require': 无法加载此类文件 -- mysql2/mysql2 (LoadError) rails 4

Question

I trying run 'rails s' in a project, but I get an unexpected error:S.
Thanks in advance.............................
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➜  backend-ror git:(dev) ✗ rails s                                                                               
/Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/mysql2-0.3.21/lib/mysql2.rb:31:in `require': cannot load such file -- mysql2/mysql2 (LoadError)
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/mysql2-0.3.21/lib/mysql2.rb:31:in `<top (required)>'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:81:in `require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:81:in `block (2 levels) in require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:76:in `each'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:76:in `block in require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:65:in `each'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler/runtime.rb:65:in `require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/bundler-1.17.3/lib/bundler.rb:114:in `require'
    from /Users/Cristopheer96/code/Cristopheer96/Guudjob/backend-ror/config/application.rb:7:in `<top (required)>'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands/commands_tasks.rb:78:in `require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands/commands_tasks.rb:78:in `block in server'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands/commands_tasks.rb:75:in `tap'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands/commands_tasks.rb:75:in `server'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-4.2.2/lib/rails/commands.rb:17:in `<top (required)>'
    from /Users/Cristopheer96/code/Cristopheer96/Guudjob/backend-ror/bin/rails:9:in `require'
    from /Users/Cristopheer96/code/Cristopheer96/Guudjob/backend-ror/bin/rails:9:in `<top (required)>'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `load'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/client/rails.rb:28:in `call'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/client/command.rb:7:in `call'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/client.rb:30:in `run'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/bin/spring:49:in `<top (required)>'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `load'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/spring-2.0.2/lib/spring/binstub.rb:31:in `<top (required)>'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
    from /Users/Cristopheer96/.rbenv/versions/2.3.1/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:68:in `require'
    from /Users/Cristopheer96/code/Cristopheer96/Gudi/backend-ror/bin/spring:13:in `<top (required)>'
    from ./bin/rails:3:in `load'
    from ./bin/rails:3:in `<main>'

Answer 1

this works for me:

´´´sudo gem install mysql2 -v 0.3.19 -- \ --with-mysql-config=/usr/local/Cellar/mysql@5.7/5.7.37/bin/mysql_config \ --with-ldflags=-L/usr/local/opt/openssl/lib \ --with-cppflags=-I/usr/local/opt/openssl/include ´´´