Include tier prices in catalog rule indexer to prevent rule from raising effective price

app/code/Magento/CatalogRule/Model/Indexer/RuleProductsSelectBuilder.php

@@ -168,9 +168,34 @@ public function buildSelect(
             sprintf($joinCondition, $tableAlias, $storeId),
             []
         );
+
+        $tierPriceTable = $this->resource->getTableName('catalog_product_entity_tier_price');
+
+        $select->joinLeft(
+            ['price_tier' => $tierPriceTable],
+            '(price_tier.' . $linkField . ' = e.' . $linkField . ')'
+            . ' AND (price_tier.website_id = ' . $websiteId . ')'
+            . ' AND (price_tier.customer_group_id = rp.customer_group_id OR price_tier.all_groups = 1)'
+            . ' AND (price_tier.qty = 1)',
+            []
+        );
+
+        $select->joinLeft(
+            ['price_tier0' => $tierPriceTable],
+            '(price_tier0.' . $linkField . ' = e.' . $linkField . ')'
+            . ' AND (price_tier0.website_id = 0)'
+            . ' AND (price_tier0.customer_group_id = rp.customer_group_id OR price_tier0.all_groups = 1)'
+            . ' AND (price_tier0.qty = 1)',
+            []
+        );
+
         $select->columns(
             [
-                'default_price' => $connection->getIfNullSql($tableAlias . '.value', 'pp_default.value'),
+                'default_price' => 'LEAST('
+                    . $connection->getIfNullSql('price_tier0.value', 'pp_default.value') . ','
+                    . $connection->getIfNullSql('price_tier.value', 'pp_default.value') . ','
+                    . $connection->getIfNullSql($tableAlias . '.value', 'pp_default.value')
+                    . ')',
             ]
         );
 

app/code/Magento/CatalogRule/Test/Unit/Model/Indexer/RuleProductsSelectBuilderTest.php

@@ -0,0 +1,170 @@
+<?php
+/**
+ * Copyright 2024 Adobe
+ * All Rights Reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\CatalogRule\Test\Unit\Model\Indexer;
+
+use Magento\Catalog\Model\Product;
+use Magento\Catalog\Model\ResourceModel\Indexer\ActiveTableSwitcher;
+use Magento\CatalogRule\Model\Indexer\IndexerTableSwapperInterface;
+use Magento\CatalogRule\Model\Indexer\RuleProductsSelectBuilder;
+use Magento\Eav\Model\Config;
+use Magento\Eav\Model\Entity\Attribute\AbstractAttribute;
+use Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend;
+use Magento\Framework\App\ResourceConnection;
+use Magento\Framework\DB\Adapter\AdapterInterface;
+use Magento\Framework\DB\Select;
+use Magento\Framework\EntityManager\EntityMetadataInterface;
+use Magento\Framework\EntityManager\MetadataPool;
+use Magento\Store\Model\StoreManagerInterface;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+
+class RuleProductsSelectBuilderTest extends TestCase
+{
+    /** @var ResourceConnection|MockObject */
+    private $resource;
+
+    /** @var Config|MockObject */
+    private $eavConfig;
+
+    /** @var StoreManagerInterface|MockObject */
+    private $storeManager;
+
+    /** @var MetadataPool|MockObject */
+    private $metadataPool;
+
+    /** @var ActiveTableSwitcher|MockObject */
+    private $activeTableSwitcher;
+
+    /** @var IndexerTableSwapperInterface|MockObject */
+    private $tableSwapper;
+
+    /** @var AdapterInterface|MockObject */
+    private $connection;
+
+    /** @var Select|MockObject */
+    private $select;
+
+    protected function setUp(): void
+    {
+        $this->resource           = $this->createMock(ResourceConnection::class);
+        $this->eavConfig          = $this->createMock(Config::class);
+        $this->storeManager       = $this->createMock(StoreManagerInterface::class);
+        $this->metadataPool       = $this->createMock(MetadataPool::class);
+        $this->activeTableSwitcher = $this->createMock(ActiveTableSwitcher::class);
+        $this->tableSwapper       = $this->createMock(IndexerTableSwapperInterface::class);
+        $this->connection         = $this->createMock(AdapterInterface::class);
+        $this->select             = $this->createMock(Select::class);
+
+        $this->resource->method('getConnection')->willReturn($this->connection);
+        $this->resource->method('getTableName')->willReturnCallback(fn($t) => $t);
+
+        $this->connection->method('select')->willReturn($this->select);
+        $this->connection->method('query')->willReturn(
+            $this->createMock(\Zend_Db_Statement_Interface::class)
+        );
+        $this->connection->method('getIfNullSql')->willReturnCallback(
+            fn($a, $b) => "IFNULL($a,$b)"
+        );
+
+        $backendMock = $this->createMock(AbstractBackend::class);
+        $backendMock->method('getTable')->willReturn('catalog_product_entity_decimal');
+
+        $priceMock = $this->createMock(AbstractAttribute::class);
+        $priceMock->method('getBackend')->willReturn($backendMock);
+        $priceMock->method('getId')->willReturn(77);
+
+        $this->eavConfig->method('getAttribute')
+            ->with(Product::ENTITY, 'price')
+            ->willReturn($priceMock);
+
+        $metadataMock = $this->createMock(EntityMetadataInterface::class);
+        $metadataMock->method('getLinkField')->willReturn('entity_id');
+        $this->metadataPool->method('getMetadata')->willReturn($metadataMock);
+
+        $websiteMock = $this->createMock(\Magento\Store\Model\Website::class);
+        $websiteMock->method('getDefaultGroup')->willReturn(null);
+        $this->storeManager->method('getWebsite')->willReturn($websiteMock);
+
+        $this->select->method('from')->willReturnSelf();
+        $this->select->method('order')->willReturnSelf();
+        $this->select->method('where')->willReturnSelf();
+        $this->select->method('join')->willReturnSelf();
+        $this->select->method('joinInner')->willReturnSelf();
+        $this->select->method('joinLeft')->willReturnSelf();
+        $this->select->method('columns')->willReturnSelf();
+    }
+
+    public function testBuildSelectJoinsTierPricesForWebsite(): void
+    {
+        $joinedTables = [];
+        $this->select->method('joinLeft')
+            ->willReturnCallback(function ($table) use (&$joinedTables) {
+                $joinedTables[] = array_key_first((array)$table);
+                return $this->select;
+            });
+
+        $this->buildModel()->buildSelect(1, []);
+
+        $this->assertContains('price_tier', $joinedTables,
+            'buildSelect must LEFT JOIN catalog_product_entity_tier_price for website-specific tier prices');
+        $this->assertContains('price_tier0', $joinedTables,
+            'buildSelect must LEFT JOIN catalog_product_entity_tier_price for global (website_id=0) tier prices');
+    }
+
+    public function testBuildSelectIncludesTierPricesInLeastExpression(): void
+    {
+        $columnsArg = null;
+        $this->select->method('columns')
+            ->willReturnCallback(function ($cols) use (&$columnsArg) {
+                $columnsArg = $cols;
+                return $this->select;
+            });
+
+        $this->buildModel()->buildSelect(1, []);
+
+        $this->assertNotNull($columnsArg);
+        $this->assertArrayHasKey('default_price', $columnsArg);
+        $this->assertStringContainsString('LEAST(', $columnsArg['default_price'],
+            'default_price must use LEAST() to ensure the rule price never exceeds the tier price');
+        $this->assertStringContainsString('price_tier', $columnsArg['default_price'],
+            'LEAST() expression must include tier price values');
+    }
+
+    public function testBuildSelectTierJoinIncludesAllGroupsCondition(): void
+    {
+        $tierJoinConditions = [];
+        $this->select->method('joinLeft')
+            ->willReturnCallback(function ($table, $condition) use (&$tierJoinConditions) {
+                $alias = array_key_first((array)$table);
+                if (in_array($alias, ['price_tier', 'price_tier0'])) {
+                    $tierJoinConditions[$alias] = $condition;
+                }
+                return $this->select;
+            });
+
+        $this->buildModel()->buildSelect(1, []);
+
+        foreach (['price_tier', 'price_tier0'] as $alias) {
+            $this->assertArrayHasKey($alias, $tierJoinConditions);
+            $this->assertStringContainsString('all_groups', $tierJoinConditions[$alias],
+                "$alias JOIN condition must handle all_groups=1 tier prices so every customer group benefits");
+        }
+    }
+
+    private function buildModel(): RuleProductsSelectBuilder
+    {
+        return new RuleProductsSelectBuilder(
+            $this->resource,
+            $this->eavConfig,
+            $this->storeManager,
+            $this->metadataPool,
+            $this->activeTableSwitcher,
+            $this->tableSwapper
+        );
+    }
+}