Better validation within similar() call.

module/VuFindSearch/src/VuFindSearch/Service.php

@@ -289,12 +289,15 @@ class Service
         $params  = $params ?: new ParamBag();
         $context = __FUNCTION__;
         $args = compact('backend', 'id', 'params', 'context');
-        $backend = $this->resolve($backend, $args);
-        $args['backend_instance'] = $backend;
+        $backendInstance = $this->resolve($backend, $args);
+        $args['backend_instance'] = $backendInstance;
 
-        $this->triggerPre($backend, $args);
+        $this->triggerPre($backendInstance, $args);
         try {
-            $response = $backend->similar($id, $params);
+            if (!($backendInstance instanceof Feature\SimilarInterface)) {
+                throw new BackendException("$backend does not support similar()");
+            }
+            $response = $backendInstance->similar($id, $params);
         } catch (BackendException $e) {
             $this->triggerError($e, $args);
             throw $e;

module/VuFindSearch/tests/unit-tests/src/VuFindTest/SearchServiceTest.php

@@ -35,6 +35,7 @@ use VuFindSearch\Backend\BackendInterface;
 use VuFindSearch\Backend\Exception\BackendException;
 use VuFindSearch\Feature\RetrieveBatchInterface;
 use VuFindSearch\Feature\RandomInterface;
+use VuFindSearch\Feature\SimilarInterface;
 use VuFindSearch\Query\Query;
 use VuFindSearch\Response\AbstractRecordCollection;
 
@@ -585,6 +586,20 @@ class SearchServiceTest extends TestCase
         unset($this->backend);
     }
 
+    /**
+     * Test similar action on bad backend.
+     *
+     * @return void
+     * @expectedException VuFindSearch\Backend\Exception\BackendException
+     * @expectedExceptionMessage foo does not support similar()
+     */
+    public function testSimilarOnNonSupportingBackend()
+    {
+        $service = $this->getService();
+        $params = new ParamBag(array('x' => 'y'));
+        $service->similar('foo', 'bar', $params);
+    }
+
     /**
      * Test exception-throwing similar action.
      *
@@ -690,9 +705,8 @@ abstract class TestClassForRetrieveBatchInterface
  * Stub class to test similar.
  */
 abstract class TestBackendClassForSimilar
-    implements BackendInterface
+    implements BackendInterface, SimilarInterface
 {
-    abstract function similar();
 }
 
 /**