Cloud

There is a special aggregation feature provided by PlayMorphia called cloud. One immediate example of cloud usage is found in the Yabe sample application.

In Yabe sample application, a Post model is defined to capture blog post data. Each Post could be associated with zero or more tags. In the following example, PostA has been tagged with “play” and “architecture”, PostB has been tagged with “play”, “mvc” and “test”.

Now we want to calculate the tag cloud, and we expect it should be like as follows:

Now let’s see some source code:

// Post model class
@Entity public class Post extends Model {
    public String title;
    public String content;
    ...
    public Set<String> tags = new HashSet<String>();
    ...
}
// calculate tag cloud
public class Tag {
    public static Map<String, Long> getCloud() {
        return Post._cloud("tags");
    }
}
// Test code to verify the tag cloud computation
Map<String, Long> cloud = Tag.getCloud();
assertEquals(cloud.get("blue"), 1);
assertEquals(cloud.get("green"), 1);
assertEquals(cloud.get("red"), 2);

And compare to the JPA version code:

// Post model class
@Entity public class Post extends Model {
    public String title;
    public String content;
    ...
    public Set<Tag> tags;
    ...
}
// Tag model class
@Entity public class Tag extends Model {
    public String name;
    ...
    public static List<Map> getCloud() {
        List<Map> result = Tag.find(
            "select new map(t.name as tag, count(p.id) as pound) from Post p join p.tags as t group by t.name"
        ).fetch();
        return result;
    }
}
// Test code to verify the tag cloud computation
List<Map> cloud = Tag.getCloud();
Collections.sort(cloud, new Comparator<Map>() {
    public int compare(Map m1, Map m2) {
        return m1.get("tag").toString().compareTo(m2.get("tag").toString());
    }
});
assertEquals("[{tag=Blue, pound=1}, {tag=Green, pound=1}, {tag=Red, pound=2}]", 
  cloud.toString());

See also

1. Understanding PlayMorphia model
2. Aggregation and Group aggregation