Pfthb

The objective is to make a Linq query to the database that gets the rest of ids that link back the input id in the method called GetFolder

Sample Data
Id parentId
10 9
11 10
12 10
13 12
14 13
15 13
100 20

Expected Output
Id parentId
10 11
12 10
13 12
14 13

 public class Folder 
 
 [Key]
 [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
 public int Id get; set; 

 [Column("Parent_Id")]
 public int? ParentId get; set; 

 public virtual ICollection<Folder> Children get; set; 
 

Have the ICollection allows me to go one view the siblings of that folder.
GetFolders methods return these:

Id parentId
10 11
11 10
12 10

 private async Task<Folder> GetFolder(int folderId)
 
 var entity = await Database.Folders
 .Where(e => e.Id == folderId)
 .Include(e => e.Children)
 .SingleOrDefaultAsync();

 return entity;
 

I tried to write a recursive function to get the rest of the Ids but it doesn't work correctly.
Does anyone have any recommends on how to write this recursive function to receive the rest of the Ids.

How about

IEnumerable<int> GetRelatedIds(Folder root)
 
 var ids = root.Children.SelectMany(c => GetRelatedIds(c)).ToList();
 ids.Add(root.Id);

 return ids;

This will also include the Id of the root folder. If you don't need it you can either just remove it from the list (it will be the last element), or run this method for all children of the root and concatenate the results.