Inverse properties

LDkit allows for resolving incoming links in linked data. This is particularly useful when you want to query entities that are related in reverse order. In typical scenarios, a property of a particular entity corresponds on a conceptual level to a triple like (entity property value), but with inverse properties, you can flip this to represent (value property entity). This feature is crucial for queries where you need to find entities that reference a given entity.

To illustrate the use of inverse properties, let’s consider a scenario where we want to list all movies directed by a particular director.

Creating context and Schemas

First, define your context, schemas and lens. Here, we create two schemas: MovieSchema and DirectorSchema.

import { createLens } from "ldkit";
import { dbo, rdfs, xsd } from "ldkit/namespaces";

// Context with SPARQL endpoint and language preference
const context = {
  sources: ["https://dbpedia.org/sparql"],
  language: "en",
};

// Movie schema
const MovieSchema = {
  "@type": dbo.Film,
  name: rdfs.label,
} as const;

// Director schema with inverse property
const DirectorSchema = {
  "@type": dbo.Person,
  name: rdfs.label,
  birthDate: {
    "@id": dbo.birthDate,
    "@type": xsd.date,
  },
  isDirectorOf: {
    "@id": dbo.director,
    "@inverse": true, // This is cool!
    "@array": true,
    "@schema": MovieSchema,
  },
} as const;

// Create Directors Lens to query data
const Directors = createLens(DirectorSchema, context);

Using inverse properties

In DirectorSchema, the isDirectorOf property is defined with @inverse modifier. This means that the library will automatically resolve the inverse relationship, effectively querying for movies where the given person is the director.

// Fetch a particular director by IRI
const tarantino = await Directors.findByIri(
  "http://dbpedia.org/resource/Quentin_Tarantino",
);

if (tarantino === null) {
  throw new Error("Tarantino not found?!");
}

// List movies directed by Tarantino
console.log("List of movies directed by Tarantino:");
for (const movie of tarantino.isDirectorOf) {
  console.log(movie.name);
}

Benefits of Inverse Properties

• Simplicity: The feature simplifies the querying of inverse relationships, making code more readable and maintainable.
• Efficiency: It enables efficient data retrieval from SPARQL endpoints by inverting the usual subject-predicate-object query pattern.
• Flexibility: Can be used in various scenarios where the relationship between entities is better expressed or only available in a reversed order.

Inverse properties in LDkit offer an elegant solution for querying linked data in reverse order. By simply adding "@inverse": true to a property in your schema, you can easily retrieve and work with entities that are related in a reverse direction. This feature is especially useful in complex data models where relationships are not always straightforward.