diff --git a/.idea/php.xml b/.idea/php.xml
index 31b8a3a..97a38d7 100644
--- a/.idea/php.xml
+++ b/.idea/php.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
-  <component name="PhpProjectSharedConfiguration" php_language_level="8.0">
+  <component name="PhpProjectSharedConfiguration" php_language_level="8.1">
     <option name="suggestChangeDefaultLanguageLevel" value="false" />
   </component>
 </project>
\ No newline at end of file
diff --git a/prop-relations/Brand.php b/prop-relations/Brand.php
new file mode 100644
index 0000000..7a3b030
--- /dev/null
+++ b/prop-relations/Brand.php
@@ -0,0 +1,10 @@
+<?php
+
+class Brand
+{
+  public function __construct (
+    public readonly int $id,
+    public readonly string $name,
+  ) {
+  }
+}
diff --git a/prop-relations/Category.php b/prop-relations/Category.php
new file mode 100644
index 0000000..2d9e35e
--- /dev/null
+++ b/prop-relations/Category.php
@@ -0,0 +1,10 @@
+<?php
+
+class Category
+{
+  public function __construct (
+    public readonly int $id,
+    public readonly string $name,
+  ) {
+  }
+}
diff --git a/prop-relations/Collection.php b/prop-relations/Collection.php
new file mode 100644
index 0000000..a7dc21e
--- /dev/null
+++ b/prop-relations/Collection.php
@@ -0,0 +1,63 @@
+<?php
+
+/**
+ * Collection<T[]>
+ */
+class Collection implements Iterator
+{
+  private int $position = 0;
+  private ?array $array;
+
+  public function __construct (
+    /** @var T[] */
+    ?array $items = null,
+  ) {
+    $this->array = $items;
+  }
+
+  public function rewind (): void
+  {
+    $this->isInitializedOrFail();
+    $this->position = 0;
+  }
+
+  public function current ()
+  {
+    $this->isInitializedOrFail();
+    return $this->array[$this->position];
+  }
+
+  public function key ()
+  {
+    $this->isInitializedOrFail();
+    return $this->position;
+  }
+
+  public function next (): void
+  {
+    $this->isInitializedOrFail();
+    ++$this->position;
+  }
+
+  public function valid (): bool
+  {
+    $this->isInitializedOrFail();
+    return isset($this->array[$this->position]);
+  }
+
+  /**
+   * @return T[]
+   */
+  public function getItems (): array
+  {
+    $this->isInitializedOrFail();
+    return $this->array;
+  }
+
+  protected function isInitializedOrFail (): void
+  {
+    if ($this->array === null) {
+      throw new CollectionNotInitializedException();
+    }
+  }
+}
diff --git a/prop-relations/CollectionNotInitializedException.php b/prop-relations/CollectionNotInitializedException.php
new file mode 100644
index 0000000..7d9b682
--- /dev/null
+++ b/prop-relations/CollectionNotInitializedException.php
@@ -0,0 +1,6 @@
+<?php
+
+class CollectionNotInitializedException extends Exception
+{
+
+}
diff --git a/prop-relations/Product.php b/prop-relations/Product.php
new file mode 100644
index 0000000..eccdbc4
--- /dev/null
+++ b/prop-relations/Product.php
@@ -0,0 +1,19 @@
+<?php
+
+class Product
+{
+  public function __construct (
+    public readonly int $id,
+    public readonly string $name,
+    /** @var Collection<Category> */
+    public readonly Collection $categories,
+    // Option: add ID
+    public readonly ?int $brandId = null,
+    // Option: add Model
+    public readonly ?Brand $brand = null,
+    // Option: schrodinger cat
+    public readonly ?Relation $brand_ = null,
+  )
+  {
+  }
+}
diff --git a/prop-relations/Relation.php b/prop-relations/Relation.php
new file mode 100644
index 0000000..bbbd5de
--- /dev/null
+++ b/prop-relations/Relation.php
@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * Relation<T>
+ */
+class Relation
+{
+  public function __construct (
+    /** @var T */
+    protected readonly ?object $value = null,
+  ) {
+  }
+
+  /**
+   * @return T
+   */
+  public function getValue (): object
+  {
+    $this->isInitializedOrFail();
+    return $this->value;
+  }
+
+  protected function isInitializedOrFail (): void
+  {
+    if ($this->value === null) {
+      throw new CollectionNotInitializedException();
+    }
+  }
+}
diff --git a/prop-relations/index.php b/prop-relations/index.php
new file mode 100644
index 0000000..e2b7761
--- /dev/null
+++ b/prop-relations/index.php
@@ -0,0 +1,68 @@
+<?php
+
+$categoryA = new Category(1, 'category a');
+$categoryB = new Category(2, 'category b');
+
+// GetProductWithCategoriesQuery
+$productWithCategories = new Product(
+  1,
+  'product a',
+  new Collection([$categoryA, $categoryB]),
+);
+
+// Works
+$productCategories = $productWithCategories->categories->getItems();
+
+// GetProductQuery
+$productWithoutCategories = new Product(
+  1,
+  'product a',
+  new Collection(),
+);
+
+// Throw Exception
+$productCategories = $productWithoutCategories->categories->getItems();
+
+// -------------------------------------------------------
+
+// how to differentiate if Brand was filled or is null
+
+// Oriol suggestion
+$productWithBrand = new Product(
+  id: 2,
+  name: 'product b',
+  categories: new Collection(),
+  brandId: 1,
+  brand: null, //new Brand(1, 'brand a'),
+);
+
+// Miguel Angel suggestion
+$productWithoutBrand = new Product(
+  id: 2,
+  name: 'product b',
+  categories: new Collection(),
+  brand: null, //new Brand(1, 'brand a'),
+);
+
+// Àlex Suggestion
+// GetProductWithBrandQuery
+$productWithOptionalBrand = new Product(
+  id: 2,
+  name: 'product b',
+  categories: new Collection(),
+  brand_: new Relation(new Brand(1, 'brand a')),
+);
+
+// Works
+$productBrand = $productWithOptionalBrand->brand_->getValue();
+
+// GetProductQuery
+$productWithoutOptionalBrand = new Product(
+  id: 2,
+  name: 'product b',
+  categories: new Collection(),
+  brand_: new Relation(),
+);
+
+// Throw Exception
+$productBrand = $productWithoutOptionalBrand->brand_->getValue();