Left join with grouping via linq2db

Question

I have the following tables

class Directory
{
    public long Id { get; set;}
    public string Path { get; set;}
    public IEnumerable<File> Files { get; set;}
}

class File
{
    public long Id { get; set;}
    public long DirectoryId { get; set; }
    public string Name { get; set; }
    public Directory Directory { get; set; }
}

How can i get directories grouped by id with appropriate files using left join and linq2db in one query?

I think it should be something like this

var query = from d in db.Direcories
            join f in db.Files on d.Id equals f.DirectoryId into items
            from i in items.DefaultIfEmpty()
            group i by new { d } into g
            select new { Directory = g.Key, Files = ????? };

 var result = query.ToList();

but i don't know how to get files from group

Thanks, I updated the tag wiki. I'd never heard of it before. — jessehouwing, May 07 '15 at 14:48

nightcoder · Accepted Answer · 2015-05-13T19:37:31.657

First of all, I strongly recommend to generate your data model classes using T4 templates, so take a look at this project: linq2db/t4models. I also recommend to look at the videos here: https://github.com/linq2db/linq2db/wiki.

After having my data model generated with T4, it will look something like this:

public partial class TestDb : LinqToDB.Data.DataConnection
{
    public ITable<Directory>    Directories   { get { return this.GetTable<Directory>(); } }
    public ITable<File>         Files         { get { return this.GetTable<File>(); } }
}

[Table(Schema="dbo", Name="Directory")]
public partial class Directory
{
    [PrimaryKey, Identity] public int    ID   { get; set; } // int
    [Column,     NotNull ] public string Path { get; set; } // varchar(max)

    #region Associations

    /// <summary>
    /// FK_File_Directory_BackReference
    /// </summary>
    [Association(ThisKey="ID", OtherKey="DirectoryID", CanBeNull=true, IsBackReference=true)]
    public List<File> Files { get; set; }

    #endregion
}

[Table(Schema="dbo", Name="File")]
public partial class File
{
    [PrimaryKey, Identity] public int    ID          { get; set; } // int
    [Column,     NotNull ] public int    DirectoryID { get; set; } // int
    [Column,     NotNull ] public string Name        { get; set; } // varchar(max)

    #region Associations

    /// <summary>
    /// FK_File_Directory
    /// </summary>
    [Association(ThisKey="DirectoryID", OtherKey="ID", CanBeNull=false, KeyName="FK_File_Directory", BackReferenceName="Files")]
    public Directory Directory { get; set; }

    #endregion
}

Now there are several ways to load directories with their files. Here is a few:

[Test] // 1 query to load directories + separate queries to load files for each directory
public void Test()
{
    LinqToDB.Common.Configuration.Linq.AllowMultipleQuery = true;
    using(var db = new TestDb())
    {
        var directoriesWithFiles = db.Directories.LoadWith(d => d.Files).ToList();
    }
}

[Test] // same as above, but manually
public void Test2()
{
    using(var db = new TestDb())
    {
        var directories = db.Directories.ToList();

        foreach (var d in directories)
        {
            d.Files = db.Files.Where(f => f.DirectoryID == d.ID).ToList();
        }
    }
}

[Test] // if you want only 2 queries to the database
public void Test3()
{
    using (var db = new TestDb())
    {
        var dict = new Dictionary<int, List<File>>();

        foreach(var file in db.Files)
        {
            if(!dict.ContainsKey(file.DirectoryID))
                dict.Add(file.DirectoryID, new List<File> { file });
            else
                dict[file.DirectoryID].Add(file);
        }

        var directories = db.Directories.ToList();

        foreach (var d in directories)
        {
            List<File> files;
            d.Files = dict.TryGetValue(d.ID, out files) ? files : new List<File>();
        }
    }
}

Or you could just make a join, load everything in 1 query and then connect files to directories manually in memory. You could write your own extension method to simplify this.

score 0 · Answer 2 · answered May 07 '15 at 12:16

The join syntax for Linq is pretty hard to wrap ones head around... I generally apply a little trick to only use the "items" variable you created as an intermediate result:

var query = from d in db.Direcories
        join f in db.Files on d.Id equals f.DirectoryId into items
        from f in items.DefaultIfEmpty()
        group f by d into g
        select new { Directory = g.Key, Files = g /* optionally add .ToList() or .ToEnumerable() */ };

var result = query.ToList();

Though I suspect that there are easier ways to accomplish what you're trying to do using db.Directories.Include(d => d.Files) or a similar construct if you have the right relational properties defined.

score 0 · Answer 3 · answered May 13 '15 at 21:01

Have a try on this:

var query = from fileAndFolder in
                 (
                  from d in db.Directories
                  from f in db.Files.Where(ff => ff.DirectoryId == d.Id).DefaultIfEmpty()
                  select new { d, f }
                 )
            group fileAndFolder by fileAndFolder.d into g
            select new { DirectoryId = g.Key, Files = g };

Left join with grouping via linq2db

3 Answers3