how to create dependent record automatically

rubyonrails-talk
1

hi 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 errors.

what can be the reason. ??

class AController < ApplicationController

def create     @a = A.new(params[:a])

    respond_to do |:a|       if @a.save ##### NEW CODE - THIS IS WHAT I HAVE ADDED   @b = B.new   @b.a_id = @a.id   @b.save #### NEW CODE END

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

end

vipin

2

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

3

Vipin,

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 => a.id   end end

4

Yeah, it really belongs in the model...

5

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

6

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

7

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

8

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;

after_create(a)   B.create :a_id => 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]   end end

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

G

9

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 say

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 validation.

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