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diff --git a/web/core/modules/views/src/Plugin/views/relationship/GroupwiseMax.php b/web/core/modules/views/src/Plugin/views/relationship/GroupwiseMax.php
new file mode 100644
index 000000000..1db8da7f7
--- /dev/null
+++ b/web/core/modules/views/src/Plugin/views/relationship/GroupwiseMax.php
@@ -0,0 +1,387 @@
+<?php
+
+namespace Drupal\views\Plugin\views\relationship;
+
+use Drupal\Core\Database\Query\AlterableInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\views\Views;
+use Drupal\views\Entity\View;
+
+/**
+ * Relationship handler that allows a groupwise maximum of the linked in table.
+ * For a definition, see:
+ * http://dev.mysql.com/doc/refman/5.0/en/example-maximum-column-group-row.html
+ * In lay terms, instead of joining to get all matching records in the linked
+ * table, we get only one record, a 'representative record' picked according
+ * to a given criteria.
+ *
+ * Example:
+ * Suppose we have a term view that gives us the terms: Horse, Cat, Aardvark.
+ * We wish to show for each term the most recent node of that term.
+ * What we want is some kind of relationship from term to node.
+ * But a regular relationship will give us all the nodes for each term,
+ * giving the view multiple rows per term. What we want is just one
+ * representative node per term, the node that is the 'best' in some way:
+ * eg, the most recent, the most commented on, the first in alphabetical order.
+ *
+ * This handler gives us that kind of relationship from term to node.
+ * The method of choosing the 'best' implemented with a sort
+ * that the user selects in the relationship settings.
+ *
+ * So if we want our term view to show the most commented node for each term,
+ * add the relationship and in its options, pick the 'Comment count' sort.
+ *
+ * Relationship definition
+ *  - 'outer field': The outer field to substitute into the correlated subquery.
+ *       This must be the full field name, not the alias.
+ *       Eg: 'term_data.tid'.
+ *  - 'argument table',
+ *    'argument field': These options define a views argument that the subquery
+ *     must add to itself to filter by the main view.
+ *     Example: the main view shows terms, this handler is being used to get to
+ *     the nodes base table. Your argument must be 'term_node', 'tid', as this
+ *     is the argument that should be added to a node view to filter on terms.
+ *
+ * A note on performance:
+ * This relationship uses a correlated subquery, which is expensive.
+ * Subsequent versions of this handler could also implement the alternative way
+ * of doing this, with a join -- though this looks like it could be pretty messy
+ * to implement. This is also an expensive method, so providing both methods and
+ * allowing the user to choose which one works fastest for their data might be
+ * the best way.
+ * If your use of this relationship handler is likely to result in large
+ * data sets, you might want to consider storing statistics in a separate table,
+ * in the same way as node_comment_statistics.
+ *
+ * @ingroup views_relationship_handlers
+ *
+ * @ViewsRelationship("groupwise_max")
+ */
+class GroupwiseMax extends RelationshipPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function defineOptions() {
+    $options = parent::defineOptions();
+
+    $options['subquery_sort'] = ['default' => NULL];
+    // Descending more useful.
+    $options['subquery_order'] = ['default' => 'DESC'];
+    $options['subquery_regenerate'] = ['default' => FALSE];
+    $options['subquery_view'] = ['default' => FALSE];
+    $options['subquery_namespace'] = ['default' => FALSE];
+
+    return $options;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
+    parent::buildOptionsForm($form, $form_state);
+
+    // Get the sorts that apply to our base.
+    $sorts = Views::viewsDataHelper()->fetchFields($this->definition['base'], 'sort');
+    $sort_options = [];
+    foreach ($sorts as $sort_id => $sort) {
+      $sort_options[$sort_id] = "$sort[group]: $sort[title]";
+    }
+    $base_table_data = Views::viewsData()->get($this->definition['base']);
+
+    // Extends the relationship's basic options, allowing the user to pick a
+    // sort and an order for it.
+    $form['subquery_sort'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Representative sort criteria'),
+      // Provide the base field as sane default sort option.
+      '#default_value' => !empty($this->options['subquery_sort']) ? $this->options['subquery_sort'] : $this->definition['base'] . '.' . $base_table_data['table']['base']['field'],
+      '#options' => $sort_options,
+      '#description' => $this->t("The sort criteria is applied to the data brought in by the relationship to determine how a representative item is obtained for each row. For example, to show the most recent node for each user, pick 'Content: Updated date'."),
+    ];
+
+    $form['subquery_order'] = [
+      '#type' => 'radios',
+      '#title' => $this->t('Representative sort order'),
+      '#description' => $this->t("The ordering to use for the sort criteria selected above."),
+      '#options' => ['ASC' => $this->t('Ascending'), 'DESC' => $this->t('Descending')],
+      '#default_value' => $this->options['subquery_order'],
+    ];
+
+    $form['subquery_namespace'] = [
+      '#type' => 'textfield',
+      '#title' => $this->t('Subquery namespace'),
+      '#description' => $this->t('Advanced. Enter a namespace for the subquery used by this relationship.'),
+      '#default_value' => $this->options['subquery_namespace'],
+    ];
+
+
+    // WIP: This stuff doesn't work yet: namespacing issues.
+    // A list of suitable views to pick one as the subview.
+    $views = ['' => '- None -'];
+    foreach (Views::getAllViews() as $view) {
+      // Only get views that are suitable:
+      // - base must the base that our relationship joins towards
+      // - must have fields.
+      if ($view->get('base_table') == $this->definition['base'] && !empty($view->getDisplay('default')['display_options']['fields'])) {
+        // TODO: check the field is the correct sort?
+        // or let users hang themselves at this stage and check later?
+        $views[$view->id()] = $view->id();
+      }
+    }
+
+    $form['subquery_view'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Representative view'),
+      '#default_value' => $this->options['subquery_view'],
+      '#options' => $views,
+      '#description' => $this->t('Advanced. Use another view to generate the relationship subquery. This allows you to use filtering and more than one sort. If you pick a view here, the sort options above are ignored. Your view must have the ID of its base as its only field, and should have some kind of sorting.'),
+    ];
+
+    $form['subquery_regenerate'] = [
+      '#type' => 'checkbox',
+      '#title' => $this->t('Generate subquery each time view is run'),
+      '#default_value' => $this->options['subquery_regenerate'],
+      '#description' => $this->t('Will re-generate the subquery for this relationship every time the view is run, instead of only when these options are saved. Use for testing if you are making changes elsewhere. WARNING: seriously impairs performance.'),
+    ];
+  }
+
+  /**
+   * Helper function to create a pseudo view.
+   *
+   * We use this to obtain our subquery SQL.
+   */
+  protected function getTemporaryView() {
+    $view = View::create(['base_table' => $this->definition['base']]);
+    $view->addDisplay('default');
+    return $view->getExecutable();
+  }
+
+  /**
+   * When the form is submitted, make sure to clear the subquery string cache.
+   */
+  public function submitOptionsForm(&$form, FormStateInterface $form_state) {
+    $cid = 'views_relationship_groupwise_max:' . $this->view->storage->id() . ':' . $this->view->current_display . ':' . $this->options['id'];
+    \Drupal::cache('data')->delete($cid);
+  }
+
+  /**
+   * Generate a subquery given the user options, as set in the options.
+   *
+   * These are passed in rather than picked up from the object because we
+   * generate the subquery when the options are saved, rather than when the view
+   * is run. This saves considerable time.
+   *
+   * @param $options
+   *   An array of options:
+   *    - subquery_sort: the id of a views sort.
+   *    - subquery_order: either ASC or DESC.
+   *
+   * @return string
+   *    The subquery SQL string, ready for use in the main query.
+   */
+  protected function leftQuery($options) {
+    // Either load another view, or create one on the fly.
+    if ($options['subquery_view']) {
+      $temp_view = Views::getView($options['subquery_view']);
+      // Remove all fields from default display
+      unset($temp_view->display['default']['display_options']['fields']);
+    }
+    else {
+      // Create a new view object on the fly, which we use to generate a query
+      // object and then get the SQL we need for the subquery.
+      $temp_view = $this->getTemporaryView();
+
+      // Add the sort from the options to the default display.
+      // This is broken, in that the sort order field also gets added as a
+      // select field. See https://www.drupal.org/node/844910.
+      // We work around this further down.
+      $sort = $options['subquery_sort'];
+      list($sort_table, $sort_field) = explode('.', $sort);
+      $sort_options = ['order' => $options['subquery_order']];
+      $temp_view->addHandler('default', 'sort', $sort_table, $sort_field, $sort_options);
+    }
+
+    // Get the namespace string.
+    $temp_view->namespace = (!empty($options['subquery_namespace'])) ? '_' . $options['subquery_namespace'] : '_INNER';
+    $this->subquery_namespace = (!empty($options['subquery_namespace'])) ? '_' . $options['subquery_namespace'] : 'INNER';
+
+    // The value we add here does nothing, but doing this adds the right tables
+    // and puts in a WHERE clause with a placeholder we can grab later.
+    $temp_view->args[] = '**CORRELATED**';
+
+    // Add the base table ID field.
+    $temp_view->addHandler('default', 'field', $this->definition['base'], $this->definition['field']);
+
+    $relationship_id = NULL;
+    // Add the used relationship for the subjoin, if defined.
+    if (isset($this->definition['relationship'])) {
+      list($relationship_table, $relationship_field) = explode(':', $this->definition['relationship']);
+      $relationship_id = $temp_view->addHandler('default', 'relationship', $relationship_table, $relationship_field);
+    }
+    $temp_item_options = ['relationship' => $relationship_id];
+
+    // Add the correct argument for our relationship's base
+    // ie the 'how to get back to base' argument.
+    // The relationship definition tells us which one to use.
+    $temp_view->addHandler('default', 'argument', $this->definition['argument table'], $this->definition['argument field'], $temp_item_options);
+
+    // Build the view. The creates the query object and produces the query
+    // string but does not run any queries.
+    $temp_view->build();
+
+    // Now take the SelectQuery object the View has built and massage it
+    // somewhat so we can get the SQL query from it.
+    $subquery = $temp_view->build_info['query'];
+
+    // Workaround until https://www.drupal.org/node/844910 is fixed:
+    // Remove all fields from the SELECT except the base id.
+    $fields = &$subquery->getFields();
+    foreach (array_keys($fields) as $field_name) {
+      // The base id for this subquery is stored in our definition.
+      if ($field_name != $this->definition['field']) {
+        unset($fields[$field_name]);
+      }
+    }
+
+    // Make every alias in the subquery safe within the outer query by
+    // appending a namespace to it, '_inner' by default.
+    $tables = &$subquery->getTables();
+    foreach (array_keys($tables) as $table_name) {
+      $tables[$table_name]['alias'] .= $this->subquery_namespace;
+      // Namespace the join on every table.
+      if (isset($tables[$table_name]['condition'])) {
+        $tables[$table_name]['condition'] = $this->conditionNamespace($tables[$table_name]['condition']);
+      }
+    }
+    // Namespace fields.
+    foreach (array_keys($fields) as $field_name) {
+      $fields[$field_name]['table'] .= $this->subquery_namespace;
+      $fields[$field_name]['alias'] .= $this->subquery_namespace;
+    }
+    // Namespace conditions.
+    $where = &$subquery->conditions();
+    $this->alterSubqueryCondition($subquery, $where);
+    // Not sure why, but our sort order clause doesn't have a table.
+    // TODO: the call to addHandler() above to add the sort handler is probably
+    // wrong -- needs attention from someone who understands it.
+    // In the meantime, this works, but with a leap of faith.
+    $orders = &$subquery->getOrderBy();
+    foreach ($orders as $order_key => $order) {
+      // But if we're using a whole view, we don't know what we have!
+      if ($options['subquery_view']) {
+        list($sort_table, $sort_field) = explode('.', $order_key);
+      }
+      $orders[$sort_table . $this->subquery_namespace . '.' . $sort_field] = $order;
+      unset($orders[$order_key]);
+    }
+
+    // The query we get doesn't include the LIMIT, so add it here.
+    $subquery->range(0, 1);
+
+    // Extract the SQL the temporary view built.
+    $subquery_sql = $subquery->__toString();
+
+    // Replace the placeholder with the outer, correlated field.
+    // Eg, change the placeholder ':users_uid' into the outer field 'users.uid'.
+    // We have to work directly with the SQL, because putting a name of a field
+    // into a SelectQuery that it does not recognize (because it's outer) just
+    // makes it treat it as a string.
+    $outer_placeholder = ':' . str_replace('.', '_', $this->definition['outer field']);
+    $subquery_sql = str_replace($outer_placeholder, $this->definition['outer field'], $subquery_sql);
+
+    return $subquery_sql;
+  }
+
+  /**
+   * Recursive helper to add a namespace to conditions.
+   *
+   * Similar to _views_query_tag_alter_condition().
+   *
+   * (Though why is the condition we get in a simple query 3 levels deep???)
+   */
+  protected function alterSubqueryCondition(AlterableInterface $query, &$conditions) {
+    foreach ($conditions as $condition_id => &$condition) {
+      // Skip the #conjunction element.
+      if (is_numeric($condition_id)) {
+        if (is_string($condition['field'])) {
+          $condition['field'] = $this->conditionNamespace($condition['field']);
+        }
+        elseif (is_object($condition['field'])) {
+          $sub_conditions = &$condition['field']->conditions();
+          $this->alterSubqueryCondition($query, $sub_conditions);
+        }
+      }
+    }
+  }
+
+  /**
+   * Helper function to namespace query pieces.
+   *
+   * Turns 'foo.bar' into '"foo_NAMESPACE".bar'.
+   * PostgreSQL doesn't support mixed-cased identifiers unless quoted, so we
+   * need to quote each single part to prevent from query exceptions.
+   */
+  protected function conditionNamespace($string) {
+    $parts = explode(' = ', $string);
+    foreach ($parts as &$part) {
+      if (strpos($part, '.') !== FALSE) {
+        $part = '"' . str_replace('.', $this->subquery_namespace . '".', $part);
+      }
+    }
+
+    return implode(' = ', $parts);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function query() {
+    // Figure out what base table this relationship brings to the party.
+    $table_data = Views::viewsData()->get($this->definition['base']);
+    $base_field = empty($this->definition['base field']) ? $table_data['table']['base']['field'] : $this->definition['base field'];
+
+    $this->ensureMyTable();
+
+    $def = $this->definition;
+    $def['table'] = $this->definition['base'];
+    $def['field'] = $base_field;
+    $def['left_table'] = $this->tableAlias;
+    $def['left_field'] = $this->field;
+    $def['adjusted'] = TRUE;
+    if (!empty($this->options['required'])) {
+      $def['type'] = 'INNER';
+    }
+
+    if ($this->options['subquery_regenerate']) {
+      // For testing only, regenerate the subquery each time.
+      $def['left_query'] = $this->leftQuery($this->options);
+    }
+    else {
+      // Get the stored subquery SQL string.
+      $cid = 'views_relationship_groupwise_max:' . $this->view->storage->id() . ':' . $this->view->current_display . ':' . $this->options['id'];
+      $cache = \Drupal::cache('data')->get($cid);
+      if (isset($cache->data)) {
+        $def['left_query'] = $cache->data;
+      }
+      else {
+        $def['left_query'] = $this->leftQuery($this->options);
+        \Drupal::cache('data')->set($cid, $def['left_query']);
+      }
+    }
+
+    if (!empty($def['join_id'])) {
+      $id = $def['join_id'];
+    }
+    else {
+      $id = 'subquery';
+    }
+    $join = Views::pluginManager('join')->createInstance($id, $def);
+
+    // use a short alias for this:
+    $alias = $def['table'] . '_' . $this->table;
+
+    $this->alias = $this->query->addRelationship($alias, $join, $this->definition['base'], $this->relationship);
+  }
+
+}