Rails first_or_create ActiveRecord method

问题

What does the first_or_create / first_or_create! method do in Rails?

According to the documentation, the method "has no description"...

回答1:

From the Guides

first_or_create

The first_or_create method checks whether first returns nil or not. If it does return nil, then create is called. This is very powerful when coupled with the where method. Let’s see an example.

Suppose you want to find a client named ‘Andy’, and if there’s none, create one and additionally set his locked attribute to false. You can do so by running:

Client.where(:first_name => 'Andy').first_or_create(:locked => false)
# => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: false, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">

The SQL generated by this method looks like this:

SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1
BEGIN
INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 0, NULL, '2011-08-30 05:22:57')
COMMIT

first_or_create returns either the record that already exists or the new record. In our case, we didn’t already have a client named Andy so the record is created and returned.

first_or_create!

You can also use first_or_create! to raise an exception if the new record is invalid. Validations are not covered on this guide, but let’s assume for a moment that you temporarily add

validates :orders_count, :presence => true

to your Client model. If you try to create a new Client without passing an orders_count, the record will be invalid and an exception will be raised:

Client.where(:first_name => 'Andy').first_or_create!(:locked => false)
# => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank

回答2:

Gets the first record that matches what you have specified or creates one if there are no matches

回答3:

If you check the source, you will see that they are almost identical. The only difference is that the first one calls the "create" method and the other one "create!". This means that the second one will raise an exception, if the creation is not successful.

回答4:

I believe first_or_create should generally be avoided these days. (Although it's still in Rails 6.1.1.) In 4.0 they added find_or_create_by and friends, which are apparently meant to replace the first_or_create methods. first_or_create was once mentioned in the guides, now it's not. And it is no longer documented in the code (since Rails 4.0). It was introduced in Rails 3.2.19.

The reasons are:

• It might seem that first_or_create(attrs) does first(attrs) || create(attrs), where in fact it does first || create(attrs). The proper usage is:

  Model.where(search_attrs).first_or_create(create_attrs)
  

  That is, it might confuse people. first(attrs) || create(attrs) is what find_or_create_by does.

• Also, using first_or_create introduces a scope, that might affect the create callbacks in an unexpected way.

More on it in the changelog (search for first_or_create).

来源：https://stackoverflow.com/questions/23078818/rails-first-or-create-activerecord-method