Add relation page propety

src/Pages/Properties/Factory.php

@@ -20,6 +20,7 @@ public static function fromArray(array $array): PropertyInterface
             Property::TYPE_SELECT    => Select::fromArray($array),
             Property::TYPE_MULTI_SELECT => MultiSelect::fromArray($array),
             Property::TYPE_FORMULA => Formula::fromArray($array),
+            Property::TYPE_RELATION => Relation::fromArray($array),
             default => throw new Exception("Invalid property type: '{$type}'"),
         };
     }

src/Pages/Properties/Property.php

@@ -101,10 +101,10 @@ public function isMultiSelect(): bool
     //     return $this->type === self::TYPE_FORMULA;
     // }
 
-    // public function isRelation(): bool
-    // {
-    //     return $this->type === self::TYPE_RELATION;
-    // }
+    public function isRelation(): bool
+    {
+        return $this->type === self::TYPE_RELATION;
+    }
 
     // public function isRollup(): bool
     // {

src/Pages/Properties/Relation.php

@@ -0,0 +1,87 @@
+<?php
+
+namespace Notion\Pages\Properties;
+
+/**
+ * @psalm-type RelationJson = array{
+ *      id: string,
+ *      type: "relation",
+ *      relation: array{ id: string }[],
+ * }
+ */
+class Relation implements PropertyInterface
+{
+    private const TYPE = Property::TYPE_RELATION;
+
+    private Property $property;
+
+    /** @var string[] */
+    private array $pageIds;
+
+    private function __construct(Property $property, string ...$pageIds)
+    {
+        $this->property = $property;
+        $this->pageIds = $pageIds;
+    }
+
+    public static function create(string ...$pageIds): self
+    {
+        $property = Property::create("", self::TYPE);
+
+        return new self($property, ...$pageIds);
+    }
+
+    public static function fromArray(array $array): self
+    {
+        /** @psalm-var RelationJson $array */
+
+        $property = Property::fromArray($array);
+
+        $pageIds = array_map(
+            function (array $pageReference): string {
+                return $pageReference["id"];
+            },
+            $array[self::TYPE],
+        );
+
+        return new self($property, ...$pageIds);
+    }
+
+    public function toArray(): array
+    {
+        $array = $this->property->toArray();
+
+        $array[self::TYPE] = array_map(
+            function (string $pageId): array {
+                return [ "id" => $pageId ];
+            },
+            $this->pageIds,
+        );
+
+        return $array;
+    }
+
+    public function property(): Property
+    {
+        return $this->property;
+    }
+
+    /** @return string[] */
+    public function pageIds(): array
+    {
+        return $this->pageIds;
+    }
+
+    public function withRelations(string ...$pageIds): self
+    {
+        return new self($this->property, ...$pageIds);
+    }
+
+    public function addRelation(string $pageId): self
+    {
+        $pageIds = $this->pageIds;
+        $pageIds[] = $pageId;
+
+        return new self($this->property, ...$pageIds);
+    }
+}

tests/Unit/Pages/Properties/RelationTest.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Notion\Test\Unit\Pages\Properties;
+
+use Notion\Pages\Properties\Factory;
+use Notion\Pages\Properties\Relation;
+use PHPUnit\Framework\TestCase;
+
+class RelationTest extends TestCase
+{
+    public function test_create(): void
+    {
+        $id1 = "5604389a-8de1-4ba6-a07f-ca346ff98f00";
+        $id2 = "03d71291-5ca3-4daa-a1e8-f012d513e8c8";
+
+        $relation = Relation::create($id1, $id2);
+
+        $this->assertEquals([$id1, $id2], $relation->pageIds());
+        $this->assertTrue($relation->property()->isRelation());
+    }
+
+    public function test_replace_relation(): void
+    {
+        $id1 = "5604389a-8de1-4ba6-a07f-ca346ff98f00";
+        $id2 = "03d71291-5ca3-4daa-a1e8-f012d513e8c8";
+
+        $relation = Relation::create($id1)->withRelations($id2);
+
+        $this->assertEquals([$id2], $relation->pageIds());
+    }
+
+    public function test_add_relation(): void
+    {
+        $id1 = "5604389a-8de1-4ba6-a07f-ca346ff98f00";
+        $id2 = "03d71291-5ca3-4daa-a1e8-f012d513e8c8";
+
+        $relation = Relation::create($id1)->addRelation($id2);
+
+        $this->assertEquals([$id1, $id2], $relation->pageIds());
+    }
+
+    public function test_array_conversion(): void
+    {
+        $array = [
+            "id" => "abc",
+            "type" => "relation",
+            "relation" => [
+                [ "id" => "264f3f43-3d87-4bb2-bb66-11812ab74eae" ],
+                [ "id" => "f3902b7f-e9e2-4406-8c3f-5d07dbc87d66" ],
+            ],
+        ];
+
+        $relation = Relation::fromArray($array);
+        $fromFactory = Factory::fromArray($array);
+
+        $this->assertEquals($array, $relation->toArray());
+        $this->assertEquals($array, $fromFactory->toArray());
+    }
+}