Infer omitted column charset from collation

src/Platforms/MySQL/CollationMetadataProvider.php

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Platforms\MySQL;
+
+/**
+ * @internal
+ */
+interface CollationMetadataProvider
+{
+    public function getCollationCharset(string $collation): ?string;
+}

src/Platforms/MySQL/CollationMetadataProvider/CachingCollationMetadataProvider.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
+
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
+
+use function array_key_exists;
+
+/**
+ * @internal
+ */
+final class CachingCollationMetadataProvider implements CollationMetadataProvider
+{
+    /** @var CollationMetadataProvider */
+    private $collationMetadataProvider;
+
+    /** @var array<string,?string> */
+    private $cache = [];
+
+    public function __construct(CollationMetadataProvider $collationMetadataProvider)
+    {
+        $this->collationMetadataProvider = $collationMetadataProvider;
+    }
+
+    public function getCollationCharset(string $collation): ?string
+    {
+        if (array_key_exists($collation, $this->cache)) {
+            return $this->cache[$collation];
+        }
+
+        return $this->cache[$collation] = $this->collationMetadataProvider->getCollationCharset($collation);
+    }
+}

src/Platforms/MySQL/CollationMetadataProvider/ConnectionCollationMetadataProvider.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
+
+use Doctrine\DBAL\Connection;
+use Doctrine\DBAL\Exception;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
+
+/**
+ * @internal
+ */
+final class ConnectionCollationMetadataProvider implements CollationMetadataProvider
+{
+    /** @var Connection */
+    private $connection;
+
+    public function __construct(Connection $connection)
+    {
+        $this->connection = $connection;
+    }
+
+    /**
+     * @throws Exception
+     */
+    public function getCollationCharset(string $collation): ?string
+    {
+        $charset = $this->connection->fetchOne(
+            <<<'SQL'
+SELECT CHARACTER_SET_NAME
+FROM information_schema.COLLATIONS
+WHERE COLLATION_NAME = ?;
+SQL
+            ,
+            [$collation]
+        );
+
+        if ($charset !== false) {
+            return $charset;
+        }
+
+        return null;
+    }
+}

src/Platforms/MySQL/Comparator.php

@@ -18,12 +18,17 @@
  */
 class Comparator extends BaseComparator
 {
+    /** @var CollationMetadataProvider */
+    private $collationMetadataProvider;
+
     /**
      * @internal The comparator can be only instantiated by a schema manager.
      */
-    public function __construct(AbstractMySQLPlatform $platform)
+    public function __construct(AbstractMySQLPlatform $platform, CollationMetadataProvider $collationMetadataProvider)
     {
         parent::__construct($platform);
+
+        $this->collationMetadataProvider = $collationMetadataProvider;
     }
 
     /**
@@ -39,28 +44,44 @@ public function diffTable(Table $fromTable, Table $toTable)
 
     private function normalizeColumns(Table $table): Table
     {
-        $defaults = array_intersect_key($table->getOptions(), [
+        $tableOptions = array_intersect_key($table->getOptions(), [
             'charset'   => null,
             'collation' => null,
         ]);
 
-        if ($defaults === []) {
-            return $table;
-        }
-
         $table = clone $table;
 
         foreach ($table->getColumns() as $column) {
-            $options = $column->getPlatformOptions();
-            $diff    = array_diff_assoc($options, $defaults);
+            $originalOptions   = $column->getPlatformOptions();
+            $normalizedOptions = $this->normalizeOptions($originalOptions);
 
-            if ($diff === $options) {
+            $overrideOptions = array_diff_assoc($normalizedOptions, $tableOptions);
+
+            if ($overrideOptions === $originalOptions) {
                 continue;
             }
 
-            $column->setPlatformOptions($diff);
+            $column->setPlatformOptions($overrideOptions);
         }
 
         return $table;
     }
+
+    /**
+     * @param array<string,string> $options
+     *
+     * @return array<string,string>
+     */
+    private function normalizeOptions(array $options): array
+    {
+        if (isset($options['collation']) && ! isset($options['charset'])) {
+            $charset = $this->collationMetadataProvider->getCollationCharset($options['collation']);
+
+            if ($charset !== null) {
+                $options['charset'] = $charset;
+            }
+        }
+
+        return $options;
+    }
 }

src/Schema/MySQLSchemaManager.php

@@ -5,6 +5,8 @@
 use Doctrine\DBAL\Platforms\AbstractMySQLPlatform;
 use Doctrine\DBAL\Platforms\MariaDb1027Platform;
 use Doctrine\DBAL\Platforms\MySQL;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider\CachingCollationMetadataProvider;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider\ConnectionCollationMetadataProvider;
 use Doctrine\DBAL\Types\Type;
 
 use function array_change_key_case;
@@ -363,7 +365,12 @@ public function listTableDetails($name)
 
     public function createComparator(): Comparator
     {
-        return new MySQL\Comparator($this->getDatabasePlatform());
+        return new MySQL\Comparator(
+            $this->getDatabasePlatform(),
+            new CachingCollationMetadataProvider(
+                new ConnectionCollationMetadataProvider($this->_conn)
+            )
+        );
     }
 
     /**

tests/Functional/Schema/MySQL/ComparatorTest.php

@@ -138,6 +138,28 @@ public function testChangeColumnCollation(): void
         ComparatorTestUtils::assertDiffNotEmpty($this->connection, $this->comparator, $table);
     }
 
+    public function testImplicitColumnCharset(): void
+    {
+        $table = new Table('comparator_test');
+        $table->addColumn('name', Types::STRING, [
+            'length' => 32,
+            'platformOptions' => ['collation' => 'utf8mb4_unicode_ci'],
+        ]);
+        $this->dropAndCreateTable($table);
+
+        self::assertFalse(ComparatorTestUtils::diffFromActualToDesiredTable(
+            $this->schemaManager,
+            $this->comparator,
+            $table
+        ));
+
+        self::assertFalse(ComparatorTestUtils::diffFromDesiredToActualTable(
+            $this->schemaManager,
+            $this->comparator,
+            $table
+        ));
+    }
+
     /**
      * @return array{Table,Column}
      *

tests/Platforms/AbstractMySQLPlatformTestCase.php

@@ -4,6 +4,7 @@
 
 use Doctrine\DBAL\Platforms\AbstractPlatform;
 use Doctrine\DBAL\Platforms\MySQL;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
 use Doctrine\DBAL\Platforms\MySQLPlatform;
 use Doctrine\DBAL\Schema\Comparator;
 use Doctrine\DBAL\Schema\ForeignKeyConstraint;
@@ -795,7 +796,12 @@ public function testIgnoresDifferenceInDefaultValuesForUnsupportedColumnTypes():
         $diffTable->changeColumn('def_blob', ['default' => null]);
         $diffTable->changeColumn('def_blob_null', ['default' => null]);
 
-        self::assertFalse((new MySQL\Comparator($this->platform))->diffTable($table, $diffTable));
+        $comparator = new MySQL\Comparator(
+            $this->platform,
+            $this->createMock(CollationMetadataProvider::class)
+        );
+
+        self::assertFalse($comparator->diffTable($table, $diffTable));
     }
 
     /**
@@ -1052,7 +1058,15 @@ public static function comparatorProvider(): iterable
         ];
 
         yield 'MySQL comparator' => [
-            new MySQL\Comparator(new MySQLPlatform()),
+            new MySQL\Comparator(
+                new MySQLPlatform(),
+                new class implements CollationMetadataProvider {
+                    public function getCollationCharset(string $collation): ?string
+                    {
+                        return null;
+                    }
+                }
+            ),
         ];
     }
 }

tests/Platforms/MySQL/CollationMetadataProvider/CachingCollationMetadataProviderTest.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\DBAL\Tests\Platforms\MySQL\CollationMetadataProvider;
+
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider\CachingCollationMetadataProvider;
+use PHPUnit\Framework\TestCase;
+
+class CachingCollationMetadataProviderTest extends TestCase
+{
+    /**
+     * @dataProvider charsetAndCollationProvider
+     */
+    public function testCharsetCaching(string $collation, ?string $charset): void
+    {
+        $underlyingProvider = $this->createMock(CollationMetadataProvider::class);
+        $underlyingProvider->expects(self::once())
+            ->method('getCollationCharset')
+            ->with($collation)
+            ->willReturn($charset);
+
+        $cachingProvider = new CachingCollationMetadataProvider($underlyingProvider);
+        self::assertSame($charset, $cachingProvider->getCollationCharset($collation));
+        self::assertSame($charset, $cachingProvider->getCollationCharset($collation));
+    }
+
+    /**
+     * @return iterable<string,array{string,?string}>
+     */
+    public static function charsetAndCollationProvider(): iterable
+    {
+        yield 'found' => ['utf8mb4_unicode_ci', 'utf8mb4'];
+        yield 'not found' => ['utf8mb5_unicode_ci', null];
+    }
+}

tests/Platforms/MySQL/ComparatorTest.php

@@ -2,6 +2,7 @@
 
 namespace Doctrine\DBAL\Tests\Platforms\MySQL;
 
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
 use Doctrine\DBAL\Platforms\MySQL\Comparator;
 use Doctrine\DBAL\Platforms\MySQLPlatform;
 use Doctrine\DBAL\Tests\Schema\ComparatorTest as BaseComparatorTest;
@@ -10,6 +11,14 @@ class ComparatorTest extends BaseComparatorTest
 {
     protected function setUp(): void
     {
-        $this->comparator = new Comparator(new MySQLPlatform());
+        $this->comparator = new Comparator(
+            new MySQLPlatform(),
+            new class implements CollationMetadataProvider {
+                public function getCollationCharset(string $collation): ?string
+                {
+                    return null;
+                }
+            }
+        );
     }
 }

tests/Schema/Platforms/MySQLSchemaTest.php

@@ -4,6 +4,7 @@
 
 use Doctrine\DBAL\Platforms\AbstractPlatform;
 use Doctrine\DBAL\Platforms\MySQL;
+use Doctrine\DBAL\Platforms\MySQL\CollationMetadataProvider;
 use Doctrine\DBAL\Platforms\MySQLPlatform;
 use Doctrine\DBAL\Schema\Comparator;
 use Doctrine\DBAL\Schema\Table;
@@ -99,7 +100,15 @@ public static function comparatorProvider(): iterable
         ];
 
         yield 'MySQL comparator' => [
-            new MySQL\Comparator(new MySQLPlatform()),
+            new MySQL\Comparator(
+                new MySQLPlatform(),
+                new class implements CollationMetadataProvider {
+                    public function getCollationCharset(string $collation): ?string
+                    {
+                        return null;
+                    }
+                }
+            ),
         ];
     }
 }