts.Add等?
最佳答案
这是我在Entity Framework Core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
方法进行构建protected override void OnModelCreating(ModelBuilder model)
{
model.Entity<Post>()
.HasMany(p => p.Comments).WithOne(c => c.Post)
.HasForeignKey(c => c.PostId);
}
提示3:急于加载收藏集
var posts = db.Posts.Include(p => p.Comments);
技巧4:如果您不急于明确加载
db.Comments.Where(c => c.PostId == post.Id).Load();