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:

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:

const indexDefinition = new IndexDefinition();
indexDefinition.name = "Animals/ByName";
indexDefinition.maps = new Set([
    "docs.Cats.Select(c => new { name = c.name})",
    "docs.Dogs.Select(c => new { name = c.name})"
]);

And query it like this:

const results = await session
    .query({ indexName: "Animals/ByName" })
    .whereEquals("name", "Mitzy")
    .all();

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:

{
    const store = new DocumentStore();
    store.conventions.findCollectionName = clazz => {
        if (clazz instanceof Animal) {
            return "Animals";
        }

        return DocumentConventions.defaultGetCollectionName(clazz);
    };
}

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.