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:
{CODE-BLOCK:csharp} from cat in docs.Cats select new { cat.name } {CODE-BLOCK/}
And for dogs:
{CODE-BLOCK:csharp} from dog in docs.Dogs select new { dog.name } {CODE-BLOCK/}
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?
The easiest way to do this is by writing a multi-map index like this one:
{CODE:java multi_map_1@Indexes\IndexingPolymorphicData.java /}
And query it like this:
{CODE-TABS} {CODE-TAB:java:Query multi_map_2@Indexes\IndexingPolymorphicData.java /} {CODE-TAB-BLOCK:sql:RQL} from index 'Animals/ByName' where name = 'Mitzy' {CODE-TAB-BLOCK/} {CODE-TABS/}
Another option would be to modify the way we generate the Collection for subclasses of Animal, like this:
{CODE:java other_ways_1@Indexes\IndexingPolymorphicData.java /}
Using this method, we can now index on all animals using:
{CODE-BLOCK:csharp} from animal in docs.Animals select new { animal.name } {CODE-BLOCK/}
But what happens when you don't want to modify the entity name of an entity itself?
You can create a polymorphic index using:
{CODE-BLOCK:csharp} from animal in docs.WhereEntityIs("Cats", "Dogs") select new { animal.Name } {CODE-BLOCK/}
It will generate an index that matches both Cats and Dogs.