Maintain branch specific databases and switch on checkout

post-checkout.rb

# frozen_string_literal: true

# rubocop:disable Metrics/LineLength

require 'yaml'
require 'digest'
require 'erb'

return if ENV['NO_DB_SWITCH'] == '1'

# Return if this is not a branch checkout.
return if ARGV[2] != '1'

def drop_existing_connections_to_database(database_name, port: 5432)
  system(%[psql -p #{port} --command="SELECT pg_terminate_backend(pg_stat_activity.pid) FROM pg_stat_activity WHERE pg_stat_activity.datname = '#{database_name}' AND pid <> pg_backend_pid();" postgres > /dev/null])
end

def branches_from_refhead(ref)
  `git show-ref --heads | grep #{ref} | awk '{print $2}'`.split("\n").map do |b|
    b.sub(%r{^refs/heads/}, '')
  end
end

def safe_name(name) = Digest::MD5.hexdigest(name)

# Get the current (destination) branch
dest_branch = `git rev-parse --abbrev-ref HEAD`.strip

# Since we're just given a commit ID referencing the branch head we're coming from,
# it could be at the head of multiple branches. We can assume the source isn't the same as the
# destination branch, so we can remove that immediately.
source_branches = branches_from_refhead(ARGV[0]).reject { |b| b == dest_branch }

project_root = `git rev-parse --show-toplevel`.strip

# Load Rails DB config and grab database name
tpl = ERB.new(File.read("#{project_root}/config/database.yml"))
rails_db_config = YAML.load(tpl.result, aliases: true)
dev_db = rails_db_config['development']['database']
dev_db_port = rails_db_config['development'].fetch('port', 5432)

# Don't do anything if the source and destination branches are the same or nonexistent
return if source_branches.include?(dest_branch) || source_branches.empty? ||
          (source_branches | [dest_branch]).any?('')

puts "Switching development database from #{source_branches.join(', ')} to #{dest_branch}..."

# Drop connections
drop_existing_connections_to_database dev_db, port: dev_db_port

# Copy dev DB to source branches
source_branches.each do |br|
  sbr = safe_name(br)
  print " -- Copying #{dev_db} to #{dev_db}_#{sbr} ..."

  if `psql -p #{dev_db_port} -tAc "SELECT 1 FROM pg_database WHERE datname='#{dev_db}_#{sbr}'"`.strip == '1'
    print ' (dropping existing one first)'
    system %(dropdb -p #{dev_db_port} -f #{dev_db}_#{sbr})
  end

  system "createdb -p #{dev_db_port} -T #{dev_db} #{dev_db}_#{sbr}"
  puts ' DONE'
end

s_dest_branch = safe_name(dest_branch)
if `psql -p #{dev_db_port} -tAc "SELECT 1 FROM pg_database WHERE datname='#{dev_db}_#{s_dest_branch}'"`.strip == '1'
  print " -- Dropping #{dev_db} ..."
  system %(dropdb -p #{dev_db_port} -f #{dev_db})
  puts ' DONE'

  # Rename destination branch DB to dev DB
  print " -- Renaming #{dev_db}_#{s_dest_branch} to #{dev_db} ..."
  system %(psql -p #{dev_db_port} --command="ALTER DATABASE #{dev_db}_#{s_dest_branch} RENAME TO #{dev_db};" postgres > /dev/null)
  puts ' DONE'
end

# rubocop:enable Metrics/LineLength

Just call this as part of a post-checkout git hook, and it will maintain independent copies of your Postgresql database for each branch you switch to.