Added findOne function to Lens (#100)

Resolves #95.

library/lens/lens.ts

@@ -235,6 +235,39 @@ export class Lens<T extends Schema> {
     return this.decode(graph);
   }
 
+  /**
+   * Find one entity that matches the given search criteria.
+   *
+   * The search criteria is a JSON object that may contain properties from the data schema.
+   *
+   * @example
+   * ```typescript
+   * import { createLens } from "ldkit";
+   * import { schema } from "ldkit/namespaces";
+   *
+   * // Create a schema
+   * const PersonSchema = {
+   *   "@type": schema.Person,
+   *   name: schema.name,
+   * } as const;
+   *
+   * // Create a resource using the data schema above
+   * const Persons = createLens(PersonSchema);
+   *
+   * // Find one person with name that starts with "Ada"
+   * const person = await Persons.findOne({
+   *   name: { $strStarts: "Ada" },
+   * });
+   * ```
+   *
+   * @param options Search criteria and pagination options
+   * @returns entities that match the given search criteria
+   */
+  async findOne(where?: SchemaSearchInterface<T>) {
+    const results = await this.find({ where, take: 1 });
+    return results.length > 0 ? results[0] : null;
+  }
+
   /**
    * Find a single entity that matches the given IRI.
    *

tests/lens_common.test.ts

@@ -129,6 +129,13 @@ Deno.test("Lens / Common / Get resource by quad condition", async () => {
   assertEquals(result[0], Tarantino);
 });
 
+Deno.test("Lens / Common / Get one resource", async () => {
+  const { directors } = init();
+  const result = await directors.findOne();
+
+  assertEquals(result, Kubrick);
+});
+
 Deno.test("Lens / Common / Count resources", async () => {
   const { directors } = init();
   const count = await directors.count();