Question

I have a data table that contains a one-to-many self referential relationship:

Plant
{ 
  ID,
  Name,
  ParentID
}

I’m trying to create a linq query that will tell me the total number of descendants stemming from one Plant.

Example:

I have 5 plants:

One {ID = 1, Name = Pine, Parent = null};// This is the root
    Two {ID = 2, Name = Evergreen, Parent = 1}
        Three {ID = 3, Name = Winter Evergreen, Parent = 2}
    Four {ID = 4, Name = Christmas Tree, Parent = 1}
Five {ID = 5, Name = Maple, Parent = null};// This is the root

When I call my LINQ query with an input of ID = 1, I want it to return 3, because there are 3 descendants of One; Two, Three and Four. Five is not a decedent of One.

The only way I can think about doing this involves nested recursive linq queries on the inner results. I have no idea how to do this and I feel as though there is an easier way.

I’m using C# 4.0 and LINQ if that matters.

Answer 1

If you were using SQL Server, the query you would want constructed would be:

DECLARE @TargetElementId int
SET @TargetElementId = 1;

WITH Plants AS
(
    SELECT ID, Name, ParentId
    FROM PlantsTable
    WHERE ParentId = @TargetElementId

    UNION ALL

    SELECT ID, Name, ParentId
    FROM PlantsTable
    INNER JOIN Plants ON PlantsTable.ParentId = Plants.ID
)
SELECT COUNT(ID) - 1 FROM Plants

I don’t believe this can be done with LinqToSql, see Common Table Expression (CTE) in linq-to-sql?, which is a question of a similar nature.