diff --git a/lib/ActiveRecord/RelationCollection.php b/lib/ActiveRecord/RelationCollection.php
index 780064e..5aa7746 100644
--- a/lib/ActiveRecord/RelationCollection.php
+++ b/lib/ActiveRecord/RelationCollection.php
@@ -1,156 +1,81 @@
 <?php
 
-/*
- * This file is part of the ICanBoogie package.
- *
- * (c) Olivier Laviale <olivier.laviale@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
 namespace ICanBoogie\ActiveRecord;
 
-use ArrayAccess;
 use Closure;
-use ICanBoogie\ActiveRecord;
-use ICanBoogie\OffsetNotWritable;
+use ICanBoogie\ActiveRecord\Config\Association;
 
 use function is_string;
 
 /**
  * Relation collection of a model.
- *
- * @implements ArrayAccess<string, Relation>
- *     Where _key_ is a getter name e.g. 'comments'
  */
-class RelationCollection implements ArrayAccess
+class RelationCollection
 {
     /**
-     * Relations.
-     *
      * @var array<string, Relation>
-     *     Where _key_ is a getter name e.g. 'comments'
+     *     Where _key_ is a getter name; for example, 'comments'
      */
-    private array $relations;
+    private readonly array $relations;
 
     public function __construct(
         public readonly Model $model,
-        ?ActiveRecord\Config\Association $association,
+        ?Association $association
     ) {
-        $this->apply_association($association);
-    }
+        $relations = [];
+
+        if ($association) {
+            foreach ($association->belongs_to as $r) {
+                $as = $r->as;
+                $relations[$as] = new BelongsToRelation(
+                    owner: $this->model,
+                    related: $r->associate,
+                    local_key: $r->local_key,
+                    foreign_key: $r->foreign_key,
+                    as: $as,
+                );
+            }
 
-    private function apply_association(?ActiveRecord\Config\Association $association): void
-    {
-        if (!$association) {
-            return;
-        }
+            if ($association->has_many) {
+                assert(is_string($this->model->primary));
+            }
 
-        foreach ($association->belongs_to as $r) {
-            $this->belongs_to(
-                related: $r->associate,
-                local_key: $r->local_key,
-                foreign_key: $r->foreign_key,
-                as: $r->as,
-            );
+            foreach ($association->has_many as $r) {
+                $as = $r->as;
+                $relations[$as] = new HasManyRelation(
+                    owner: $this->model,
+                    related: $r->associate,
+                    foreign_key: $r->foreign_key,
+                    as: $as,
+                    through: $r->through,
+                );
+            }
         }
 
-        foreach ($association->has_many as $r) {
-            $this->has_many(
-                related: $r->associate,
-                foreign_key: $r->foreign_key,
-                as: $r->as,
-                through: $r->through,
-            );
-        }
+        $this->relations = $relations;
     }
 
     /**
-     * Checks if a relation exists.
+     * Whether a relation exists.
      *
-     * @param string $offset Relation name.
+     * @param non-empty-string $relation_name
      */
-    public function offsetExists(mixed $offset): bool
+    public function has(string $relation_name): bool
     {
-        return isset($this->relations[$offset]);
+        return isset($this->relations[$relation_name]);
     }
 
     /**
-     * Returns a relation.
+     * Returns an existing relation.
      *
-     * @param string $offset Relation name.
+     * @param non-empty-string $relation_name
      *
-     * @throws RelationNotDefined if the relation is not defined.
-     */
-    public function offsetGet(mixed $offset): Relation
-    {
-        return $this->relations[$offset]
-            ?? throw new RelationNotDefined($offset, $this);
-    }
-
-    /**
-     * @throws OffsetNotWritable because relations cannot be set.
+     * @throws RelationNotDefined if the relation doesn't exist.
      */
-    public function offsetSet(mixed $offset, mixed $value): void
+    public function get(string $relation_name): ?Relation
     {
-        throw new OffsetNotWritable($offset, $this);
-    }
-
-    /**
-     * @throws OffsetNotWritable because relations cannot be unset.
-     */
-    public function offsetUnset(mixed $offset): void
-    {
-        throw new OffsetNotWritable($offset, $this);
-    }
-
-    /**
-     * Adds a {@link BelongsToRelation} relation.
-     *
-     * @param class-string<ActiveRecord> $related
-     * @param non-empty-string $local_key
-     * @param non-empty-string $foreign_key
-     * @param non-empty-string $as
-     */
-    public function belongs_to(
-        string $related,
-        string $local_key,
-        string $foreign_key,
-        string $as,
-    ): void {
-        $this->relations[$as] = new BelongsToRelation(
-            owner: $this->model,
-            related: $related,
-            local_key: $local_key,
-            foreign_key: $foreign_key,
-            as: $as,
-        );
-    }
-
-    /**
-     * Adds a {@link HasManyRelation} relation.
-     *
-     * @param class-string<ActiveRecord> $related
-     * @param non-empty-string $foreign_key
-     * @param non-empty-string $as
-     * @param class-string<ActiveRecord>|null $through
-     */
-    public function has_many(
-        string $related,
-        string $foreign_key,
-        string $as,
-        ?string $through = null,
-    ): void {
-        assert(is_string($this->model->primary));
-
-        $this->relations[$as] = new HasManyRelation(
-            owner: $this->model,
-            related: $related,
-            foreign_key: $foreign_key,
-            as: $as,
-            through: $through,
-        );
+        return $this->relations[$relation_name]
+            ?? throw new RelationNotDefined($relation_name, $this);
     }
 
     /**
diff --git a/tests/ActiveRecord/HasManyRelationTest.php b/tests/ActiveRecord/HasManyRelationTest.php
index fb9d752..735fe5e 100644
--- a/tests/ActiveRecord/HasManyRelationTest.php
+++ b/tests/ActiveRecord/HasManyRelationTest.php
@@ -59,7 +59,7 @@ public function test_relations(): void
         $relations = $this->articles->relations;
         $this->assertInstanceOf(RelationCollection::class, $relations);
 
-        $relation = $relations['comments'];
+        $relation = $relations->get('comments');
         $this->assertInstanceOf(HasManyRelation::class, $relation);
         $this->assertSame('comments', $relation->as);
         $this->assertSame($this->articles, $relation->owner);
@@ -71,7 +71,7 @@ public function test_relations(): void
     public function test_undefined_relation(): void
     {
         $this->expectException(RelationNotDefined::class);
-        $this->articles->relations['undefined_relation'];
+        $this->articles->relations->get('undefined_relation');
     }
 
     public function test_getter(): void
diff --git a/tests/ActiveRecord/HasManyRelationThroughTest.php b/tests/ActiveRecord/HasManyRelationThroughTest.php
index 5768c19..34676b4 100644
--- a/tests/ActiveRecord/HasManyRelationThroughTest.php
+++ b/tests/ActiveRecord/HasManyRelationThroughTest.php
@@ -57,10 +57,10 @@ public function test_through_is_set(): void
     {
         $r = $this->physicians->relations;
 
-        $this->assertArrayHasKey('appointments', $r);
-        $this->assertArrayHasKey('patients', $r);
+        $this->assertTrue($r->has('appointments'));
+        $this->assertTrue($r->has('patients'));
 
-        $ra = $r['appointments'];
+        $ra = $r->get('appointments');
         assert($ra instanceof HasManyRelation);
         $this->assertInstanceOf(HasManyRelation::class, $ra);
         $this->assertEquals('appointments', $ra->as);
@@ -68,7 +68,7 @@ public function test_through_is_set(): void
         $this->assertEquals('physician_id', $ra->foreign_key);
         $this->assertNull($ra->through);
 
-        $rp = $r['patients'];
+        $rp = $r->get('patients');
         assert($rp instanceof HasManyRelation);
         $this->assertInstanceOf(HasManyRelation::class, $rp);
         $this->assertEquals('patients', $rp->as);
diff --git a/tests/ActiveRecord/ModelBelongsToTest.php b/tests/ActiveRecord/ModelBelongsToTest.php
index 4538067..a106a38 100644
--- a/tests/ActiveRecord/ModelBelongsToTest.php
+++ b/tests/ActiveRecord/ModelBelongsToTest.php
@@ -99,9 +99,9 @@ public function getter_is_created_from_the_column_name_without_id_suffix(): void
 
         $people = $models->model_for_record(Person::class);
 
-        $this->assertArrayHasKey('dance_session', $people->relations);
-        $this->assertArrayHasKey('hire_skill', $people->relations);
-        $this->assertArrayHasKey('summon_skill', $people->relations);
-        $this->assertArrayHasKey('teach_skill', $people->relations);
+        $this->assertTrue($people->relations->has('dance_session'));
+        $this->assertTrue($people->relations->has('hire_skill'));
+        $this->assertTrue($people->relations->has('summon_skill'));
+        $this->assertTrue($people->relations->has('teach_skill'));
     }
 }
diff --git a/tests/ActiveRecord/RelationNotDefinedTest.php b/tests/ActiveRecord/RelationNotDefinedTest.php
index 86068ee..5e7df54 100644
--- a/tests/ActiveRecord/RelationNotDefinedTest.php
+++ b/tests/ActiveRecord/RelationNotDefinedTest.php
@@ -20,16 +20,14 @@ final class RelationNotDefinedTest extends TestCase
     public function test_exception(): void
     {
         $relation_name = uniqid();
-        $collection = $this
-            ->getMockBuilder(RelationCollection::class)
-            ->disableOriginalConstructor()
-            ->getMock();
+        $relations = new class() extends RelationCollection {
+            public function __construct() {
+            }
+        };
 
-        /* @var $collection RelationCollection */
-
-        $exception = new RelationNotDefined($relation_name, $collection);
+        $exception = new RelationNotDefined($relation_name, $relations);
 
         $this->assertSame($relation_name, $exception->relation_name);
-        $this->assertSame($collection, $exception->collection);
+        $this->assertSame($relations, $exception->collection);
     }
 }