diff --git a/JSONAPI.playground/Sources/Entities.swift b/JSONAPI.playground/Sources/Entities.swift
index c47dd34..14c2a89 100644
--- a/JSONAPI.playground/Sources/Entities.swift
+++ b/JSONAPI.playground/Sources/Entities.swift
@@ -25,7 +25,7 @@ extension String: CreatableRawIdType {
 
 // MARK: - typealiases for convenience
 public typealias ExampleEntity<Description: EntityDescription> = Entity<Description, NoMetadata, NoLinks, String>
-public typealias ToOne<E: OptionalRelatable> = ToOneRelationship<E, NoMetadata, NoLinks>
+public typealias ToOne<E: Identifiable> = ToOneRelationship<E, NoMetadata, NoLinks>
 public typealias ToMany<E: Relatable> = ToManyRelationship<E, NoMetadata, NoLinks>
 
 // MARK: - A few resource objects (entities)