FIX Preselect based on SS_List values

src/TagField.php

@@ -274,11 +274,13 @@ class TagField extends MultiSelectField
         }
 
         // Convert an array of values into a datalist of options
-        if (is_array($values) && !empty($values)) {
-            $values = DataList::create($dataClass)
-                ->filter($this->getTitleField(), $values);
-        } else {
-            $values = ArrayList::create();
+        if (!$values instanceof SS_List) {
+            if (is_array($values) && !empty($values)) {
+                $values = DataList::create($dataClass)
+                    ->filter($this->getTitleField(), $values);
+            } else {
+                $values = ArrayList::create();
+            }
         }
 
         // Prep a function to parse a dataobject into an option

tests/TagFieldTest.php

@@ -360,6 +360,24 @@ class TagFieldTest extends SapphireTest
         $this->assertEquals('Name', $readOnlyField->getTitleField());
     }
 
+    public function testItDisplaysWithSelectedValuesFromDataList()
+    {
+        $source = TagFieldTestBlogTag::get();
+        $selectedTag = $source->First();
+        $unselectedTag = $source->Last();
+        $value = $source->filter('ID', $selectedTag->ID); // arbitrary subset
+        $field = new TagField('TestField', null, $source, $value);
+
+        // Not the cleanest way to assert this, but getOptions() is protected
+        $schema = $field->getSchemaDataDefaults();
+        $this->assertTrue(
+            $this->getFromOptionsByTitle($schema['options'], $selectedTag->Title)['Selected']
+        );
+        $this->assertFalse(
+            $this->getFromOptionsByTitle($schema['options'], $unselectedTag->Title)['Selected']
+        );
+    }
+
     public function testGetSchemaDataDefaults()
     {
         $form = new Form(null, 'Form', new FieldList(), new FieldList());
@@ -403,4 +421,20 @@ class TagFieldTest extends SapphireTest
         $attributes = $field->getAttributes();
         $this->assertNotEmpty($attributes['data-schema']);
     }
+
+    /**
+     * @param array $options
+     * @param string $title
+     * @return array|null
+     */
+    protected function getFromOptionsByTitle(array $options, $title)
+    {
+        foreach ($options as $option) {
+            if ($option['Title'] == $title) {
+                return $option;
+            }
+        }
+
+        return null;
+    }
 }