Fix perform readonly transformation for lazyloaded searchabledropdown

src/Forms/SearchableDropdownTrait.php

@@ -586,4 +586,18 @@ private function updateOptionsForSchema(
         }
         return $options;
     }
+
+    public function performReadonlyTransformation()
+    {
+        // This is calling a non-static method statically
+        // using call_user_func() here to prevent an IDE warning
+        // The reason we're calling FormField::castedCopy directly is to prevent an ancestor
+        // call to $this->getSource() which will load the entire DataList into memory which
+        // causes issues with very large datasets and isn't needed when the field is read-only
+        $field = call_user_func('SilverStripe\\Forms\\FormField::castedCopy', SearchableLookupField::class);
+        $field->setSource($this->sourceList);
+        $field->setReadonly(true);
+
+        return $field;
+    }
 }

src/Forms/SearchableLookupField.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace SilverStripe\Forms;
+
+use SilverStripe\Core\Convert;
+use SilverStripe\ORM\ArrayLib;
+use SilverStripe\ORM\DataObjectInterface;
+use SilverStripe\ORM\FieldType\DBField;
+use SilverStripe\ORM\ArrayList;
+use SilverStripe\ORM\DataList;
+
+/**
+ * Read-only complement of {@link SearchableDropdownField} and {@link SearchableMultiDropdownField}.
+ *
+ * Shows the "human value" of the SearchableDropdownField for the currently selected
+ * values.
+ */
+class SearchableLookupField extends LookupField
+{
+    private ?DataList $sourceList = null;
+
+    /**
+     * To retain compatibility with ancestor getSource() this returns an array of only the selected values
+     */
+    public function getSource(): array
+    {
+        $values = $this->getValueArray();
+        if (empty($values) || $this->sourceList === null) {
+            $selectedValuesList = ArrayList::create();
+        } else {
+            $selectedValuesList = $this->sourceList->filterAny(['ID' => $values]);
+        }
+        return $this->getListMap($selectedValuesList);
+    }
+
+    /**
+     * @param mixed $source
+     */
+    public function setSource($source): static
+    {
+        // Setting to $this->sourceList instead of $this->source because SelectField.source
+        // docblock type is array|ArrayAccess i.e. does not allow DataList
+        if ($source instanceof DataList) {
+            $this->sourceList = $source;
+        } else {
+            $this->sourceList = null;
+        }
+        return $this;
+    }
+}