通过 linq2db 左加入分组

Question

我有以下表格

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; }
}

如何在一个查询中使用左连接和 linq2db 获取按 ID 分组的目录和适当的文件？

我觉得应该是这样的

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();

但我不知道如何从组中获取文件

Answer 1

Linq 的连接语法很难让人理解...我通常会应用一些小技巧来仅使用您创建的 "items" 变量作为中间结果：

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();

尽管我怀疑如果您定义了正确的关系属性，使用 db.Directories.Include(d => d.Files) 或类似结构可以通过更简单的方法来完成您尝试做的事情。

Answer 2

首先，我强烈建议使用 T4 模板生成您的数据模型类，所以请看一下这个项目：linq2db/t4models. I also recommend to look at the videos here: https://github.com/linq2db/linq2db/wiki.

用 T4 生成我的数据模型后，它看起来像这样：

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
}

现在有几种方法可以加载目录及其文件。这是一些：

[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>();
        }
    }
}

或者您可以只进行连接，在 1 个查询中加载所有内容，然后将文件手动连接到内存中的目录。您可以编写自己的扩展方法来简化此操作。

Answer 3

试一试：

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 };

通过 linq2db 左加入分组

Left join with grouping via linq2db

c#

linq2db