--- /dev/null
+<?php
+
+namespace Drupal\aggregator\Plugin\Validation\Constraint;
+
+use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
+
+/**
+ * Supports validating feed titles.
+ *
+ * @Constraint(
+ * id = "FeedTitle",
+ * label = @Translation("Feed title", context = "Validation")
+ * )
+ */
+class FeedTitleConstraint extends UniqueFieldConstraint {
+
+ public $message = 'A feed named %value already exists. Enter a unique title.';
+
+}