diff --git a/tests/test-functions.php b/tests/test-functions.php
index 68140e0..da85ef8 100644
--- a/tests/test-functions.php
+++ b/tests/test-functions.php
@@ -1,9 +1,80 @@
 <?php
 class Test_Functions extends ActivityPub_TestCase_Cache_HTTP {
+	public $user_id;
+	public $post_id;
+
 	public function test_get_remote_metadata_by_actor() {
 		$metadata = \ActivityPub\get_remote_metadata_by_actor( 'pfefferle@notiz.blog' );
 		$this->assertEquals( 'https://notiz.blog/author/matthias-pfefferle/', $metadata['url'] );
 		$this->assertEquals( 'pfefferle', $metadata['preferredUsername'] );
 		$this->assertEquals( 'Matthias Pfefferle', $metadata['name'] );
 	}
+
+	public function set_up() {
+		$this->post_id = \wp_insert_post(
+			array(
+				'post_author' => $this->user_id,
+				'post_content' => 'test',
+			)
+		);
+	}
+
+	public function test_object_id_to_comment_basic() {
+		$single_comment_source_id = 'https://example.com/single';
+		$content = 'example';
+		$comment_id = \wp_new_comment(
+			array(
+				'comment_post_ID' => $this->post_id,
+				'comment_author' => 'Example User',
+				'comment_author_url' => 'https://example.com/user',
+				'comment_content' => $content,
+				'comment_type' => '',
+				'comment_author_email' => '',
+				'comment_parent' => 0,
+				'comment_meta' => array(
+					'source_id' => $single_comment_source_id,
+					'source_url' => 'https://example.com/123',
+					'avatar_url' => 'https://example.com/icon',
+					'protocol' => 'activitypub',
+				),
+			),
+			true
+		);
+		$query_result = \Activitypub\object_id_to_comment( $single_comment_source_id );
+		$this->assertInstanceOf( WP_Comment::class, $query_result );
+		$this->assertEquals( $comment_id, $query_result->comment_ID );
+		$this->assertEquals( $content, $query_result->comment_content );
+	}
+
+	public function test_object_id_to_comment_none() {
+		$single_comment_source_id = 'https://example.com/none';
+		$query_result = \Activitypub\object_id_to_comment( $single_comment_source_id );
+		$this->assertFalse( $query_result );
+	}
+
+	public function test_object_id_to_comment_duplicate() {
+		$duplicate_comment_source_id = 'https://example.com/duplicate';
+		for ( $i = 0; $i < 2; ++$i ) {
+			\wp_new_comment(
+				array(
+					'comment_post_ID' => $this->post_id,
+					'comment_author' => 'Example User',
+					'comment_author_url' => 'https://example.com/user',
+					'comment_content' => 'example',
+					'comment_type' => '',
+					'comment_author_email' => '',
+					'comment_parent' => 0,
+					'comment_meta' => array(
+						'source_id' => $duplicate_comment_source_id,
+						'source_url' => 'https://example.com/123',
+						'avatar_url' => 'https://example.com/icon',
+						'protocol' => 'activitypub',
+					),
+				),
+				true
+			);
+		}
+		$query_result = \Activitypub\object_id_to_comment( $duplicate_comment_source_id );
+		$this->assertFalse( $query_result );
+	}
 }