diff -u b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
--- b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -103,6 +103,24 @@
     $this->query->addTag('migrate_' . $this->migration->id());
     $this->query->addMetaData('migration', $this->migration);
 
+    // If an ID list is provided, filter on it. This only works for single-value
+    // keys, so throw an exception if the key is multi-value.
+    if (isset($this->configuration['idlist'])) {
+      $key = $this->getIds();
+      if (sizeof($key) == 1) {
+        $field = key($key);
+        // If a table alias was provided, prepend it in order to prevent
+        // the possibility of 'ambiguous column' errors.
+        if (isset($key[$field]['alias'])) {
+          $field = $key[$field]['alias'] . '.' . $field;
+        }
+        $this->query->condition($field, $this->configuration['idlist'], 'IN');
+      }
+      else {
+        throw new \LogicException('Cannot filter on multi-value key.');
+      }
+    }
+
     return $this->query;
   }
 
@@ -116,72 +134,63 @@
     $this->prepareQuery();
     $high_water_property = $this->migration->get('highWaterProperty');
 
-    // The rules for determining what conditions to add to the query are as
-    // follows (applying first applicable rule)
-    // 1. If idlist is provided, then only process items in that list (AND key
-    //    IN (idlist)). Only applicable with single-value keys.
-    if (isset($this->configuration['idlist'])) {
-      $this->query->condition(array_keys($this->getIds())[0], $this->configuration['idlist'], 'IN');
-    }
-    else {
-      // 2. If the map is joinable, join it. We will want to accept all rows
-      //    which are either not in the map, or marked in the map as NEEDS_UPDATE.
-      //    Note that if high water fields are in play, we want to accept all rows
-      //    above the high water mark in addition to those selected by the map
-      //    conditions, so we need to OR them together (but AND with any existing
-      //    conditions in the query). So, ultimately the SQL condition will look
-      //    like (original conditions) AND (map IS NULL OR map needs update
-      //      OR above high water).
-      $conditions = $this->query->orConditionGroup();
-      $condition_added = FALSE;
-      if ($this->mapJoinable()) {
-        // Build the join to the map table. Because the source key could have
-        // multiple fields, we need to build things up.
-        $count = 1;
-        $map_join = '';
-        $delimiter = '';
-        foreach ($this->getIds() as $field_name => $field_schema) {
-          if (isset($field_schema['alias'])) {
-            $field_name = $field_schema['alias'] . '.' . $field_name;
-          }
-          $map_join .= "$delimiter$field_name = map.sourceid" . $count++;
-          $delimiter = ' AND ';
+    // 2. If the map is joinable, join it. We will want to accept all rows
+    //    which are either not in the map, or marked in the map as NEEDS_UPDATE.
+    //    Note that if high water fields are in play, we want to accept all rows
+    //    above the high water mark in addition to those selected by the map
+    //    conditions, so we need to OR them together (but AND with any existing
+    //    conditions in the query). So, ultimately the SQL condition will look
+    //    like (original conditions) AND (map IS NULL OR map needs update
+    //      OR above high water).
+    $conditions = $this->query->orConditionGroup();
+    $condition_added = FALSE;
+    if ($this->mapJoinable()) {
+      // Build the join to the map table. Because the source key could have
+      // multiple fields, we need to build things up.
+      $count = 1;
+      $map_join = '';
+      $delimiter = '';
+      foreach ($this->getIds() as $field_name => $field_schema) {
+        if (isset($field_schema['alias'])) {
+          $field_name = $field_schema['alias'] . '.' . $field_name;
         }
+        $map_join .= "$delimiter$field_name = map.sourceid" . $count++;
+        $delimiter = ' AND ';
+      }
 
-        $alias = $this->query->leftJoin($this->migration->getIdMap()->getQualifiedMapTableName(), 'map', $map_join);
-        $conditions->isNull($alias . '.sourceid1');
-        $conditions->condition($alias . '.source_row_status', MigrateIdMapInterface::STATUS_NEEDS_UPDATE);
-        $condition_added = TRUE;
-
-        // And as long as we have the map table, add its data to the row.
-        $n = count($this->getIds());
+      $alias = $this->query->leftJoin($this->migration->getIdMap()->getQualifiedMapTableName(), 'map', $map_join);
+      $conditions->isNull($alias . '.sourceid1');
+      $conditions->condition($alias . '.source_row_status', MigrateIdMapInterface::STATUS_NEEDS_UPDATE);
+      $condition_added = TRUE;
+
+      // And as long as we have the map table, add its data to the row.
+      $n = count($this->getIds());
+      for ($count = 1; $count <= $n; $count++) {
+        $map_key = 'sourceid' . $count;
+        $this->query->addField($alias, $map_key, "migrate_map_$map_key");
+      }
+      if ($n = count($this->migration->get('destinationIds'))) {
         for ($count = 1; $count <= $n; $count++) {
-          $map_key = 'sourceid' . $count;
+          $map_key = 'destid' . $count++;
           $this->query->addField($alias, $map_key, "migrate_map_$map_key");
         }
-        if ($n = count($this->migration->get('destinationIds'))) {
-          for ($count = 1; $count <= $n; $count++) {
-            $map_key = 'destid' . $count++;
-            $this->query->addField($alias, $map_key, "migrate_map_$map_key");
-          }
-        }
-        $this->query->addField($alias, 'source_row_status', 'migrate_map_source_row_status');
       }
-      // 3. If we are using high water marks, also include rows above the mark.
-      //    But, include all rows if the high water mark is not set.
-      if (isset($high_water_property['name']) && ($high_water = $this->migration->getHighWater()) !== '') {
-        if (isset($high_water_property['alias'])) {
-          $high_water = $high_water_property['alias'] . '.' . $high_water_property['name'];
-        }
-        else {
-          $high_water = $high_water_property['name'];
-        }
-        $conditions->condition($high_water, $high_water, '>');
-        $condition_added = TRUE;
+      $this->query->addField($alias, 'source_row_status', 'migrate_map_source_row_status');
+    }
+    // 3. If we are using high water marks, also include rows above the mark.
+    //    But, include all rows if the high water mark is not set.
+    if (isset($high_water_property['name']) && ($high_water = $this->migration->getHighWater()) !== '') {
+      if (isset($high_water_property['alias'])) {
+        $high_water = $high_water_property['alias'] . '.' . $high_water_property['name'];
       }
-      if ($condition_added) {
-        $this->query->condition($conditions);
+      else {
+        $high_water = $high_water_property['name'];
       }
+      $conditions->condition($high_water, $high_water, '>');
+      $condition_added = TRUE;
+    }
+    if ($condition_added) {
+      $this->query->condition($conditions);
     }
 
     return new \IteratorIterator($this->query->execute());
@@ -196,7 +205,7 @@
    * {@inheritdoc}
    */
   public function count() {
-    return $this->query()->countQuery()->execute()->fetchField();
+    return $this->prepareQuery()->countQuery()->execute()->fetchField();
   }
 
   /**
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Unit/source/d6/NodeIdListTest.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Tests\migrate_drupal\Unit\source\d6\NodeIdListTest.
+ */
+
+namespace Drupal\Tests\migrate_drupal\Unit\source\d6;
+
+/**
+ * Tests D6 node source plugin.
+ *
+ * @group migrate_drupal
+ */
+class NodeIdListTest extends NodeTest {
+
+  // The fake Migration configuration entity.
+  protected $migrationConfiguration = array(
+    'id' => 'test',
+    // The fake configuration for the source.
+    'source' => array(
+      'plugin' => 'd6_node',
+      'idlist' => array(1),
+    ),
+  );
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->expectedResults = array_slice($this->expectedResults, 0, 1);
+  }
+
+}