Use environment variable to set subsite_id in YML

src/Service/IndexConfiguration.php

@@ -3,6 +3,7 @@
 namespace SilverStripe\SearchService\Service;
 
 use SilverStripe\Core\Config\Configurable;
+use SilverStripe\Core\Environment;
 use SilverStripe\Core\Extensible;
 use SilverStripe\Core\Injector\Injectable;
 use SilverStripe\SearchService\Interfaces\DocumentInterface;
@@ -95,6 +96,11 @@ public function getIndexes(): array
     {
         $indexes = $this->config()->get('indexes');
 
+        // Convert environment variable defined in YML config to their value
+        array_walk($indexes, function (array &$configuration) {
+            $configuration = $this->environmentVariableToValue($configuration);
+        });
+
         if (!$this->onlyIndexes) {
             return $indexes;
         }
@@ -267,4 +273,25 @@ public function getFieldsForIndex(string $index): array
         return $fields;
     }
 
+    /**
+     * For every configuration item if value is environment variable then convert it to its value
+     */
+    protected function environmentVariableToValue(array $configuration): array
+    {
+        foreach ($configuration as $name => $value) {
+            if (!is_string($value)) {
+                continue;
+            }
+
+            $environmentValue = Environment::getEnv($value);
+            if (!$environmentValue) {
+                continue;
+            }
+
+            $configuration[$name] = $environmentValue;
+        }
+
+        return $configuration;
+    }
+
 }