select_for_update.rb

# When you use select * for update, other process only could touch the query when it be `committed`

# Query 1
User.transaction do
  User.find(1).groups.joins(:owner).lock.each(&:destroy)
end

User.transaction do
  # If the groups has same owner, it will wait the query 1
  # If not, it will continue
  User.find(2).groups.joins(:owner).lock.each(&:destroy)
end

# -----------------
User.transaction do
  # if the query could not find any record, it will lock whole table
  User.where(credits: 1005).lock.first
  sleep 60
end

# It will hang over there
User.find(1653).update_columns(credits: 1003)