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Rails 2.1 introduced in the MySQL Adapter “smart integer columns.” The idea was to use the :limit option to determine whether a smallint, int, or bigint should be used. This is something that the Postgres adapter had already previously implemented. The relevant code in activerecord/lib/active_record/connection_adapters/mysql_adapter.rb is:

  # Maps logical Rails types to MySQL-specific data types.
  def type_to_sql(type, limit = nil, precision = nil, scale = nil)
    return super unless type.to_s == 'integer'
 
    case limit
    when 0..3
      "smallint(#{limit})"
    when 4..8
      "int(#{limit})"
    when 9..20
      "bigint(#{limit})"
    else
      'int(11)'
    end
  end

Mirko Froehlich suggests monkey patching this function. Timothy Jones blogged about it.

To monkey-patch this, just drop a file (fix_mysql_adapter.rb) into your initializers/ directory, as such:

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