public function getDropSchemaSQL(array $classes)
    {
        $visitor = new DropSchemaSqlCollector($this->platform);
        $schema  = $this->getSchemaFromMetadata($classes);

        $sm         = $this->em->getConnection()->getSchemaManager();
        $fullSchema = $sm->createSchema();

        foreach ($fullSchema->getTables() as $table) {
            if (! $schema->hasTable($table->getName())) {
                foreach ($table->getForeignKeys() as $foreignKey) {
                    /** @var $foreignKey \Doctrine\DBAL\Schema\ForeignKeyConstraint */
                    if ($schema->hasTable($foreignKey->getForeignTableName())) {
                        $visitor->acceptForeignKey($table, $foreignKey);
                    }
                }
            } else {
                $visitor->acceptTable($table);
                foreach ($table->getForeignKeys() as $foreignKey) {
                    $visitor->acceptForeignKey($table, $foreignKey);
                }
            }
        }

        if ($this->platform->supportsSequences()) {
            foreach ($schema->getSequences() as $sequence) {
                $visitor->acceptSequence($sequence);
            }

            foreach ($schema->getTables() as $table) {
                /** @var $sequence Table */
                if ($table->hasPrimaryKey()) {
                    $columns = $table->getPrimaryKey()->getColumns();
                    if (count($columns) === 1) {
                        $checkSequence = $table->getName() . '_' . $columns[0] . '_seq';
                        if ($fullSchema->hasSequence($checkSequence)) {
                            $visitor->acceptSequence($fullSchema->getSequence($checkSequence));
                        }
                    }
                }
            }
        }

        return $visitor->getQueries();
    }