如何在实体框架中的多对多关系中按 id 查询？

Question

I have Group entity like

public class Group
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public string CategoryId { get; set; }
        public ICollection<GroupTag> TagsLink { get; set; }
    }

I have Tag entity like

 public class Tag
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public ICollection<GroupTag> GroupLink { get; set; }
    }

and I mapped

 public class GroupTag
    {
        public string GroupId { get; set; }
        public Group Group { get; set; }
        public string TagId { get; set; }
        public Tag Tag { get; set; }
        public byte Order { get; set; }
    }

and so how to retrieve group by groupId and tag by tagId?

Answer 1

if you are using net core 5 or 6 you can add some more extension to your classes:

public class Group
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public ICollection<GroupTag> GroupTags { get; set; }
        
         public string CategoryId { get; set; }
          public virtual Category Category { get; set; }

         public  virtual ICollection<Tag> Tags { get; set; }
    }

 public class Tag
    {
        public string Id { get; set; }
        public string Name { get; set; }

        public virtual ICollection<GroupTag> GroupTags { get; set; }
        
        public virtual ICollection<Group> Groups { get; set; }
    }

If you already created dbcontext you can get a group by groupId

var group= context.Groups
 .Include(i=> i.Tags) // if you don't need tags, ommit this
 .FirstOrDefault(i=> i.GroupId==groupId);

and the same way you can get a tag