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:
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 });
}
}
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.