macOS下Rails 3项目连接MySQL报错：undefined method `sub' for nil:NilClass

Hey there, I’ve run into this exact error before when setting up older Rails projects with MySQL—let’s walk through the most common fixes that should get you up and running:

Possible Causes & Solutions #

• Check for conflicting DATABASE_URL environment variable
  Rails prioritizes the DATABASE_URL env var over your database.yml config. If this variable is set to an invalid value or is nil, it’ll throw that sub error.

  • Run this in your terminal to check:

    echo $DATABASE_URL
    

  • If you get any output (or even an empty line that indicates it’s set but empty), unset it with:

    unset DATABASE_URL
    

  • Restart your Rails server after doing this.

• Pin the mysql2 gem to a Rails 3.2-compatible version
  Newer versions of the mysql2 gem (0.4.x and above) aren’t compatible with Rails 3.2. You need to use a 0.3.x release.

  • Update your Gemfile to include:

    gem 'mysql2', '~> 0.3.21'
    

  • Run bundle install to install the correct version.

• Replace localhost with 127.0.0.1 in your database config
  Sometimes macOS’s localhost resolution can cause issues with MySQL connections. Switching to the direct IP address often fixes this:
  Update your config/database.yml default section:

  default: &default
    adapter: mysql2
    pool: 5
    encoding: utf8
    min_messages: warning
    timeout: 5000
    username: root
    password:
    database: heart_development
    host: 127.0.0.1
  

• Verify MySQL is running
  If MySQL isn’t active, Rails can’t connect at all.

  • If you installed MySQL via Homebrew, check its status with:

    brew services list
    

  • If it’s not running, start it with:

    brew services start mysql
    

  • Alternatively, check the MySQL icon in System Preferences to start the service.

• Confirm your MySQL root user permissions
  Make sure the root user can connect without a password from 127.0.0.1/localhost:

  • Try logging into MySQL directly with:

    mysql -u root
    

  • If this fails, you’ll need to reset your MySQL root password (follow the official MySQL docs for your version to do this).

内容的提问来源于stack exchange，提问作者Nikita Kirilov