StackOverFlowQuestion about how to express a question for left self join in Raven DB. To explain what I want I use T-SQL. Below Example of RavenDB map reduce index that almost does it.

RavenDB Map Reduce index Almost

  public class PageTextElement
    {
        public string Page;
        public string Token;
        public string Webtext;
        public string Language;
        public string Translator;
        public DateTime CreationTime;
        
    }


    public class ComparePageTextElementCount
    {
        public string Page;
        public string Token;
        public string Webtext;
        public string WebtextCompare;
        public int Count;
    }


    public class LeftJoinPageTextTranslationsCount : AbstractMultiMapIndexCreationTask<ComparePageTextElementCount>
    {
        public LeftJoinPageTextTranslationsCount()
        {
            AddMap<PageTextElement>(baseElements =>
                                    from baseElement in baseElements.Where(l => l.Language == "en")
                                    select
                                        new
                                        {
                                            baseElement.Page,
                                            baseElement.Token,
                                            baseElement.Webtext,
                                            WebtextCompare = (string)null,
                                            Count = 0
                                        });

            AddMap<PageTextElement>(compareElements =>
                                    from compareElement in compareElements.Where(l => l.Language == "sv")
                                    select
                                        new
                                        {
                                            compareElement.Page,
                                            compareElement.Token,
                                            Webtext = (string)null,
                                            WebtextCompare = compareElement.Webtext,
                                            Count = 1
                                        }
                );
            Reduce = results => from result in results
                                group result by
                                    new { result.Page, result.Token }
                                    into g
                                    select new
                                    {
                                        g.Key.Page,
                                        g.Key.Token,
                                        Webtext = g.Select(x => x.Webtext).FirstOrDefault(x => x != null),
                                        WebtextCompare = g.Select(x => x.WebtextCompare).FirstOrDefault(x => x != null),
                                        Count = g.Sum(x => x.Count)
                                    };
            Index(x => x.Webtext, FieldIndexing.Analyzed);
        }
    }

Self Left join Example

DROP TABLE PageTextElement_TB
CREATE TABLE PageTextElement_TB (
[Page] CHAR(10)NOT NULL
,[Token] CHAR(10)NOT NULL
,[Webtext] NVARCHAR(Max) NOT NULL
,LANGUAGE CHAR(2) NOT NULL
,Translator NVARCHAR(10) NOT NULL
,CreationTime DATETIME NOT NULL
)
ALTER TABLE [dbo].[PageTextElement_TB] ADD  CONSTRAINT [DF_PageTextElement_TB_CreationTime]  DEFAULT (getdate()) FOR [CreationTime]
GO


USE [test]
GO

IF  EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DF_PageTextElement_TB_CreationTime]') AND type = 'D')
BEGIN
ALTER TABLE [dbo].[PageTextElement_TB] DROP CONSTRAINT [DF_PageTextElement_TB_CreationTime]
END

GO

USE [test]
GO

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[PageTextElement_TB]') AND type in (N'U'))
DROP TABLE [dbo].[PageTextElement_TB]

CREATE TABLE [dbo].[PageTextElement_TB](
  [Page] [char](10) NOT NULL,
	[Token] [char](15) NOT NULL,
	[Language] [char](2) NOT NULL,
	[Webtext] [nvarchar](max) NOT NULL,
	[Translator] [nvarchar](10) NOT NULL,
	[CreationTime] [datetime] NOT NULL,
 CONSTRAINT [PK_PageTextElement_TB] PRIMARY KEY CLUSTERED 
(
	[Page] ASC,
	[Token] ASC,
	[Language] ASC
))

ALTER TABLE [dbo].[PageTextElement_TB] ADD  CONSTRAINT [DF_PageTextElement_TB_CreationTime]  DEFAULT (getdate()) FOR [CreationTime]
GO
INSERT INTO dbo.PageTextElement_TB
        ( Page ,
          Token ,
          [Language] ,
          Webtext ,
          Translator 
        )
VALUES 
-- English text 
 ( 'home' ,'Welcome' ,'en' , 
          N'Welcome to Aniara' ,  N'robot'  
        ),
        ( 'home' ,'RulesOfBoarding' , 'en' ,
          N'Do not break line' ,  N'robot' 
        )
        ,
        ( 'home' , 'PriceModel' , 'en' ,
          N'Based on weight and oxygen consumption' , N'robot'  
        )
        -- Here comes the one line that is missing in other language should be visible in left join
        ,( 'home' , 'RebateModel' , 'en' ,
          N'Truly Unique talent cant reduce price with 50%' , N'robot'  
        )
      -- Swedish text 
 ,( 'home' ,'Welcome' ,'sv' , 
          N'Välkommen till Aniara' ,  N'robot'  
        ),
        ( 'home' ,'RulesOfBoarding' , 'sv' ,
          N'Träng dig ej i kön' ,  N'robot' 
        )
        ,
        ( 'home' , 'PriceModel' , 'sv' ,
          N'Priset baseras på vikt och syreförbrukning' , N'robot'  
        )

GO

-- I can express this in RavenDB
SELECT bt.Page,bt.Token,bt.Webtext AS baseText,COUNT(ct.Webtext) FROM dbo.PageTextElement_TB bt
LEFT JOIN dbo.PageTextElement_TB AS ct ON bt.Page=ct.Page AND bt.Token = ct.Token AND ct.Language='sv' 
WHERE bt.Language='en'
GROUP BY bt.Page,bt.Token,bt.Webtext

-- But I want to express this in RavenDB
SELECT bt.Page,bt.Token,bt.Webtext AS baseText,ct.Webtext AS compareText FROM dbo.PageTextElement_TB bt
LEFT JOIN dbo.PageTextElement_TB AS ct ON bt.Page=ct.Page AND bt.Token = ct.Token AND ct.Language='sv' 
WHERE bt.Language='en'

It is based on the http://ayende.com/blog/89089/ravendb-multi-maps-reduce-indexes.
The project with my experiment are here : https://github.com/patriklindstrom/LeftJoinRavenDB/tree/StackOverFlowQuestion

I added the answer as a new revision. http://stackoverflow.com/a/18953783/648076