Mysql::Error: Lost connection to MySQL server during query:

解决这个问题的步骤是：
1.下载 mysql/ruby 2.7.1


2解压其中的mysql.so到%RUBYARCHDIR%目录下（例如：C:\ruby\lib\ruby\site_ruby\1.8\i386-msvcrt）

3.MySQL/Ruby needs a DLL dependency satisfied in order to run. You can do this by either:

------(Recommended) Including the MySQL bindir in your path (e.g. C:\Program Files\MySQL\MySQL Server 5.0\bin).

------Copying libmysql.dll from your MySQL bindir or source package to your Ruby bindir (e.g. C:\ruby\bin).

引用

Instant Rails users: You will find that the libmysql.dll from the included PHP distribution won’t work. Instead, grab a libmysql.dll from a real MySQL distribution. InstantRails 1.3a comes with MySQL 4.1.9, which you can download here

4.To verify, try to run the following code:

require 'mysql'
puts Mysql::VERSION

If it outputs 20700, then you’re all set up