Fix a raw SQL for MySQL2

[ozovalihasan]

Description

ActiveRecord::Base.connection.execute(<<-SQL).first
   SELECT (
     (SELECT COUNT(*) FROM passwords) +
     (SELECT COUNT(*) FROM urls) +
     (SELECT COUNT(*) FROM file_pushes)
   ) as total_count
SQL

is returning [0] for MySQL2 and {"total_count" => 0} for SQLite3 and PostgreSQL. So, it is causing an error.

It is fixed to return 0;

ActiveRecord::Base.connection.select_value(<<-SQL)
     SELECT (
       (SELECT COUNT(*) FROM passwords) +
       (SELECT COUNT(*) FROM urls) +
       (SELECT COUNT(*) FROM file_pushes)
     )
SQL

What is the problem?

SQLite2 returns result of high-level queries in a hash. So, it doesn't cause any issue when working with ActiveRecord. But, the result of lower-level #execute method is returned in an array.

Related Issue

#3458

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created. (rake test)
  -- Tested manually
• I've added documentation as necessary so users can easily use and understand this feature/fix.
  -- No need to document

[Copilot]

Pull Request Overview

This PR fixes the raw SQL check in the DropLegacyModels migration so it works correctly with MySQL2 by using select_value instead of execute(...).first.

• Replace execute(...).first["total_count"] with select_value(...)
• Remove the unused SQL alias total_count in the query