Active Record in Docker

app.rb

$stdout = IO.new(IO.sysopen("/proc/1/fd/1", "w"), "w")
$stdout.sync = true
require "active_record"
I18n.enforce_available_locales = false
require "uri"
ActiveRecord::Base.establish_connection ENV["DATABASE_URL"]

# Write your code
class YourModel < ActiveRecord::Base; end
puts YourModel.count

Dockerfile

FROM ruby
COPY ["Gemfile", "Gemfile.lock", "app.rb", "/"]
RUN bundle
CMD ["ruby", "/app.rb"]

Gemfile

source "https://rubygems.org"

gem "activerecord"
gem "pg"

note.md

About first 2 lines in app.rb

To enable stdout flush.

ref. https://hawksnowlog.blogspot.com/2019/04/dose-not-show-puts-output-on-docker.html

About require "uri"

Parse DATABASE_URL URI for establish_connection.

run.sh

# to generate Gemfile.lock
bundle

docker image build . -t your-image-name

docker container run --rm -e DATABASE_URL=postgres://username:password@host:5432/databasename your-image-name