how to create dependent record automatically

I am facing a problem. I have two models A and B which have relations
as follows

A has_one B
B belongs_to A

Now I want to ensure that whenever an 'A' record is created,
automatically a 'B' record is also created.
I was trying to achieve this by doing the following but it is not
making the entry in B table, otherwise code runs fine without any

what can be the reason. ??

class AController < ApplicationController

def create
    @a =[:a])

    respond_to do |:a|
  @b =
  @b.a_id =

        flash[:notice] = 'A was successfully created.'
        format.html { redirect_to(@a) }
        format.xml { render :xml => @a, :status
=> :created, :location => @a }
        formatat.html { render :action => "new" }
        format.xml { render :xml => @a.errors, :status
=> :unprocessable_entity }



I wonder if is returning false? Maybe a validation issue?
Have you tried throwing a debugger statement in there?


wouldn't it be better moving this out of the controller altogether?

If every A should have a B then I'd create an observer - "AObserver"

and add an after_create callback:

class AObserver < ActiveRecord::Observer

  def after_create(a)
    B.create! :a_id =>

Yeah, it really belongs in the model...

yea it turned out to be exactly the 'save' failure due to validation.

I would go with the observer so you can keep the logic in the model
(observer)...skinny controllers, fat models and all.

but won't t also have the issue of validation.

yes and no

after_create is called after A has been saved, so it will be valid.

You'll have to make sure that the params for B are valid though

By calling create! instead of create, rails will raise an exception
if B is not valid but you could also write;

  B.create :a_id =>
  if a.b # => tests if a now has a valid B
    return true
  else # if it could not create B
    [do something here]

Perhaps if you give us a little more information on A and B, their
attributes etc., I can offer a little more advice?


Perhaps if you give us a little more information on A and B, their
attributes etc., I can offer a little more advice?

Well, what I am trying to achieve here is that no way two records are
created in B such that they belong to same record in A. For clear
explanation let me try to map the problem in more realistic way. Let's

A => Student {roll_no}
B => Marksheet ( maths, science, sst, student_id )

Now every student has only one marksheet. I want to make sure that for
every student only one mark sheet is created. For this I have
maintained a relationship of 1:1 between STudent:Marksheet. Marksheet
blongs_to Student and Student has_one marksheet.

Marksheet also has a field student_id. And
<<<<validates_uniqueness_of :student_id >>.>>will ensure that no two
records are created in a normal situation.

But I am concerned of a rare possibility when through two different
sessions same user is trying to create two marksheets.
Based on my understanding in such cases, this validation might allow
to create two records with the same "stident_id" if in both the
sessions database is fetched when no record was saved.

My approach was to not even to let the user create the record in
Marksheet/ As soon as a Student record is created, I add the
Marksheet record as well. Now I can create and save marksheet record
in normal case but if Marksheet record has validations for NIL fields
( to be left NIL initially for subject fields ) will fail the

Is it the situation where "locks" are used ??