How to skip transaction in ActiveRecord for INSERT ONLY statement?

Question

Look at this example:

2.1.3 :001 > Stat.create!
   (0.1ms)  BEGIN
  SQL (0.3ms)  INSERT INTO `stats` (`created_at`, `updated_at`) VALUES ('2015-03-16 11:20:08', '2015-03-16 11:20:08')
   (0.4ms)  COMMIT
 => #<Stat id: 1, uid: nil, country: nil, city: nil, created_at: "2015-03-16 11:20:08", updated_at: "2015-03-16 11:20:08">

As you can see the create! method execute insert statement inside useless transaction. How to disable transation in this case only (without disabling them in whole application)?

Answer 1

How it works:

The persistence module define create: https://github.com/rails/rails/blob/4-2-stable/activerecord/lib/active_record/persistence.rb#L46

def create!(attributes = nil, &block)
  if attributes.is_a?(Array)
    attributes.collect { |attr| create!(attr, &block) }
  else
    object = new(attributes, &block)
    object.save!
    object
  end
end

It create an object and call #save!

It is not documented in the public api, but calls https://github.com/rails/rails/blob/4-2-stable/activerecord/lib/active_record/transactions.rb#L290

def save!(*) #:nodoc:
  with_transaction_returning_status { super }
end

At this point the transaction wrap the save (super), which is at Persistence module again: https://github.com/rails/rails/blob/4-2-stable/activerecord/lib/active_record/persistence.rb#L141

def save!(*)
  create_or_update || raise(RecordNotSaved.new(nil, self))
end

Let's hack this with some new methods:

module ActiveRecord
  module Persistence
    module ClassMethods

      def atomic_create!(attributes = nil, &block)
        if attributes.is_a?(Array)
          raise "An array of records can't be atomic"
        else
          object = new(attributes, &block)
          object.atomic_save!
          object
        end
      end

    end

    alias_method :atomic_save!, :save!
  end
end

module ActiveRecord
  module Transactions

    def atomic_save!(*)
      super
    end

  end
end

Perhaps you want to use the standard create! method, then you need to redefine it. I define a first optional parameter :atomic, and when it's present means you want to use the atomic_save! method.

module ActiveRecord
  module Persistence
    module ClassMethods

      def create_with_atomic!(first = nil, second = nil, &block)
        attributes, atomic = second == nil ? [first, second] : [second, first]
        if attributes.is_a?(Array)
          create_without_atomic!(attributes, &block)
        else
          object = new(attributes, &block)
          atomic == :atomic ? object.atomic_save! : object.save!
          object
        end
      end
      alias_method_chain :create!, :atomic

    end
  end
end

With this in config/initializers/<any_name>.rb it can work.

How it runs at console:

~/rails/r41example (development) > Product.atomic_create!(name: 'atomic_create')
  SQL (99.4ms)  INSERT INTO "products" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", "2015-03-22 03:50:07.558473"], ["name", "atomic_create"], ["updated_at", "2015-03-22 03:50:07.558473"]]
=> #<Product:0x000000083b1340> {
            :id => 1,
          :name => "atomic_create",
    :created_at => Sun, 22 Mar 2015 03:50:07 UTC +00:00,
    :updated_at => Sun, 22 Mar 2015 03:50:07 UTC +00:00
}
~/rails/r41example (development) > Product.create!(name: 'create with commit')
  (0.1ms)  begin transaction
  SQL (0.1ms)  INSERT INTO "products" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", "2015-03-22 03:50:20.790566"], ["name", "create with commit"], ["updated_at", "2015-03-22 03:50:20.790566"]]
  (109.3ms)  commit transaction
=> #<Product:0x000000082f3138> {
            :id => 2,
          :name => "create with commit",
    :created_at => Sun, 22 Mar 2015 03:50:20 UTC +00:00,
    :updated_at => Sun, 22 Mar 2015 03:50:20 UTC +00:00
}
~/rails/r41example (development) > Product.create!(:atomic, name: 'create! atomic')
  SQL (137.3ms)  INSERT INTO "products" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", "2015-03-22 03:51:03.001423"], ["name", "create! atomic"], ["updated_at", "2015-03-22 03:51:03.001423"]]
=> #<Product:0x000000082a0bb8> {
            :id => 3,
          :name => "create! atomic",
    :created_at => Sun, 22 Mar 2015 03:51:03 UTC +00:00,
    :updated_at => Sun, 22 Mar 2015 03:51:03 UTC +00:00
}

Caveat: You will lose after_rollback and after_commit callbacks!

Note: on 4.1 the methods create! and save! are in module Validations. On Rails 4.2 are in Persistence.

Edit: Perhaps you think you can earn the transaction elapsed time. In my examples the commit time goes to the inserts (I have a standard HD and I think you have an SSD).