diff -u b/core/modules/migrate/config/migrate.migration.d6_system_site.yml b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
--- b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
+++ b/core/modules/migrate/config/migrate.migration.d6_system_site.yml
@@ -3,38 +3,24 @@
-  plugin: drupal6_variable
-  variables:
-    - site_name
-    - site_mail
-    - site_slogan
-    - site_frontpage
-    - site_403
-    - site_404
-    - drupal_weight_select_max
-    - admin_compact_mode
+    plugin: drupal6_variable
+    variables:
+        - site_name
+        - site_mail
+        - site_slogan
+        - site_frontpage
+        - site_403
+        - site_404
+        - drupal_weight_select_max
+        - admin_compact_mode
 process:
-  -
-    source: site_name
-    destination: name
-  -
-    source: site_mail
-    destination: mail
-  -
-    source: site_slogan
-    destination: slogan
-  -
-    source: site_frontpage
-    destination: page:front
-  -
-    source: site_403
-    destination: page:403
-  -
-    source: site_404
-    destination: page:404
-  -
-    source: drupal_weight_select_max
-    destination: weight_select_max
-  -
-    source: admin_compact_mode
-    destination: admin_compact_mode
+    -
+        plugin: copy_from_source
+        site_name: name
+        site_mail: mail
+        site_slogan: slogan
+        site_frontpage: 'page:front'
+        site_403: 'page:403'
+        site_404: 'page:404'
+        drupal_weight_select_max: weight_select_max
+        admin_compact_mode: admin_compact_mode
 destination:
-  plugin: d8_config
-  config_name: system.site
+    plugin: d8_config
+    config_name: system.site
diff -u b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
--- b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
@@ -8,13 +8,15 @@
 namespace Drupal\migrate\Entity;
 
 use Drupal\Core\Config\Entity\ConfigEntityBase;
-use Drupal\Core\Database\Database;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\migrate\Plugin\MigrateProcessBag;
 
 /**
  * Defines the Migration entity.
  *
+ * The migration entity stores the information about a single migration, like
+ * the source, process and destination plugins.
+ *
  * @EntityType(
  *   id = "migration",
  *   label = @Translation("Migration"),
@@ -128,6 +130,15 @@
    * @var array
    */
   public $sourceIds = array();
+
+  /**
+   * The destination identifiers.
+   *
+   * An array of destination identifiers: the keys are the name of the
+   * properties, the values are dependent on the id map plugin.
+   *
+   * @var array
+   */
   public $destinationIds = array();
 
   /**
@@ -178,7 +189,7 @@
   }
 
   /**
-   * @return \Drupal\migrate\Plugin\MigrateProcessBag
+   * {@inheritdoc}
    */
   public function getProcess() {
     if (!$this->migrateProcessBag) {
@@ -188,7 +199,7 @@
   }
 
   /**
-   * @return \Drupal\migrate\Plugin\MigrateDestinationInterface
+   * {@inheritdoc}
    */
   public function getDestination() {
     if (!isset($this->destinationPlugin)) {
@@ -198,7 +209,7 @@
   }
 
   /**
-   * @return \Drupal\migrate\Plugin\MigrateIdMapInterface
+   * {@inheritdoc}
    */
   public function getIdMap() {
     if (!isset($this->idMapPlugin)) {
@@ -212,7 +223,7 @@
   /**
    * @return \Drupal\Core\KeyValueStore\KeyValueStoreInterface
    */
-  protected function getHigherWaterStorage() {
+  protected function getHighWaterStorage() {
     if (!isset($this->highwaterStorage)) {
       $this->highwaterStorage = \Drupal::keyValue('migrate:highwater');
     }
@@ -221,9 +232,9 @@
 
   public function getHighwater() {
-    return $this->getHigherWaterStorage()->get($this->id());
+    return $this->getHighWaterStorage()->get($this->id());
   }
 
   public function saveHighwater($highwater) {
-    $this->getHigherWaterStorage()->set($this->id(), $highwater);
+    $this->getHighWaterStorage()->set($this->id(), $highwater);
   }
 }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
--- b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/MigrationInterface.php
@@ -13,8 +13,10 @@
  * Interface for migrations.
  */
 interface MigrationInterface extends ConfigEntityInterface {
+
   const SOURCE = 'source';
   const DESTINATION = 'destination';
+
   /**
    * Codes representing the current status of a migration, and stored in the
    * migrate_status table.
@@ -27,9 +29,9 @@
 
   /**
    * Message types to be passed to saveMessage() and saved in message tables.
-   * MESSAGE_INFORMATIONAL represents a condition that did not prevent the operation
-   * from succeeding - all others represent different severities of conditions
-   * resulting in a source record not being imported.
+   * MESSAGE_INFORMATIONAL represents a condition that did not prevent the
+   * operation from succeeding - all others represent different severities of
+   * conditions resulting in a source record not being imported.
    */
   const MESSAGE_ERROR = 1;
   const MESSAGE_WARNING = 2;
@@ -49,21 +51,29 @@
   const RESULT_DISABLED = 6;    // This migration is disabled, skipping
 
   /**
+   * Returns the initialized source plugin.
+   *
    * @return \Drupal\migrate\Plugin\MigrateSourceInterface
    */
   public function getSource();
 
   /**
+   * Returns the initialized process plugin.
+   *
    * @return \Drupal\migrate\Plugin\MigrateProcessBag
    */
   public function getProcess();
 
   /**
+   * Returns the initialized destination plugin.
+   *
    * @return \Drupal\migrate\Plugin\MigrateDestinationInterface
    */
   public function getDestination();
 
   /**
+   * Returns the initialized id_map plugin.
+   *
    * @return \Drupal\migrate\Plugin\MigrateIdMapInterface
    */
   public function getIdMap();
@@ -76,2 +86,3 @@
   public function saveHighwater($highwater);
+
 }
diff -u b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
--- b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
+++ b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
@@ -66,6 +66,13 @@
   protected $processed_since_feedback = 0;
 
   /**
+   * The translation manager.
+   *
+   * @var \Drupal\Core\StringTranslation\TranslationInterface
+   */
+  protected $translationManager;
+
+  /**
    * @param MigrationInterface $migration
    * @param MigrateMessageInterface $message
    *
@@ -92,7 +99,7 @@
             $limit *= 1024;
             break;
           default:
-            throw new MigrateException(t('Invalid PHP memory_limit !limit',
+            throw new MigrateException($this->t('Invalid PHP memory_limit !limit',
               array('!limit' => $limit)));
         }
       }
@@ -158,7 +165,7 @@
     }
     catch (\Exception $e) {
       $this->message->display(
-        t('Migration failed with source plugin exception: !e',
+        $this->t('Migration failed with source plugin exception: !e',
           array('!e' => $e->getMessage())));
       return MigrationInterface::RESULT_FAILED;
     }
@@ -182,7 +189,7 @@
         else {
           $id_map->saveIdMapping($row, array(), MigrateIdMapInterface::STATUS_FAILED, $this->rollbackAction);
           if ($id_map->messageCount() == 0) {
-            $message = t('New object was not saved, no error provided');
+            $message = $this->t('New object was not saved, no error provided');
             $this->saveMessage($message);
             $this->message->display($message);
           }
@@ -227,7 +234,7 @@
       }
       catch (\Exception $e) {
         $this->message->display(
-          t('Migration failed with source plugin exception: !e',
+          $this->t('Migration failed with source plugin exception: !e',
             array('!e' => $e->getMessage())));
         return MigrationInterface::RESULT_FAILED;
       }
@@ -365,7 +372,7 @@
     $pct_memory = $usage / $this->memoryLimit;
     if ($pct_memory > $this->memoryThreshold) {
       $this->message->display(
-        t('Memory usage is !usage (!pct% of limit !limit), resetting statics',
+        $this->t('Memory usage is !usage (!pct% of limit !limit), resetting statics',
           array('!pct' => round($pct_memory*100),
                 '!usage' => format_size($usage),
                 '!limit' => format_size($this->memoryLimit))),
@@ -379,7 +386,7 @@
       // coming back here and trimming a tiny amount
       if ($pct_memory > (.90 * $this->memoryThreshold)) {
         $this->message->display(
-          t('Memory usage is now !usage (!pct% of limit !limit), not enough reclaimed, starting new batch',
+          $this->t('Memory usage is now !usage (!pct% of limit !limit), not enough reclaimed, starting new batch',
             array('!pct' => round($pct_memory*100),
                   '!usage' => format_size($usage),
                   '!limit' => format_size($this->memoryLimit))),
@@ -388,7 +395,7 @@
       }
       else {
         $this->message->display(
-          t('Memory usage is now !usage (!pct% of limit !limit), reclaimed enough, continuing',
+          $this->t('Memory usage is now !usage (!pct% of limit !limit), reclaimed enough, continuing',
             array('!pct' => round($pct_memory*100),
                   '!usage' => format_size($usage),
                   '!limit' => format_size($this->memoryLimit))),
@@ -442,2 +449,23 @@
 
+  /**
+   * Translates a string to the current language or to a given language.
+   *
+   * See the t() documentation for details.
+   */
+  protected function t($string, array $args = array(), array $options = array()) {
+    return $this->translationManager()->translate($string, $args, $options);
+  }
+
+  /**
+   * Gets the translation manager.
+   *
+   * @return \Drupal\Core\StringTranslation\TranslationInterface
+   *   The translation manager.
+   */
+  protected function translationManager() {
+    if (!$this->translationManager) {
+      $this->translationManager = \Drupal::translation();
+    }
+    return $this->translationManager;
+  }
 }
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateIdMapInterface.php
@@ -147,12 +147,12 @@
    * Given a (possibly multi-field) source key, return the (possibly multi-field)
    * destination key it is mapped to.
    *
-   * @param array $source_id
-   *  Array of source key values.
+   * @param array $source_id_values
+   *  Array of source identifier values.
    * @return array
    *  Array of destination key values, or NULL on failure.
    */
-  public function lookupDestinationID(array $source_id);
+  public function lookupDestinationID(array $source_id_values);
 
   /**
    * Remove any persistent storage used by this map (e.g., map and message tables)
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateProcessBag.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateProcessBag.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateProcessBag.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/MigrateProcessBag.php
@@ -12,19 +12,40 @@
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\migrate\Entity\MigrationInterface;
 
+/**
+ * Contains all active process plugins on a migration.
+ */
 class MigrateProcessBag extends DefaultPluginBag {
 
   /**
+   * The migration entity this process bag is attached to.
+   *
    * @var \Drupal\migrate\Entity\MigrationInterface
    */
   protected $migration;
 
+  /**
+   * The key within the plugin configuration that contains the plugin ID.
+   *
+   * @var string
+   */
   protected $pluginKey = 'plugin';
 
   /**
-   * @param PluginManagerInterface $manager
+   * @var \Drupal\migrate\Plugin\MigratePluginManager
+   */
+  protected $manager;
+
+  /**
+   * Constructs a new MigrateProcessBag.
+   *
+   * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
+   *   The plugin manager to initialize process plugins.
    * @param array $configurations
-   * @param MigrationInterface $migration
+   *   (optional) An associative array containing the initial configuration for
+   *   each plugin in the bag, keyed by plugin instance ID.
+   * @param \Drupal\migrate\Entity\MigrationInterface $migration
+   *   The migration entity this process bag is attached to.
    */
   public function __construct(PluginManagerInterface $manager, array $configurations = array(), MigrationInterface $migration) {
     parent::__construct($manager, $configurations);
@@ -34,22 +55,17 @@
   /**
    * {@inheritdoc}
    *
-   * @return \Drupal\migrate\Plugin\ProcessInterface
-   */
-  public function &get($instance_id) {
-    return parent::get($instance_id);
-  }
-
-  /**
-   * {@inheritdoc}
+   * Extends in order to pass the migration entity to the plugin manager.
    */
   public function initializePlugin($instance_id) {
-    $this->configurations[$instance_id] += array('plugin' => 'property_map');
+    $this->configurations[$instance_id] += array('plugin' => 'copy_from_source');
     $configuration = isset($this->configurations[$instance_id]) ? $this->configurations[$instance_id] : array();
     if (!isset($configuration[$this->pluginKey])) {
       throw new UnknownPluginException($instance_id);
     }
-    $this->pluginInstances[$instance_id] = $this->manager->createInstance($configuration[$this->pluginKey], $configuration, $this->migration);
+    $plugin_key = $configuration[$this->pluginKey];
+    unset($configuration[$this->pluginKey]);
+    $this->pluginInstances[$instance_id] = $this->manager->createInstance($plugin_key, $configuration, $this->migration);
     $this->addInstanceID($instance_id);
   }
 
diff -u b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/id_map/Sql.php
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/id_map/Sql.php
@@ -7,7 +7,8 @@
 
 namespace Drupal\migrate\Plugin\migrate\id_map;
 
-use Drupal\Core\Database\Database;
+use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\migrate\Entity\MigrationInterface;
 use Drupal\migrate\MigrateMessageInterface;
@@ -16,6 +17,11 @@
 use Drupal\migrate\Row;
 
 /**
+ * Defines the sql based ID map implementation.
+ *
+ * It creates one map and one message table per migration entity to store the
+ * relevant information.
+ *
  * @PluginID("sql")
  */
 class Sql extends PluginBase implements MigrateIdMapInterface {
@@ -40,6 +46,8 @@
   }
 
   /**
+   * The database connection for the map/message tables on the destination.
+   *
    * @var \Drupal\Core\Database\Connection
    */
   protected $database;
@@ -86,7 +94,9 @@
   }
 
   /**
-   * We don't need to check the tables more than once per request.
+   * Stores whether the the tables (map/message) already exist.
+   *
+   * This is determined just once per request/instance of the class.
    *
    * @var boolean
    */
@@ -94,19 +104,20 @@
 
   public function __construct($configuration, $plugin_id, $plugin_definition, MigrationInterface $migration) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
+
     $this->migration = $migration;
     $machine_name = $migration->id();
 
-    // Default generated table names, limited to 63 characters
+    // Default generated table names, limited to 63 characters.
     $prefixLength = strlen($this->getDatabase()->tablePrefix()) ;
-    $this->mapTable = 'migrate_map_' . drupal_strtolower($machine_name);
-    $this->mapTable = drupal_substr($this->mapTable, 0, 63 - $prefixLength);
-    $this->messageTable = 'migrate_message_' . drupal_strtolower($machine_name);
-    $this->messageTable = drupal_substr($this->messageTable, 0, 63 - $prefixLength);
+    $this->mapTable = 'migrate_map_' . Unicode::strtolower($machine_name);
+    $this->mapTable = Unicode::substr($this->mapTable, 0, 63 - $prefixLength);
+    $this->messageTable = 'migrate_message_' . Unicode::strtolower($machine_name);
+    $this->messageTable = Unicode::substr($this->messageTable, 0, 63 - $prefixLength);
     $this->sourceIds = $migration->get('sourceIds');
     $this->destinationIds = $migration->get('destinationIds');
 
-    // Build the source and destination key maps
+    // Build the source and destination key maps.
     $this->sourceIdFields = array();
     $count = 1;
     foreach ($this->sourceIds as $field => $schema) {
@@ -124,6 +135,9 @@
     return SqlBase::getDatabaseConnection($this->migration->id(), $this->configuration);
   }
 
+  /**
+   * {@inheritdoc}
+   */
   public function setMessage(MigrateMessageInterface $message) {
     $this->message = $message;
   }
@@ -252,9 +266,13 @@
   }
 
   /**
-   * Retrieve a row from the map table, given a source ID
+   * Retrieve a row from the map table, given a source ID.
    *
    * @param array $source_id
+   *   A list of source IDs, even there is just one source ID.
+   *
+   * @return array
+   *   The raw row data as associative array.
    */
   public function getRowBySource(array $source_id) {
     $query = $this->getDatabase()->select($this->mapTable, 'map')
@@ -270,6 +288,7 @@
    * Retrieve a row from the map table, given a destination ID
    *
    * @param array $destination_id
+   *   A list of destination IDs, even there is just one destination ID.
    *
    * @return mixed
    *   The row(s) of data
reverted:
--- b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/PropertyMap.php
+++ /dev/null
@@ -1,180 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\migrate\Plugin\migrate\process\PropertyMap.
- */
-
-namespace Drupal\migrate\Plugin\migrate\process;
-use Drupal\Core\Plugin\PluginBase;
-use Drupal\migrate\MigrateException;
-use Drupal\migrate\MigrateExecutable;
-use Drupal\migrate\Plugin\ProcessInterface;
-use Drupal\migrate\Row;
-
-/**
- * This class tracks mappings between source and destination.
- *
- * @PluginId("property_map")
- */
-class PropertyMap extends PluginBase implements ProcessInterface {
-
-  /**
-   * Destination field name for the mapping. If empty, the mapping is just a
-   * stub for annotating the source field.
-   *
-   * @var string
-   */
-  protected $destination;
-
-  /**
-   * Source field name for the mapping. If empty, the defaultValue will be
-   * applied.
-   *
-   * @var string
-   */
-  protected $source;
-
-  /**
-   * @var int
-   */
-  const MAPPING_SOURCE_CODE = 1;
-  const MAPPING_SOURCE_DB = 2;
-  protected $mappingSource = self::MAPPING_SOURCE_CODE;
-
-  /**
-   * Default value for simple mappings, when there is no source mapping or the
-   * source field is empty. If both this and the sourceProperty are omitted, the
-   * mapping is just a stub for annotating the destination field.
-   *
-   * @var mixed
-   */
-  protected $default;
-
-  /**
-   * Separator string. If present, the destination field will be set up as an
-   * array of values exploded from the corresponding source field.
-   *
-   * @var string
-   */
-  protected $separator;
-
-  /**
-   * Array of callbacks to be called on a source value.
-   *
-   * @var array
-   */
-  protected $callbacks = array();
-
-  /**
-   * An associative array with keys:
-   *   - table: The table for querying for a duplicate.
-   *   - property: The property for querying for a duplicate.
-   *
-   * @todo: Let fields declare this data and a replacement pattern. Then
-   * developers won't have to specify this.
-   *
-   * @var string
-   */
-  protected $dedupe;
-
-  /**
-   * Optional notes about a mapping.
-   *
-   * @var string
-   */
-  protected $description = '';
-
-  protected $issueGroup;
-
-  /**
-   * An optional issue ID corresponding to a mapping.
-   *
-   * @var string
-   */
-  protected $issueNumber;
-
-  /**
-   * An optional priority corresponding to a mapping.
-   *
-   * @var string
-   */
-  protected $issuePriority = self::ISSUE_PRIORITY_OK;
-
-  /**
-   * Priority levels that are available for mappings.
-   *
-   * @var string
-   */
-  const ISSUE_PRIORITY_OK = 1;
-  const ISSUE_PRIORITY_LOW = 2;
-  const ISSUE_PRIORITY_MEDIUM = 3;
-  const ISSUE_PRIORITY_BLOCKER = 4;
-
-  public static $priorities = array();
-
-  protected $configuration = array();
-
-  protected $source_migration = array();
-
-  public function __construct(array $configuration, $plugin_id, array $plugin_definition) {
-    // Must have one or the other
-    if (empty($configuration['destination'])) {
-      throw new MigrateException('Property mappings must have a destination property.');
-    }
-    if (!isset($configuration['default']) && empty($configuration['source'])) {
-      throw new MigrateException('Property mappings must have a source property or a default.');
-    }
-    parent::__construct($configuration, $plugin_id, $plugin_definition);
-  }
-
-  /**
-   * Apply field mappings to a data row received from the source, returning
-   * a populated destination object.
-   */
-  public function apply(Row $row, MigrateExecutable $migrate_executable) {
-    $destination_values = NULL;
-
-    // If there's a source mapping, and a source value in the data row, copy
-    // to the destination
-    if (!empty($this->configuration['source']) && $row->hasSourceProperty($this->configuration['source'])) {
-      $destination_values = $row->getSourceProperty($this->configuration['source']);
-    }
-    // Otherwise, apply the default value (if any)
-    elseif (isset($this->configuration['default'])) {
-      $destination_values = $this->configuration['default'];
-    }
-
-    // If there's a separator specified for this destination, then it
-    // will be populated as an array exploded from the source value
-    if (!empty($this->configuration['separator']) && isset($destination_values)) {
-      $destination_values = explode($this->configuration['separator'], $destination_values);
-    }
-
-    // If a source migration is supplied, use the current value for this property
-    // to look up a destination ID from the provided migration
-    if (!empty($this->configuration['source_migration']) && isset($destination_values)) {
-      $destination_values = $migrate_executable->handleSourceMigration($this->configuration['source_migration'], $destination_values, $this->configuration['default'], $this);
-    }
-
-    // Call any designated callbacks
-    if (!empty($this->configuration['callbacks'])) {
-      foreach ($this->callbacks as $callback) {
-        if (isset($destination_values) && is_callable($callback)) {
-          $destination_values = call_user_func($callback, $destination_values);
-        }
-      }
-    }
-
-    // If specified, assure a unique value for this property.
-    if (!empty($this->configuration['dedupe'])&& isset($destination_values)) {
-      $destination_values = $migrate_executable->handleDedupe($this->configuration['dedupe'], $destination_values);
-    }
-
-    // Store the destination together with possible configuration.
-    if (isset($destination_values)) {
-      $keys = explode(':', $this->configuration['destination']);
-      $row->setDestinationPropertyDeep($keys, $destination_values);
-    }
-  }
-}
diff -u b/core/modules/migrate/lib/Drupal/migrate/Row.php b/core/modules/migrate/lib/Drupal/migrate/Row.php
--- b/core/modules/migrate/lib/Drupal/migrate/Row.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Row.php
@@ -16,18 +16,18 @@
 class Row {
 
   /**
+   * The actual values of the source row.
+   *
    * @var array
    */
   protected $source = array();
 
   /**
-   * The value of the source identifiers.
-   *
-   * This is a subset of the $source array.
+   * The source identifiers.
    *
    * @var array
    */
-  protected $sourceIdValues = array();
+  protected $sourceIds = array();
 
   /**
    * The destination values.
@@ -57,24 +57,25 @@
   protected $frozen = FALSE;
 
   /**
-   * Constructs a Migrate>Row object.
+   * Constructs a \Drupal\Migrate\Row object.
    *
+   * @param array $values
+   *   An array of values to add as properties on the object.
    * @param array $source_ids
    *   An array containing the ids of the source using the keys as the field
    *   names.
-   * @param array $values
-   *   An array of values to add as properties on the object.
+   * @param array $destination_ids
+   *   An array containing the ids of the destination using the keys as the field
+   *   names.
    *
    * @throws \InvalidArgumentException
    *   Thrown when a source id property does not exist.
    */
-  public function __construct(array $source_ids, array $values) {
+  public function __construct(array $values, array $source_ids) {
     $this->source = $values;
+    $this->sourceIds = $source_ids;
     foreach (array_keys($source_ids) as $id) {
-      if ($this->hasSourceProperty($id)) {
-        $this->sourceIdValues[$id] = $values[$id];
-      }
-      else {
+      if (!$this->hasSourceProperty($id)) {
         throw new \InvalidArgumentException("$id has no value");
       }
     }
@@ -87,7 +88,7 @@
    *   An array containing the values of the source identifiers.
    */
   public function getSourceIdValues() {
-    return $this->sourceIdValues;
+    return array_intersect_key($this->source, $this->sourceIds);
   }
 
   /**
@@ -155,50 +156,43 @@
   }
 
   /**
-   * Determines whether a destination has a property.
-   *
-   * @param string $property
-   *   A property on the destination.
-   *
-   * @return bool
-   *   TRUE if the destination has property; FALSE otherwise.
+   * @param array $property_keys
+   *   An array of properties on the destination.
    */
   public function hasDestinationProperty($property) {
-    return isset($this->destination[$property]) || array_key_exists($property, $this->destination);
+    return NestedArray::keyExists($this->destination, explode(':', $property));
   }
 
   /**
-   * Sets a destination property.
+   * Sets destination properties.
    *
    * @param string $property
-   *   A property on the destination.
+   *   The name of the destination property.
    * @param mixed $value
    *   The property value to set on the destination.
    */
   public function setDestinationProperty($property, $value) {
-    $this->destination[$property] = $value;
+    NestedArray::setValue($this->destination, explode(':', $property), $value, TRUE);
   }
 
   /**
-   * Sets destination properties.
+   * This returns the whole destination array.
    *
-   * @param array $property_keys
-   *   An array of properties on the destination.
-   * @param mixed $value
-   *   The property value to set on the destination.
+   * @return array
+   *   An array of destination values.
    */
-  public function setDestinationPropertyDeep(array $property_keys, $value) {
-    NestedArray::setValue($this->destination, $property_keys, $value, TRUE);
+  public function getDestination() {
+    return $this->destination;
   }
 
   /**
-   * This returns the whole destination array.
+   * Return a the value of a destination property.
    *
-   * @return array
-   *   An array of destination plugins.
+   * @return mixed
+   *  The destination value.
    */
-  public function getDestination() {
-    return $this->destination;
+  public function getDestinationProperty($property) {
+    return NestedArray::getValue($this->destination, explode(':', $property));
   }
 
   /**
diff -u b/core/modules/migrate/lib/Drupal/migrate/Source.php b/core/modules/migrate/lib/Drupal/migrate/Source.php
--- b/core/modules/migrate/lib/Drupal/migrate/Source.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Source.php
@@ -11,10 +11,12 @@
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
 
 /**
- * A base class for migrate sources.
+ * Source is a caching / decision making wrapper around the source plugin.
  *
  * Derived classes are expected to define __toString(), returning a string
  * describing the source and significant options, i.e. the query.
+ *
+ * @see \Drupal\migrate\MigrateSourceInterface
  */
 class Source implements \Iterator, \Countable {
 
@@ -272,7 +274,7 @@
     while ($this->getIterator()->valid()) {
       $row_data = $this->getIterator()->current();
       $this->getIterator()->next();
-      $row = new Row($this->migration->get('sourceIds'), $row_data);
+      $row = new Row($row_data, $this->migration->get('sourceIds'), $this->migration->get('destinationIds'));
 
       // Populate the source key for this row
       $this->currentIds = $row->getSourceIdValues();
diff -u b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
--- b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateSqlSourceTestCase.php
@@ -70,23 +70,6 @@
   }
 
   /**
-   * Sets attribute values for a source test.
-   *
-   * @param object $object
-   *   The destination of the written attribute value.
-   * @param string $attribute_name
-   *   The name of the attribute to write.
-   * @param mixed $value
-   *   The value of the written attribute.
-   */
-  protected function writeAttribute($object, $attribute_name, $value) {
-    $reflection = new \ReflectionClass($object);
-    $reflection_property = $reflection->getProperty($attribute_name);
-    $reflection_property->setAccessible(TRUE);
-    $reflection_property->setValue($object, $value);
-  }
-
-  /**
    * Tests retrieval.
    */
   public function testRetrieval() {
diff -u b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateTestCase.php b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateTestCase.php
--- b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateTestCase.php
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/MigrateTestCase.php
@@ -64,2 +64,19 @@
 
+  /**
+   * Sets attribute values for a source test.
+   *
+   * @param object $object
+   *   The destination of the written attribute value.
+   * @param string $attribute_name
+   *   The name of the attribute to write.
+   * @param mixed $value
+   *   The value of the written attribute.
+   */
+  protected function writeAttribute($object, $attribute_name, $value) {
+    $reflection = new \ReflectionClass($object);
+    $reflection_property = $reflection->getProperty($attribute_name);
+    $reflection_property->setAccessible(TRUE);
+    $reflection_property->setValue($object, $value);
+  }
+
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/lib/Drupal/migrate/MigrateMessage.php
@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\MigrateMessage.
+ */
+
+namespace Drupal\migrate;
+
+/**
+ * Defines a migrate message class.
+ */
+class MigrateMessage implements MigrateMessageInterface {
+
+  /**
+   * Displays a migrate message.
+   *
+   * @param string $message
+   *   The message to display.
+   * @param string $type
+   *   The type of message, for example: status or warning.
+   */
+  function display($message, $type = 'status') {
+    drupal_set_message($message, $type);
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/CopyFromSource.php
@@ -0,0 +1,34 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Plugin\migrate\process\CopyFromSource.
+ */
+
+namespace Drupal\migrate\Plugin\migrate\process;
+
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\migrate\MigrateExecutable;
+use Drupal\migrate\Plugin\ProcessInterface;
+use Drupal\migrate\Row;
+
+/**
+ * This plugin copies from the source to the destination.
+ *
+ * @PluginId("copy_from_source")
+ */
+class CopyFromSource extends PluginBase implements ProcessInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function apply(Row $row, MigrateExecutable $migrate_executable) {
+    foreach ($this->configuration as $from => $to) {
+      if ($row->hasSourceProperty($from)) {
+        $destination_values = $row->getSourceProperty($from);
+        $row->setDestinationProperty($to, $destination_values);
+      }
+    }
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/lib/Drupal/migrate/Plugin/migrate/process/DefaultValue.php
@@ -0,0 +1,35 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Plugin\migrate\process\DefaultValue.
+ */
+
+
+namespace Drupal\migrate\Plugin\migrate\process;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\migrate\MigrateExecutable;
+use Drupal\migrate\Plugin\ProcessInterface;
+use Drupal\migrate\Row;
+
+
+/**
+ * This plugin sets missing values on the destination.
+ *
+ * @PluginId("default_value")
+ */
+class DefaultValue extends PluginBase implements ProcessInterface {
+
+ /**
+   * {@inheritdoc}
+   */
+  public function apply(Row $row, MigrateExecutable $migrate_executable) {
+    foreach ($this->configuration as $key => $default_value) {
+      if (!$row->hasDestinationProperty($key)) {
+        $row->setDestinationProperty($key, $default_value);
+      }
+    }
+  }
+
+}
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/tests/Drupal/migrate/Tests/CopyFromSourceTest.php
@@ -0,0 +1,91 @@
+<?php
+/**
+ * @file
+ * Contains
+ */
+
+namespace Drupal\migrate\Tests;
+
+
+use Drupal\migrate\Plugin\migrate\process\CopyFromSource;
+use Drupal\migrate\Row;
+
+/**
+ * Tests for CopyFroumSource class.
+ *
+ * @group migrate
+ * @group Drupal
+ *
+ * @covers \Drupal\migrate\Plugin\migrate\process\CopyFromSource
+ */
+class CopyFromSourceTest extends MigrateTestCase {
+
+  protected $sourceIds = array(
+    'nid' => 'Node ID',
+  );
+  protected $destinationIds = array(
+    'nid' => 'Destination Id',
+  );
+  protected $values = array(
+    'nid' => 1,
+    'title' => 'node 1',
+  );
+
+  /**
+   * @var \Drupal\migrate\MigrateExecutable
+   */
+  protected $migrateExecutable;
+
+  /**
+   * @var \Drupal\migrate\Row
+   */
+  protected $row;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'PropertyMap class functionality',
+      'description' => 'Tests PropertyMap class functionality.',
+      'group' => 'Migrate',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    $this->migrateExecutable = $this->getMockBuilder('Drupal\migrate\MigrateExecutable')
+      ->disableOriginalConstructor()
+      ->getMock();
+    $this->row = new Row($this->values, $this->sourceIds, $this->destinationIds);
+  }
+
+  /**
+   * Tests sub destination.
+   */
+  public function testSubDestination() {
+    $configuration = array(
+      'nid' => 'testproperty:sub',
+    );
+    $map = new CopyFromSource($configuration, 'copy_from_source', array());
+    $map->apply($this->row, $this->migrateExecutable);
+    $destination = $this->row->getDestination();
+    $this->assertSame(1, $destination['testproperty']['sub']);
+  }
+
+  /**
+   * Tests missing source.
+   */
+  public function testNoSourceValue() {
+    $configuration = array(
+      'foo' => 'testproperty:sub',
+    );
+    $map = new CopyFromSource($configuration, 'copy_from_source', array());
+    $map->apply($this->row, $this->migrateExecutable);
+    $destination = $this->row->getDestination();
+    $this->assertSame(array(), $destination);
+  }
+
+}