Ruby on Rails: Building a child with default values when its parent is created

Question

I have a parent and child model relationship. In the child's migration.rb, the child model's columns each have default values (except the parent_id column).

When I make a new parent object, how can I make it so that a child object is created and saved into its table with the data from the default values along with the parent_id?

I'm thinking that it will have to do with something like an after_create on the parent model, but I'm not sure how to set it up.

Answer 1

Revised: I revised the answer to use before_create and building, not creating, the associated models. The ActiveRecord machinery then takes care of saving the associated models once the parent is saved.

I even tested this code!

# in your Room model...
has_many :doors

before_create :build_main_door

private

def build_main_door
  # Build main door instance. Will use default params. One param (:main) is
  # set explicitly. The foreign key to the owning Room model is set
  doors.build(:main => true)
  true # Always return true in callbacks as the normal 'continue' state
end

####### has_one case:

# in your Room model...
has_one :door
before_create :build_main_door
private
def build_main_door
  # Build main door instance. Will use default params. One param (:main) is
  # set explicitly. The foreign key to the owning Room model is set
  build_door(:main => true)
  true # Always return true in callbacks as the normal 'continue' state
end

Added...

The build method is added by the owning model's machinery by the has_many statement. Since the example uses has_many :doors (model name Door), the build call is doors.build

See the docs for has_many and has_one to see all of the additional methods that are added.

# If the owning model has
has_many :user_infos   # note: use plural form

# then use
user_infos.build(...) # note: use plural form

# If the owning model has
has_one :user_info     # note: use singular form

# then use
build_user_info(...) # note: different form of build is added by has_one since
                     # has_one refers to a single object, not to an 
                     # array-like object (eg user_infos) that can be 
                     # augmented with a build method

Rails 2.x introduced the autosave option for associations. I don't think it applies to the above (I'm using default). Autosave testing results.