post-checkout git hook to switch Rails DB along with the branch

post-checkout.rb

#!/usr/bin/env ruby

require 'yaml'

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

def drop_existing_connections_to_database(database_name)
  system(%[psql --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

# 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
rails_db_config = YAML.load_file("#{project_root}/config/database.yml")
dev_db = rails_db_config['development']['database']

# 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

# Copy dev DB to source branches
source_branches.each do |br|
  print " -- Copying #{dev_db} to #{dev_db}__#{br} ..."

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

  system "createdb -T #{dev_db} #{dev_db}__#{br}"
  puts ' DONE'
end

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

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