How can I query this hierarchical data using LINQ?

Answer 1

This StackOverflow post might provide some insight:

http://stackoverflow.com/questions/236575/linq-to-sql-hierarchical-query-to-find-ancestors

Answer 2

If you are doing this with direct LINQ to SQL, there is no way to do this without some kind of recursion, whether you do it yourself or you hide it behind an extension method. Recursive SQL is very bad (many round trips, many single queries).

There are two options here. One is to pull the entire table(s) with the hierarchy into memory and use LINQ to Objects on it. Leave the "details" tables in SQL. If you have less than several thousand entities, this is probably the most efficient way to go. You can keep a single copy of the table(s) in cache and refresh them when necessary. When you need to fetch more detailed data from the DB for a single record, you can reattach that entity from your cached hierarchy to a new DataContext and fetch it.

The other option is to use a more complex relationship model in your database. Storing parent only by nature demands recursion, but you can use the adjacency list model to construct a single query which can span many levels of inheritance. This will mean your LINQ to SQL queries become less intuitive (querying against Entity.Right and Entity.Left isn't quite as pretty as Parent or Children...) but you can do in one query what might take hundreds or thousands in the literal recursive approach.

Answer 3

If you drop all four tables (Agency, BusinessUnit, Client, Map) on the linq to sql designer, and draw relationships from Map to the other three, there will be some useful properties on Map.

  //construct a query to fetch the row/column shaped results.
var query = 
  from m in db.map
  //where m.... ?
  let a = m.Agency
  let b = m.BusinessUnit
  let c = m.Client
  // where something about a or b or c ?
  select new {
    AgencyID = a.AgencyID,
    AgencyName = a.Name,
    BusinessUnitID = b.BusinessUnitID,
    ClientID = c.ClientID,
    NumberOfAccounts = c.NumberOfAccounts,
    Score = c.Score
  };
  //hit the database
var rawRecords = query.ToList();

  //shape the results further into a hierarchy.    
List<Agency> results = rawRecords
  .GroupBy(x => x.AgencyID)
  .Select(g => new Agency()
  {
    Name = g.First().AgencyName,
    BusinessUnits = g
    .GroupBy(y => y.BusinessUnitID)
    .Select(g2 => new BusinessUnit()
    {
      Clients = g2
      .Select(z => new Client()
      {
        NumberOfAccounts = z.NumberOfAccounts,
        Score = z.Score
      })
    })
  })
  .ToList();

If approriate filters are supplied (see the commented out where clauses), then only the needed portions of the tables will be pulled into memory. This is standard SQL joining at work here.