accepts_nested_attributes_for is Creating New Records; Gotcha!

accepts_nested_attributes_for is a really powerful method in Rails because it allows a model to alter related models through itself. However, it has a pretty big gotcha.

An example using accepts_nested_attributes_for is where a user model which belongs_to an alias model, i.e. #user.rb class User < ActiveRecord::Base end``#alias.rb class Alias < ActiveRecord::Base belongs_to :user accepts_nested_attributes_for :user end

This allows the Alias model to change the user by passing a hash key user_attributes i.e. `Alias.first.user.name

“Alice”
Alias.first.update_attributes(
{
:user_attributes => {
:id => 1,
:name => “Bob”
}
})``Alias.first.user.name
“Bob”`

The gotcha exists if you do not pass the :id symbol in the attributes hash, i.e. `Alias.first.user_id

1
Alias.first.update_attributes(
{
:user_attributes => {
:name => “Bob”
}
})``Alias.first.user_id
2`

This behaviour is documented, it is just not what I would have expected.

To “fix” this (if you do not want to pass the id every time) you can set the :update_only flag to true, i.e. #alias.rb class Alias < ActiveRecord::Base belongs_to :user accepts_nested_attributes_for :user, :update_only => true end