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Indexes: Indexing Polymorphic Data

By default, RavenDB indexes operate only on a specific entity type, or a Collection, that ignores the inheritance hierarchy.

For example, let's assume that we have the following inheritance hierarchy:

Figure 1: Polymorphic indexes

If we saved a Cat, it would have a collection set to "Cats" and if we saved a Dog, it would be in collection "Dogs".

If we wanted to index cats by name, we would write:

from cat in docs.Cats
select new { cat.Name }

And for dogs:

from dog in docs.Dogs
select new { dog.Name }

Although it works, each index would only give us results for the animal it has been defined on. But what if we wanted to query across all animals?

Multi-Map Indexes

The easiest way to do this is by writing a multi-map index like this one:

public class Animals_ByName : AbstractMultiMapIndexCreationTask
{
    public Animals_ByName()
    {
        AddMap<Cat>(cats => from c in cats select new { c.Name });

        AddMap<Dog>(dogs => from d in dogs select new { d.Name });
    }
}
public class Animals_ByName : AbstractJavaScriptIndexCreationTask
{
    public Animals_ByName()
    {
        Maps = new HashSet<string>()
        {
            @"map('cats', function (c){ return {Name: c.Name}})",
            @"map('dogs', function (d){ return {Name: d.Name}})"
        };
    }
}

And query it like this:

IList<IAnimal> results = session
    .Query<IAnimal, Animals_ByName>()
    .Where(x => x.Name == "Mitzy")
    .ToList();
List<IAnimal> results = session
    .Advanced
    .DocumentQuery<IAnimal, Animals_ByName>()
    .WhereEquals("Name", "Mitzy")
    .ToList();
from index 'Animals/ByName'
where Name = 'Mitzy'

Other Options

Another option would be to modify the way we generate the Collection for subclasses of Animal, like this:

DocumentStore store = new DocumentStore()
{
    Conventions =
    {
        FindCollectionName = type =>
        {
            if (typeof(Animal).IsAssignableFrom(type))
                return "Animals";
            return DocumentConventions.DefaultGetCollectionName(type);
        }
    }
};

Using this method, we can now index on all animals using:

from animal in docs.Animals
select new { animal.Name }

But what happens when you don't want to modify the entity name of an entity itself?

You can create a polymorphic index using:

from animal in docs.WhereEntityIs("Cats", "Dogs")
select new { animal.Name }

It will generate an index that matches both Cats and Dogs.