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I work in C# and Entity framework. I have a table in my database named Genre. Here are its attributes: idGenre, name, idParentGenre.

For example. the values would be:

(idGenre = 1, name = "acoustic", idParentGenre=2)

(idGenre = 2, name = "rock", idParentGenre=2)

(idGenre = 3, name = "country", idParentGenre=4)

(idGenre = 4, name = "folk", idParentGenre=5)

(idGenre = 5, name = "someOtherGenre", idParentGenre=5)

As you can see, it's kind of a tree.

Now, I have a method for searching through this table. The input parameter is idGenre and idParentGenre. I should return if the genre (idGenre) is a son/grandchild/grandgrandchild/... of idParentGenre.

For example, I get idGenre=3, idParentGenre=5, I should return true.

However, there isn't recursion in Linq. Is there a way I can do this?

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  • What do you mean by "recursion in Linq". Can you show the method that you have? What is wrong with it? Commented Dec 28, 2011 at 20:57
  • 1
    Do it the old fashion way. LINQ is great, but it cannot solve every problem. Commented Dec 28, 2011 at 20:59

3 Answers 3

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4

I would make a method to handle this instead of using LINQ:

bool HasParent(int genre, int parent)
{
    Genre item = db.Genres.FirstOrDefault(g => g.IdGenre == genre);
    if (item == null)
        return false;

    // If there is no parent, return false, 
    // this is assuming it's defined as int?
    if (!item.idParentGenre.HasValue)
        return false;

    if (item.idParentGenre.Value == parent)
        return true;

    return HasParent(item.idParentGenre, parent);
}

This lets you handle this in a single recursive function.

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Comments

2

It looks like you're trying to implement a tree without using a tree.

Have you considered...using a tree? Here's a great question and some answers you could build off (including one with code:

delegate void TreeVisitor<T>(T nodeData);

class NTree<T>
{
    T data;
    LinkedList<NTree<T>> children;

    public NTree(T data)
    {
        this.data = data;
        children = new LinkedList<NTree<T>>();
    }

    public void addChild(T data)
    {
        children.AddFirst(new NTree<T>(data));
    }

    public NTree<T> getChild(int i)
    {
        foreach (NTree<T> n in children)
            if (--i == 0) return n;
        return null;
    }

    public void traverse(NTree<T> node, TreeVisitor<T> visitor)
    {
        visitor(node.data);
        foreach (NTree<T> kid in node.children)
            traverse(kid, visitor);
    }        
}
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1 Comment

I can't use a tree. I have a table in my database for this purpose.
1

Bring the table of genres in the memory (it cannot be that big), and recursively traverse it to create a mapping between an idGenre and a transitive closure of its descendents, like this:

1: {1, 2}
2: {2}
3: {3, 4, 5}
4: {4, 5}
5: {5}

The data above stays only in memory. You recompute it every time on start-up, and on updates to the genres table.

When it is time to query for all songs in a specific genre, use the pre-calculated table in an idGenre in ... query, like this:

IEnumerable<Song> SongsWithGenreId(int idGenre) {
    var idClosure = idToIdClosure[idGenre];
    return context.Songs.Where(song => idClosure.Contains(song.idGenre));
}
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2 Comments

But this isn't a perfect solution. I already have a place where I place the id of the parent (this place is the table), I don't think another place for the same purpose is good.
@Srcee It isn't "another place" to store your data, it's a cache of the same data, only preprocessed. Your queries would be very fast - much faster than anything else suggested so far, because there will be only one roundtrip. All the recursion would be baked into the structure of the idToIdClosure dictionary, which is good, because genres do not change all that often.

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