Rails/ActiveRecord: save changes to a model's associated collections

Question

Do I have to save modifications to individual items in a collection for a model, or is there a method I can call to save them when I save the model.

#save doesn't seem to do it. For example:

irb> rental = #...
#=> #<Rental id: 18737, customer_id: 61, dvd_id: 3252, date_rented: "2008-12-16 05:00:00", date_shipped: "2008-12-16 05:00:00", date_returned: "2008-12-22 05:00:00">
irb> rental.dvd
#=> #<Dvd id: 3252, title: "The Women of Summer", year: 1986, copies: 20, is_new: false, is_discontinued: false, list_price: #<BigDecimal:1a48f0c,'0.1599E2',8(8)>, sale_price: #<BigDecimal:1a48ed0,'0.1599E2',8(8)>>
irb> rental.dvd.copies += 1
#=> 21
irb> rental.save
#=> true
irb> rental.dvd
#=> #<Dvd id: 3252, title: "The Women of Summer", year: 1986, copies: 21, is_new: false, is_discontinued: false, list_price: #<BigDecimal:1a2e9cc,'0.1599E2',8(8)>, sale_price: #<BigDecimal:1a2e97c,'0.1599E2',8(8)>>
irb> Dvd.find_by_title('The Women of Summer')
#=> #<Dvd id: 3252, title: "The Women of Summer", year: 1986, copies: 20, is_new: false, is_discontinued: false, list_price: #<BigDecimal:1a30164,'0.1599E2',8(8)>, sale_price: #<BigDecimal:1a30128,'0.1599E2',8(8)>>

In the above example, the copy of the DVD that the rental has doesn't seem to update the copy in the DB (note the differing number of copies).

Answer 1

You have to do this yourself. Active Record does not cascade save operations in has_many relations after the initial save.

You could automate the process with a before_save callback.

Answer 2

You have to do this yourself

This isnt entirely true. You can use the "build" method which will force a save. For the sake of example assume that you have a Company model and Employees (Company has_many Employees). You could do something like:

acme = Company.new({:name => "Acme, Inc"})
acme.employees.build({:first_name => "John"})
acme.employees.build({:first_name => "Mary"})
acme.employees.build({:first_name => "Sue"})
acme.save

Would create all 4 records, the Company record and the 3 Employee records and the company_id would be pushed down to the Employee object appropriately.

Answer 3

just do a rental.dvd.save after you increment the value or in the above case you could use

rental.dvd.increment!(:copies)

which will also automatically save, note the '!' on increment!