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RavenDB – Map Reduce

December 22nd, 2011 Leave a comment Go to comments

So, learning Map Reduce in RavenDB I decided that to take what I learnt from the index created in my previous post. I think I picked something rather difficult to begin with, but I’ve succeeded Smile

Given a document Article which has a collection of Tags.

I want to get a Count of each Tag across all Articles.

public class Content
{
    public int Id { get; set; }
    public string Title { get; set; }

    public IEnumerable<Tag> Tags { get; set; }
}

public class Tag
{
    public string Name { get; set; }
}

Note: Tag is it’s own class because I added additional properties to it.

Now I insert some data:

using (var session = documentStore.OpenSession())
{
    session.Store(new Content
    {
        Title = "Test Title for a Video",
        Tags = new List<Tag>
        {
            new Tag() {Name = "c#"},
            new Tag() {Name = "autofac"},
            new Tag() {Name = "asp.net"},
        }
    });
    session.Store(new Content
    {
        Title = "Test Title for an Article",
        Tags = new List<Tag>
        {
            new Tag() {Name = "c#"},
            new Tag() {Name = "nhibernate"},
            new Tag() {Name = "fluent-nhibernate"},
            new Tag() {Name = "mvc"}
        }
    });
    session.Store(new Content
    {
        Title = "Test Title for an Article",
        Tags = new List<Tag>
        {
            new Tag() {Name = "ravendb"},
            new Tag() {Name = "asp.net"},
            new Tag() {Name = "autofac"},
            new Tag() {Name = "c#"}
        }
    });

    session.SaveChanges();
}

 

So I’m expecting a count of:

  • 3 x c#
  • 2 x autofac
  • 2 x asp.net
  • 1 x ravendb
  • 1 x mvc
  • 1 x nhibernate
  • 1 x fluent-nhibernate

    • I’m going to pull these out with a defined type rather than dynamic/object, so I’ve created a new class with Count and Name:

    public class TagResult
    {
        public int      Count   { get; set; }
        public string   Name    { get; set; }
    }

So creating a new Index:

public class All_Tags : AbstractMultiMapIndexCreationTask<TagResult>
{
    public All_Tags()
    {
    }
}

The first thing I need to do is map out ONLY the Tag’s, when I select out the Tag’s, I’m also going to include another field called Count, with a default value of 1. This is so I can re-use it to sum the total number of times the tag is used.

AddMap<Content>(contents => from content in contents
                            from tag in content.Tags
                            select new
                            {
                                Name = tag.Name,
                                Count = 1
                            });

This would give me a result that contains duplicates for the tags. Along the lines of:

c# 1
c# 1
c# 1
autofac 1
autofac 1
asp.net 1
asp.net 1
ravendb 1
mvc 1
nhibernate 1
fluent-nhibernate 1

So what I need to do in the Reduce, is group the tags together by their Name.

Reduce = results => from result in results
                    group result by result.Name into tag
                    select new
                    {
                        Count = tag.Sum(x => x.Count),
                        Name = tag.Key,
                    };

So here, I group all the tags together by their name, but I also sum the ‘count’ value together to get the total number of times the tag is used.

Now run up the app and view the index:

image

Now if I query the index:

image

Awesome. Now to query this, I have to use the TagResult class defined previously, and the All_Tags index just created.

using (var session = documentStore.OpenSession())
{
    var result = session.Query<TagResult, All_Tags>()
                        .ToList();

    foreach (var tag in result)
    {
        Console.WriteLine(tag.Count + " x " + tag.Name);
    }

    session.SaveChanges();
}

Running this I get the following result:

image

The results I expected previously.

So there you have it. Map Reduce.

Categories: RavenDB Tags:
  1. Wal
    April 27th, 2012 at 00:26 | #1
    Reply | Quote

    Very nice practical example; is this really the work of a kiwi? ;)

  2. Phillip
    April 27th, 2012 at 15:45 | #2
    Reply | Quote

    Haha, it sure is :)

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