db:schema:load vs db:migrate with capistrano - mysql

Question

I have a rails app that I'm moving to another server and I figure I should use db:schema:load to create the mysql database because it's recommended. My problem is that I'm using capistrano to deploy and it seems to be defaulting to rake db:migrate instead. Is there a way to change this or is capistrano using db:migrate for a good reason?

Answer 1

Why to use db:schema:load
I find that my own migrations eventually do some shuffling of data (suppose I combine first_name and last_name columns into a full_name column, for instance). As soon as I do any of this, I start using ActiveRecord to sift through database records, and your models eventually make assumptions about certain columns. My "Person" table, for instance, was later given a "position" column by which people are sorted. Earlier migrations now fail to select data, because the "position" column doesn't exist yet.
How to change the default behavior in Capistrano
In conclusion, I believe deploy:cold should use db:schema:load instead of db:migrate. I solved this problem by changing the middle step which Capistrano performs on a cold deploy. For Capistrano v2.5.9, the default task in the library code looks like this.
namespace :deploy do
...
task :cold do
update
migrate # This step performs `rake db:migrate`.
start
end
...
end
I overrode the task in my deploy.rb as follows.
namespace :deploy do
task :cold do # Overriding the default deploy:cold
update
load_schema # My own step, replacing migrations.
start
end
task :load_schema, :roles => :app do
run "cd #{current_path}; rake db:schema:load"
end
end

Answer 2

Climbing up on the shoulders of Andres Jaan Tack, Adam Spiers, and Kamiel Wanrooij, I've built the following task to overwrite deploy:cold.
task :cold do
transaction do
update
setup_db #replacing migrate in original
start
end
end
task :setup_db, :roles => :app do
raise RuntimeError.new('db:setup aborted!') unless Capistrano::CLI.ui.ask("About to `rake db:setup`. Are you sure to wipe the entire database (anything other than 'yes' aborts):") == 'yes'
run "cd #{current_path}; bundle exec rake db:setup RAILS_ENV=#{rails_env}"
end
My enhancements here are...
wrap it in transaction do, so that Capistrano will do a proper rollback after aborting.
doing db:setup instead of db:schema:load, so that if the database doesn't already exist, it will be created before loading the schema.

Answer 3

That's a great answer from Andres Jaan Tack. I just wanted to add a few comments.
Firstly, here's an improved version of Andres' deploy:load_schema task which includes a warning, and more importantly uses bundle exec and RAILS_ENV to ensure that the environment is set up correctly:
namespace :deploy do
desc 'Load DB schema - CAUTION: rewrites database!'
task :load_schema, :roles => :app do
run "cd #{current_path}; bundle exec rake db:schema:load RAILS_ENV=#{rails_env}"
end
end
I have submitted a feature request to have deploy:load_schema implemented in Capistrano. In that request, I noted that the 'db:schema:load vs. db:migrate' debate has already been covered in the Capistrano discussion group, and there was some reluctance to switch the deploy:cold task to using db:schema:load over db:migrate, since if run unintentionally, the former nukes the entire database whereas the latter would probably complain and bail harmlessly. Nevertheless db:schema:load is technically the better approach, so if the risk of accidental data loss could be mitigated, it would be worth switching.

Answer 4

In Capistrano 3 / Rails 4, the default deploy syntax has changed. You can do this instead:
desc 'Deploy app for first time'
task :cold do
invoke 'deploy:starting'
invoke 'deploy:started'
invoke 'deploy:updating'
invoke 'bundler:install'
invoke 'deploy:db_setup' # This replaces deploy:migrations
invoke 'deploy:compile_assets'
invoke 'deploy:normalize_assets'
invoke 'deploy:publishing'
invoke 'deploy:published'
invoke 'deploy:finishing'
invoke 'deploy:finished'
end
desc 'Setup database'
task :db_setup do
on roles(:db) do
within release_path do
with rails_env: (fetch(:rails_env) || fetch(:stage)) do
execute :rake, 'db:setup' # This creates the database tables AND seeds
end
end
end
end
If you're cautious of invoking the standard deploy tasks manually in the :cold task (as they may change in upcoming version or if you have a custom deploy task), you can also simply call deploy:db_setup before running deploy.
To perform db:schema:load instead of db:setup, you can simply change the rake task, like so:
desc 'Load DB Schema'
task :db_schema_load do
...
execute :rake, 'db:schema:load'
...
end