diff --git a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
index 7c5688d..439e2e1 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -193,6 +193,15 @@ public function getIterator() {
     return $this->iterator;
   }
 
+  /**
+   * Check if we can join against the map table.
+   *
+   * This function specifically catches issues when we're migrating with
+   * unique sets of credentials for the source and destination database.
+   *
+   * @return bool
+   *   TRUE if we can join against the map table otherwise FALSE.
+   */
   protected function mapJoinable() {
     if (!$this->getIds()) {
       return FALSE;
@@ -210,4 +219,5 @@ protected function mapJoinable() {
     }
     return TRUE;
   }
+
 }