diff --git a/src/Query/GroupOption.php b/src/Query/GroupOption.php
index d51a20c..ef4ce30 100644
--- a/src/Query/GroupOption.php
+++ b/src/Query/GroupOption.php
@@ -92,6 +92,10 @@ class GroupOption implements QueryOptionInterface, QueryOptionTreeItemInterface
       $this->{$prop}[$option->id()] = $option;
       return TRUE;
     }
+    // Direct child.
+    elseif (isset($this->childGroups[$target_id])) {
+      return $this->childGroups[$target_id]->insert($target_id, $option);
+    }
     elseif ($proper_child = array_reduce($this->childGroups, $find_proper_id, NULL)) {
       return $this->childGroups[$proper_child]->insert($target_id, $option);
     }
@@ -115,15 +119,15 @@ class GroupOption implements QueryOptionInterface, QueryOptionTreeItemInterface
     }
 
     if (!empty($this->childOptions)) {
-      $group = array_reduce($this->childOptions, function ($group, $child) {
-        return $child->apply($group);
-      }, $group);
+      foreach ($this->childOptions as $child) {
+        $child->apply($group);
+      }
     }
 
     if (!empty($this->childGroups)) {
-      $group = array_reduce($this->childGroups, function ($group, $child) {
-        return $child->apply($group);
-      }, $group);
+      foreach ($this->childGroups as $child) {
+        $child->apply($group);
+      }
     }
 
     return $query->condition($group);
@@ -143,11 +147,16 @@ class GroupOption implements QueryOptionInterface, QueryOptionTreeItemInterface
       return TRUE;
     }
 
-    // If any child GroupOptions or their children have the id return TRUE.
-    return array_reduce($this->groupOptions, function ($has_child, $group) use ($id) {
-      // If we already know that we have the child, skip evaluation and return.
-      return $has_child || $group->id() == $id || $group->hasChild($id);
-    }, FALSE);
+    if (is_array($this->childGroups)) {
+      // If any child Groups or their children have the id return TRUE.
+      return array_reduce($this->childGroups, function ($has_child, $group) use ($id) {
+        // If we already know that we have the child, skip evaluation and return.
+        return $has_child || $group->id() == $id || $group->hasChild($id);
+      }, FALSE);
+    }
+    else {
+      return FALSE;
+    }
   }
 
 }
diff --git a/src/Query/QueryOptionTreeItemInterface.php b/src/Query/QueryOptionTreeItemInterface.php
index 102921c..632f8af 100644
--- a/src/Query/QueryOptionTreeItemInterface.php
+++ b/src/Query/QueryOptionTreeItemInterface.php
@@ -21,7 +21,7 @@ interface QueryOptionTreeItemInterface {
   public function insert($target_id, QueryOptionInterface $option);
 
   /**
-   * Returns whether or the given id is a (grand)child of the object.
+   * Returns whether the given id is a (grand)child of the object.
    */
   public function hasChild($id);
 
diff --git a/tests/src/Functional/JsonApiFunctionalTest.php b/tests/src/Functional/JsonApiFunctionalTest.php
index 366ab7e..9a6a0e5 100644
--- a/tests/src/Functional/JsonApiFunctionalTest.php
+++ b/tests/src/Functional/JsonApiFunctionalTest.php
@@ -259,6 +259,122 @@ class JsonApiFunctionalTest extends JsonApiFunctionalTestBase {
     // With the 'Accept' header we can know we want the 404 error formatted as
     // JSON API.
     $this->assertSession()->responseHeaderContains('Content-Type', 'application/vnd.api+json');
+
+    // Test documentation filtering examples.
+    // 1. Only get published nodes.
+    $filter = [
+      'status-filter' => [
+        'condition' => [
+          'path' => 'status',
+          'value' => 1,
+        ],
+      ],
+    ];
+    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
+      'query' => ['filter' => $filter],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertGreaterThanOrEqual(OffsetPage::$maxSize, count($collection_output['data']));
+    // 2. Nested Filters: Get nodes created by user admin.
+    $filter = [
+      'name-filter' => [
+        'condition' => [
+          'path' => 'uid.name',
+          'value' => $this->user->getAccountName(),
+        ],
+      ],
+    ];
+    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
+      'query' => ['filter' => $filter],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertGreaterThanOrEqual(OffsetPage::$maxSize, count($collection_output['data']));
+    // 3. Filtering with arrays: Get nodes created by users [admin, john].
+    $filter = [
+      'name-filter' => [
+        'condition' => [
+          'path' => 'uid.name',
+          'operator' => 'IN',
+          'value' => [
+            $this->user->getAccountName(),
+            $this->getRandomGenerator()->name(),
+          ],
+        ],
+      ],
+    ];
+    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
+      'query' => ['filter' => $filter],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertGreaterThanOrEqual(OffsetPage::$maxSize, count($collection_output['data']));
+    // 4. Grouping filters: Get nodes that are published and create by admin.
+    $filter = [
+      'and-group' => [
+        'group' => [
+          'conjunction' => 'AND',
+        ],
+      ],
+      'name-filter' => [
+        'condition' => [
+          'path' => 'uid.name',
+          'value' => $this->user->getAccountName(),
+          'memberOf' => 'and-group',
+        ],
+      ],
+      'status-filter' => [
+        'condition' => [
+          'path' => 'status',
+          'value' => 1,
+          'memberOf' => 'and-group',
+        ],
+      ],
+    ];
+    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
+      'query' => ['filter' => $filter],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertGreaterThanOrEqual(OffsetPage::$maxSize, count($collection_output['data']));
+    // 5. Grouping grouped filters: Get nodes that are promoted or sticky and
+    //    created by admin.
+    $filter = [
+      'and-group' => [
+        'group' => [
+          'conjunction' => 'AND',
+        ],
+      ],
+      'or-group' => [
+        'group' => [
+          'conjunction' => 'OR',
+          'memberOf' => 'and-group',
+        ],
+      ],
+      'admin-filter' => [
+        'condition' => [
+          'path' => 'uid.name',
+          'value' => $this->user->getAccountName(),
+          'memberOf' => 'and-group',
+        ],
+      ],
+      'sticky-filter' => [
+        'condition' => [
+          'path' => 'sticky',
+          'value' => 1,
+          'memberOf' => 'or-group',
+        ],
+      ],
+      'promote-filter' => [
+        'condition' => [
+          'path' => 'promote',
+          'value' => 0,
+          'memberOf' => 'or-group',
+        ],
+      ],
+    ];
+    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
+      'query' => ['filter' => $filter],
+    ]));
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertEquals(0, count($collection_output['data']));
   }
 
   /**