move pre alter table alter index SQL to private method

lib/Doctrine/DBAL/Platforms/MySqlPlatform.php

@@ -646,30 +646,48 @@ class MySqlPlatform extends AbstractPlatform
             }
         }
 
-        // handle columns that are removed from changed indexes
-        foreach ($diff->changedIndexes as $chgKey => $chgIndex) {
-            // Dropping primary keys requires to unset autoincrement attribute on the particular column first
-            if ($chgIndex->isPrimary() && $diff->fromTable instanceof Table) {
+        $sql = array_merge(
+            $sql,
+            $this->getPreAlterTableAlterIndexForeignKeySQL($diff),
+            parent::getPreAlterTableIndexForeignKeySQL($diff)
+        );
+
+        return $sql;
+    }
 
-                // when chgIndex->isPrimary we can be sure fromTable->hasPrimaryKey is true
+    /**
+     * @param TableDiff $diff The table diff to gather the SQL for.
+     *
+     * @return array
+     */
+    private function getPreAlterTableAlterIndexForeignKeySQL(TableDiff $diff)
+    {
+        $sql = array();
+        $table = $diff->name;
+
+        foreach ($diff->changedIndexes as $changedIndex) {
+            // Changed primary key
+            if ($changedIndex->isPrimary() && $diff->fromTable instanceof Table) {
                 foreach ($diff->fromTable->getPrimaryKeyColumns() as $columnName) {
                     $column = $diff->fromTable->getColumn($columnName);
 
-                    if ($column->getAutoincrement() === true && in_array($columnName, $chgIndex->getColumns()) === false) {
+                    // Check if an autoincrement column was dropped from the primary key.
+                    if ($column->getAutoincrement() && ! in_array($columnName, $changedIndex->getColumns())) {
+                        // The autoincrement attribute needs to be removed from the dropped column
+                        // before we can drop and recreate the primary key.
                         $column->setAutoincrement(false);
 
                         $sql[] = 'ALTER TABLE ' . $table . ' MODIFY ' .
                             $this->getColumnDeclarationSQL($column->getQuotedName($this), $column->toArray());
 
-                        // original autoincrement information might be needed later on by other parts of the table alteration
+                        // Restore the autoincrement attribute as it might be needed later on
+                        // by other parts of the table alteration.
                         $column->setAutoincrement(true);
                     }
                 }
             }
         }
 
-        $sql = array_merge($sql, parent::getPreAlterTableIndexForeignKeySQL($diff));
-
         return $sql;
     }