inner join inside extension method (linq)

Question

I am trying to write an extention method for a repository. The repository has a method IQueryable<parent> GetAll(). I want to create an extention method FilterByChildId(int childId) so that I can write:

List<parent> data = from d in repository.getAll().FilterByChildId(33).ToList()

Not sure how to do the join on the parent and child inside the extension method. I thoudht it would be something like:

public static IQueryable<parent> FilterByChildId(
  this IQueryable<parent> query, 
  int id)
{
  return from data in query where data.child.id == id select data
}

but not a chance. I tried all sorts of variations using join, groupjoin but not clicking. Any help would be appreciated.

Answer 1

I would essentially write the same as you did.

public static IQueryable<Parent> FilterByChildId(
   this IQueryable<Parent> parents, Int32 id)
{
   return parents.Where(p => p.Child.Id == id);
}

Just checked it with one of my Entity Framework models and it works. I assumed a simple many (parent) to one (child) relationship as your code indicates. Note that this is a bit confusing - many parents for one child. Could it be that you have a many to many relationship? One parent might have many childs and one child might have multiple parents? Then the proeprty Childs (child in your code) would be a collection and the code must be changed as follows.

public static IQueryable<Parent> FilterByChildId(
   this IQueryable<Parent> parents, Int32 id)
{
   return parents.Where(p => p.Childs.Any(c => c.Id == id));
}

The third posibility is one parent with many childs. The second method will work for this case but always return at most one parent assuming the child ids are unique.