Order a category listing by the position set in the browsed category

src/Adapter/MySQL.php

@@ -380,8 +380,10 @@ protected function computeOrderByField(array $filterToTableMapping)
             return $orderField;
         }
 
-        // If we have an initial population, add the field into initial population selects, so we can use it in the outer query for sorting
-        if ($this->getInitialPopulation() !== null) {
+        // If we have an initial population, add the field into initial population selects, so we can use it in the outer query for sorting.
+        // An expression is skipped: it is evaluated against the outer query's joins, which the initial population does not have,
+        // so selecting it there refers to tables that are not joined at that level.
+        if ($this->getInitialPopulation() !== null && strpos($orderField, '(') === false) {
             $this->getInitialPopulation()->addSelectField($orderField);
         }
 

src/Filters/Products.php

@@ -71,6 +71,18 @@ public function getProductByFilters(
         $orderWay = Validate::isOrderWay($orderWay) ? $orderWay : 'ASC';
         $orderBy = Validate::isOrderBy($orderBy) ? $orderBy : 'position';
 
+        // A product has one position per category it belongs to. When the listing spans a subtree
+        // (PS_LAYERED_FULL_TREE), the rows are restricted with nleft/nright rather than to the browsed
+        // category, so `position` is not functionally dependent on the product the query groups by and
+        // the value picked is whichever row the server happens to keep. Pin the ordering to the browsed
+        // category; a product reached only through a subcategory has no position here and is listed after
+        // the ones the merchant did arrange.
+        if ($orderBy === 'position' && $query->getIdCategory()) {
+            $positionInCategory = 'MIN(IF(cp.id_category = ' . (int) $query->getIdCategory() . ', cp.position, NULL))';
+            $this->searchAdapter->addSelectField('position');
+            $orderBy = 'ISNULL(' . $positionInCategory . ') ASC, ' . $positionInCategory;
+        }
+
         // Apply it to the filter
         $this->searchAdapter->setOrderField($orderBy);
         $this->searchAdapter->setOrderDirection($orderWay);

tests/php/FacetedSearch/Adapter/MySQLTest.php

@@ -70,6 +70,30 @@ protected function setUp()
         Configuration::setStaticExpectations($configurationMock);
     }
 
+    /**
+     * A product has one position per category, so ordering a subtree listing by position has to
+     * aggregate over the browsed category. The expression is evaluated against the outer query's
+     * joins, so it must not be selected in the initial population, which does not have them.
+     */
+    public function testGetQueryOrderedByAnExpressionKeepsItOutOfTheInitialPopulation()
+    {
+        $this->adapter->useFiltersAsInitialPopulation();
+        $this->adapter->addFilter('nleft', [9], '>=');
+        $this->adapter->addFilter('nright', [14], '<=');
+        $this->adapter->addGroupBy('id_product');
+        $this->adapter->addSelectField('position');
+        $this->adapter->setOrderField('ISNULL(MIN(IF(cp.id_category = 6, cp.position, NULL))) ASC, MIN(IF(cp.id_category = 6, cp.position, NULL))');
+        $this->adapter->setOrderDirection('asc');
+
+        $query = $this->adapter->getQuery();
+        $innerQuery = substr($query, strpos($query, '(') + 1, strrpos($query, ') p INNER JOIN') - strpos($query, '(') - 1);
+
+        $this->assertNotContains('cp.id_category', $innerQuery, 'The initial population does not join category_product, so it cannot select an expression using it.');
+        $this->assertNotContains('ps_category_product', $innerQuery, 'The expression is read in the outer query, so the initial population does not need the join at all.');
+        $this->assertContains(') p INNER JOIN ps_category_product cp ON (p.id_product = cp.id_product)', $query, 'The outer query joins category_product, which is what the expression reads.');
+        $this->assertContains('ORDER BY ISNULL(MIN(IF(cp.id_category = 6, cp.position, NULL))) ASC, MIN(IF(cp.id_category = 6, cp.position, NULL)) ASC, p.id_product DESC', $query);
+    }
+
     public function testGetEmptyQuery()
     {
         $this->assertEquals(