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Migrating a Rails project from MySQL to PostgreSQL
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README.md

Migrating a Rails project from MySQL to PostgreSQL

This brief guide is written from my own experience with migrating a large (~5GB) MySQL database to PostgreSQL for a Rails project.

No warranties, guarantees, support etc. Use at your own risk and, as always, ENSURE YOU MAKE BACKUPS FIRST!

I chose pgloader because it's extremely fast. YMMV.

  1. Replace mysql2 gem with pg in Gemfile.
  2. Update config/database.yml for PostgreSQL. I used Rails' template as a starting point.
  3. Run rake db:create to generate your shiney new PostgreSQL DB.
  4. Go through db/schema.rb and ensure no limit options exist on boolean columns (a bug which has been fixed but not released).
  5. Run rake db:schema:load to setup the schema.
  6. Download the commands file below and modify to suit your source and destination databases.
  7. Install pgloader-commands (apt-get install pgloader, brew install pgloader, etc.).
  8. Run pgloader --verbose commands, sit back and enjoy the show.

Please leave comments with your own experiences!

Raw
pgloader-commands
-- See https://github.com/dimitri/pgloader/blob/master/pgloader.1.md for
-- connection string options.
LOAD DATABASE
FROM mysql://root@localhost/source_database
INTO postgresql:///destination_database
-- data-only: We don't need pgloader to touch the schema as Rails does a better
-- job using rake db:schema:load.
-- truncate: Ensure all tables are empty first (especially schema_migrations).
-- WARNING: THIS WILL SMOKE YOUR DATABASE!
WITH data only, truncate;
@dhinakaranS
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dhinakaranS commented Dec 28, 2017

Hello, Thanks for this, I am trying to convert mysql2 to postgres. When i run rake db:create I am getting the following error.

rake aborted!
Gem::LoadError: You have already activated rake 12.3.0, but your Gemfile requires rake 10.4.2. Prepending bundle exec to your command may solve this.
/home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler/runtime.rb:313:in check_for_activated_spec!' /home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler/runtime.rb:31:in block in setup'
/home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler/runtime.rb:26:in map' /home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler/runtime.rb:26:in setup'
/home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler.rb:107:in setup' /home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/bundler-1.16.1/lib/bundler/setup.rb:10:in <top (required)>'
/home/supranimbus12/tracks-2.3.0/config/boot.rb:4:in <top (required)>' /home/supranimbus12/tracks-2.3.0/config/application.rb:1:in <top (required)>'
/home/supranimbus12/tracks-2.3.0/Rakefile:4:in <top (required)>' /home/supranimbus12/.rvm/gems/ruby-2.2.3@global/gems/rake-12.3.0/exe/rake:27:in <top (required)>'
(See full trace by running task with --trace)

@everaldo
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everaldo commented Jan 26, 2018

@dhinakaranS, you have to prepend your command with bundle exec:

bundle exec rake db:create

@prestidigitation
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prestidigitation commented Feb 9, 2021

Thank you for the useful guide!

Just to let you know, I think that the Rails PostgreSQL template URL has been changed to https://github.com/rails/rails/blob/main/railties/lib/rails/generators/rails/app/templates/config/databases/postgresql.yml.tt

@stevebatcup
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stevebatcup commented Jun 8, 2021 •
edited
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ALTER SCHEMA 'destination_database' RENAME TO 'public'

This was the missing trick for me. Thanks so much.

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