Add Nested definition to support nested result structures

README.md

@@ -133,3 +133,48 @@ $resolver = new QueryResolver(
 $resolver->resolve($someShipWithoutAPilot);
 // ['pilotName' => 'Unknown pilot']
 ```
+
+### Nested
+
+The `Nested` definition lets you return nested data structures instead of a
+flat associative array. It navigates to a sub-source using any other
+definition (typically a `Path`) and then applies its own set of queries to
+it.
+
+If the navigation resolves to a single object, `Nested` returns a nested
+associative array. If it resolves to a collection, `Nested` applies its
+queries to every entry and returns a list of nested associative arrays.
+`Nested` definitions can be combined to build arbitrarily deep structures.
+
+```php
+<?php
+
+use Lemonade\ObjectQuery\Query\Query;
+use Lemonade\ObjectQuery\QueryResolver;
+use Lemonade\ObjectQuery\Definition\Nested;
+use Lemonade\ObjectQuery\Definition\Path;
+
+$resolver = new QueryResolver(
+    new Query('name', (new Path())->get('name')),
+    new Query('pilot', new Nested(
+        (new Path())->get('pilot'),
+        new Query('name', (new Path())->get('name')),
+        new Query('id', (new Path())->get('id'))
+    )),
+    new Query('starShips', new Nested(
+        (new Path())->get('starShips'),
+        new Query('name', (new Path())->get('name'))
+    ))
+);
+
+$resolver->resolve($someCharacter);
+// [
+//     'name' => 'Han Solo',
+//     'pilot' => ['name' => 'Han Solo', 'id' => 1002],
+//     'starShips' => [['name' => 'Millenium Falcon'], ['name' => 'Imperial shuttle']],
+// ]
+```
+
+If the navigation definition is `optional()` and cannot be resolved, the
+`Nested` definition behaves like any other and the corresponding key is
+omitted from the result set.

src/Definition/Nested.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Lemonade\ObjectQuery\Definition;
+
+use Lemonade\ObjectQuery\DefinitionInterface;
+use Lemonade\ObjectQuery\QueryInterface;
+use Lemonade\ObjectQuery\QueryResolver;
+use Lemonade\ObjectQuery\Source\ArraySource;
+use Lemonade\ObjectQuery\Source\ObjectSource;
+use Lemonade\ObjectQuery\SourceInterface;
+
+/**
+ * Resolves one or more queries against a sub-source, allowing the resolver
+ * to build nested data structures instead of a flat associative array.
+ *
+ * The given definition (typically a Path) is used to navigate to the
+ * sub-source first. Depending on what it resolves to, the nested queries
+ * are then applied either to a single object (resulting in a nested
+ * associative array) or to every entry of a collection (resulting in a
+ * list of nested associative arrays).
+ */
+final class Nested implements DefinitionInterface
+{
+    /** @var QueryInterface[] */
+    private array $queries;
+
+    public function __construct(private readonly DefinitionInterface $definition, QueryInterface ...$queries)
+    {
+        $this->queries = $queries;
+    }
+
+    public function getValue(SourceInterface $source): mixed
+    {
+        $target = $this->definition->getValue($source);
+
+        if ($target instanceof UnresolvedValue) {
+            return $target;
+        }
+
+        if ($target instanceof ArraySource) {
+            return array_map(
+                fn (mixed $entry) => $this->resolveEntry($entry),
+                $target->getSource()
+            );
+        }
+
+        if ($target instanceof SourceInterface) {
+            return $this->resolver()->resolve($target);
+        }
+
+        return $target;
+    }
+
+    private function resolveEntry(mixed $entry): mixed
+    {
+        if (is_object($entry)) {
+            return $this->resolver()->resolve(new ObjectSource($entry));
+        }
+
+        if (is_array($entry)) {
+            return $this->resolver()->resolve(new ArraySource($entry));
+        }
+
+        return $entry;
+    }
+
+    private function resolver(): QueryResolver
+    {
+        return new QueryResolver(...$this->queries);
+    }
+}

tests/Unit/Definition/NestedTest.php

@@ -0,0 +1,134 @@
+<?php
+
+namespace Lemonade\ObjectQuery\Tests\Unit\Definition;
+
+use Lemonade\ObjectQuery\Definition\Nested;
+use Lemonade\ObjectQuery\Definition\Path;
+use Lemonade\ObjectQuery\DefinitionInterface;
+use Lemonade\ObjectQuery\Query\Query;
+use Lemonade\ObjectQuery\QueryResolver;
+use Lemonade\ObjectQuery\Source\ObjectSource;
+use PHPUnit\Framework\TestCase;
+
+class NestedTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function itCanBeConstructed(): void
+    {
+        $actual = new Nested(new Path());
+
+        $this->assertInstanceOf(DefinitionInterface::class, $actual);
+    }
+
+    /**
+     * @test
+     */
+    public function itShouldResolveANestedObject(): void
+    {
+        $pilot = (object) ['name' => 'Han Solo', 'id' => 1002];
+        $ship  = (object) ['name' => 'Millenium Falcon', 'pilot' => $pilot];
+
+        $resolver = new QueryResolver(
+            new Query('name', (new Path())->get('name')),
+            new Query('pilot', new Nested(
+                (new Path())->get('pilot'),
+                new Query('name', (new Path())->get('name')),
+                new Query('id', (new Path())->get('id'))
+            ))
+        );
+
+        $result = $resolver->resolve(new ObjectSource($ship));
+
+        $this->assertSame(
+            [
+                'name'  => 'Millenium Falcon',
+                'pilot' => ['name' => 'Han Solo', 'id' => 1002],
+            ],
+            $result
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function itShouldResolveANestedListOfObjects(): void
+    {
+        $ship = (object) [
+            'name' => 'Millenium Falcon',
+            'crew' => [
+                (object) ['name' => 'Han Solo'],
+                (object) ['name' => 'Chewbacca'],
+            ],
+        ];
+
+        $resolver = new QueryResolver(
+            new Query('crew', new Nested(
+                (new Path())->get('crew'),
+                new Query('name', (new Path())->get('name'))
+            ))
+        );
+
+        $result = $resolver->resolve(new ObjectSource($ship));
+
+        $this->assertSame(
+            ['crew' => [['name' => 'Han Solo'], ['name' => 'Chewbacca']]],
+            $result
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function itShouldSupportDeeplyNestedStructures(): void
+    {
+        $engine = (object) ['power' => 'strong'];
+        $pilot  = (object) ['name' => 'Han Solo', 'ship' => (object) ['name' => 'Falcon', 'engine' => $engine]];
+
+        $resolver = new QueryResolver(
+            new Query('name', (new Path())->get('name')),
+            new Query('ship', new Nested(
+                (new Path())->get('ship'),
+                new Query('name', (new Path())->get('name')),
+                new Query('engine', new Nested(
+                    (new Path())->get('engine'),
+                    new Query('power', (new Path())->get('power'))
+                ))
+            ))
+        );
+
+        $result = $resolver->resolve(new ObjectSource($pilot));
+
+        $this->assertSame(
+            [
+                'name' => 'Han Solo',
+                'ship' => [
+                    'name'   => 'Falcon',
+                    'engine' => ['power' => 'strong'],
+                ],
+            ],
+            $result
+        );
+    }
+
+    /**
+     * @test
+     */
+    public function itShouldPropagateAnUnresolvedNavigationValue(): void
+    {
+        $ship = (object) ['name' => 'Millenium Falcon'];
+
+        $resolver = new QueryResolver(
+            new Query('name', (new Path())->get('name')),
+            new Query('pilot', new Nested(
+                (new Path())->get('pilot')->get('name')->optional(),
+                new Query('name', (new Path())->get('name'))
+            ))
+        );
+
+        $result = $resolver->resolve(new ObjectSource($ship));
+
+        $this->assertSame(['name' => 'Millenium Falcon'], $result);
+    }
+}