diff --git a/app/Http/Controllers/Api/CustomFieldsController.php b/app/Http/Controllers/Api/CustomFieldsController.php
index fb0493046f..da27ba02d2 100644
--- a/app/Http/Controllers/Api/CustomFieldsController.php
+++ b/app/Http/Controllers/Api/CustomFieldsController.php
@@ -125,6 +125,38 @@ class CustomFieldsController extends Controller
 
     }
 
+    public function associate(Request $request, $field_id)
+    {
+        $this->authorize('edit', CustomFieldset::class);
+        $field = CustomField::findOrFail($field_id);
+
+        $fieldset_id = $request->input('fieldset_id');
+        foreach ($field->fieldset as $fieldset) {
+            if ($fieldset->id == $fieldset_id) {
+                return response()->json(Helper::formatStandardApiResponse('success', $fieldset, trans('admin/custom_fields/message.fieldset.update.success')));
+            }
+        }
+
+        $fieldset = CustomFieldset::findOrFail($fieldset_id);
+        $fieldset->fields()->attach($field->id, ["required" => ($request->input('required') == "on"), "order" => $request->input('order', $fieldset->fields->count())]);
+        return response()->json(Helper::formatStandardApiResponse('success', $fieldset, trans('admin/custom_fields/message.fieldset.update.success')));
+    }
+
+    public function disassociate(Request $request, $field_id)
+    {
+        $this->authorize('edit', CustomFieldset::class);
+        $field = CustomField::findOrFail($field_id);
+
+        $fieldset_id = $request->input('fieldset_id');
+        foreach ($field->fieldset as $fieldset) {
+            if ($fieldset->id == $fieldset_id) {
+                $fieldset->fields()->detach($field->id);
+                return response()->json(Helper::formatStandardApiResponse('success', $fieldset, trans('admin/custom_fields/message.fieldset.update.success')));
+            }
+        }
+        $fieldset = CustomFieldset::findOrFail($fieldset_id);
+        return response()->json(Helper::formatStandardApiResponse('success', $fieldset, trans('admin/custom_fields/message.fieldset.update.success')));
+    }
 
     /**
      * Delete a custom field.
diff --git a/app/Http/Controllers/Api/CustomFieldsetsController.php b/app/Http/Controllers/Api/CustomFieldsetsController.php
index 3b82d0e5eb..915518e945 100644
--- a/app/Http/Controllers/Api/CustomFieldsetsController.php
+++ b/app/Http/Controllers/Api/CustomFieldsetsController.php
@@ -126,7 +126,7 @@ class CustomFieldsetsController extends Controller
     {
         $this->authorize('delete', CustomFieldset::class);
         $fieldset = CustomFieldset::findOrFail($id);
-        
+
         $modelsCount = $fieldset->models->count();
         $fieldsCount = $fieldset->fields->count();
 
@@ -141,7 +141,23 @@ class CustomFieldsetsController extends Controller
         return response()->json(Helper::formatStandardApiResponse('error', null, 'Unspecified error'));
 
 
-        
+
+    }
+
+    /**
+     * Return JSON containing a list of fields belonging to a fieldset.
+     *
+     * @author [V. Cordes] [<volker@fdatek.de>]
+     * @since [v4.1.10]
+     * @param $fieldsetId
+     * @return string JSON
+     */
+    public function fields($id)
+    {
+        $this->authorize('view', CustomFieldset::class);
+        $set = CustomFieldset::findOrFail($id);
+        $fields = $set->fields->get();
+        return (new CustomFieldsTransformer)->transformCustomFields($fields, $fields->count());
     }
 
 }
diff --git a/routes/api.php b/routes/api.php
index 36646956c3..5b40657a74 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -219,11 +219,32 @@ Route::group(['prefix' => 'v1','namespace' => 'Api'], function () {
                 'uses' => 'CustomFieldsController@postReorder'
             ]
         );
+        Route::post('{field}/associate',
+            [
+                'as' => 'api.customfields.associate',
+                'uses' => 'CustomFieldsController@associate'
+            ]
+        );
+        Route::post('{field}/disassociate',
+            [
+                'as' => 'api.customfields.disassociate',
+                'uses' => 'CustomFieldsController@disassociate'
+            ]
+        );
     }); // Fields group
 
 
     /*--- Fieldsets API ---*/
 
+    Route::group(['prefix' => 'fieldsets'], function () {
+        Route::get('{fieldset}/fields',
+            [
+                'as' => 'api.fieldsets.fields',
+                'uses' => 'CustomFieldsetsController@fields'
+            ]
+        );
+    });
+
     Route::resource('fieldsets', 'CustomFieldsetsController',
         [
             'names' =>
@@ -240,7 +261,6 @@ Route::group(['prefix' => 'v1','namespace' => 'Api'], function () {
     ); // Custom fieldset resource
 
 
-
     /*--- Groups API ---*/
 
     Route::resource('groups', 'GroupsController',