How to use MySQL on Docker while developing a Rails project locally

Question

I am trying to use MySQL on Docker (instead of Homebrew) for a Rails project. This is because I need different versions of MySQL for each project, and it's a pain to switch versions with brew. However, I'm having trouble pointing Rails to the MySQL server on Docker.

I started the Docker container like this (5.6 is the version I need):

docker run -p 3306:3306 --name project-db -e MYSQL_ROOT_PASSWORD=mysql_pw -d mysql:5.6

The container seems to be up and running fine, because

• docker ps tells me it is running
• I can connect to it via mysql -u root -p -h 127.0.0.1

But I can't get Rails to create its DB in Docker; executing rails db:setup creates the DB in the locally installed MySQL instead.

I tried uninstalling MySQL locally with brew uninstall mysql, thinking perhaps it was somehow overriding the Docker one. But then, trying to set up the DB results in this error:

Mysql2::Error: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)

I'm not very familiar with Docker so I'm probably misunderstanding something. I googled around, but most of the information I could find was for Dockerizing the entire Rails project including MySQL, which is not what I'm trying to do.

Thanks for any advice in advance!

Answer 1

Your MySQL instance is exposed on localhost via port 3306. You must configure Rails to use it, something like:

development:
  adapter: mysql2
  encoding: utf8
  database: my_db_name
  username: root
  password: my_password
  host: 127.0.0.1
  port: 3306

Or by using environment variable:

export DATABASE_URL=mysql2://root:@127.0.0.1

See this post and rails doc for details

Currently you are trying to access MySQL via a socket at /tmp/mysql.sock but there is nothing available here as your MySQL instance runs into its container.