# An object that initializes multiple objects?

**URL:** <https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235>\
**Category:** Rails\
**Created:** [January 18, 2006, 1:22am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235 "2006-01-18T01:22:43Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 1:22am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/1 "2006-01-18T01:22:43Z")

</div>

Hi,

Something that I have noticed is that every class that inherits from  
ActiveRecord::Base has a database table of its own.

However what if I don’t want a class to have its own table, but to  
actually serve as a proxy to initialize other entries?

For instance as an example, let’s have a form with the fields for  
“student name”, “student id number”, “parent’s name”, “street 1”,  
“street 2”, “town”, “postcode”, “state”.

There will be (suppose) 2 underlying database tables that will be  
populated as a result of this form submission, a table “student\_record”  
that has “id”, “name”, and “id\_number” (not to be confused with “id”, it  
is more like “passport number”), as well as another table “contact” with  
the fields “parent\_name”, “street\_1”, “street\_2”, “town”, “postcode”,  
“state” as well as “student\_record\_id” which is a foreign key pointing  
to the id of the associated “student\_record”.

Now, strictly speaking it can be argued that all of these entries can be  
put into a single table for simplicity’s sake, but supposing there are  
students with multiple parental contacts (perhaps separated, or they  
have holiday addresses, etc) and the system is meant to accomodate them,  
hence the provision for multiple contacts.

How is it possible to have a “front end” class that acts and works just  
like an object by itself (ie, maybe a “student” class), but actually  
have no underlying database table of its own and instead exists by  
pulling records from associated databases?

---

<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 1:48am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/2 "2006-01-18T01:48:41Z")

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I’m not sure if this matches what you need, but this is the first thing  
that  
popped in my head. So this means it’s probably more complicated than it  
needs to be 🙂

In the student model use:

class Student \< ActiveRecord::Base  
#this should handle the student\_id reference as long as  
#the contact table has a student\_id field…  
#but you can change this default association  
#[http://api.rubyonrails.com/classes/ActiveRecord/Associations/ClassMethods.html#M000471](http://api.rubyonrails.com/classes/ActiveRecord/Associations/ClassMethods.html#M000471)  
has\_many :contact

…rest of model code  
end

In the controller you can use:

def edit  
@student = Student.find(@params[:student\_id])  
end

In the view you can use something like:  
\<%=text\_field\_tag(“student\_name”,@student.name)%\>

loop through the contacts:

\<%  
@student.contact.each{|contact|  
%\>  
\<%=text\_field\_tag(“street\_#{contact.id}”,contact.street\_1)%\>  
\<%  
}  
%\>

using the #{contact\_id} to differentiate between the params.

any help at all?

-andy

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<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 2:07am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/3 "2006-01-18T02:07:34Z")

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Thanks, that’s what I suspected too.

Initially I thought rails has a way of doing it so I can get something  
for free, guess not ☹

Just to clear up something, so in the contact class there’d be a  
statement belongs\_to :student, and when I add a new contact to a student  
I use the statement student.contact \<\< Contact.new(:street1 =\> "etc  
etc…)

Will I need to specify a :Student =\> student when creating the new  
contact? Or will rails do that for free?

---

<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 2:18am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/4 "2006-01-18T02:18:50Z")

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> I use the statement student.contact \<\< Contact.new(:street1 =\> "etc  
> etc…)

I believe that using the above statement will handle student association  
in  
the new contact for you. I know this works for has\_and\_belongs\_to\_many,  
just haven’t had the chance to use has\_many yet.

-andy

---

<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 3:31pm UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/5 "2006-01-18T15:31:59Z")

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> The trouble with using \<\< is it seems like error messages that results  
> from a child object failing to save does not seem to show up to the  
> parent class. Is there a way to actually communicate the validation  
> error messages between the different .save methods?

I haven’t ran into this scenario, so I don’t have a well thought out  
solution. Unless there’s a mechanism for this in Rails already, I would  
think you would have to test for error messages after each save of the  
child  
and then add a message to the parent. There is probably a slick way to  
do  
this, but I don’t know it. ☹

-andy

---

<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 10:40pm UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/6 "2006-01-18T22:40:46Z")

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Andrew S. wrote:

> I haven’t ran into this scenario, so I don’t have a well thought out  
> solution. Unless there’s a mechanism for this in Rails already, I would  
> think you would have to test for error messages after each save of the  
> child  
> and then add a message to the parent. There is probably a slick way to  
> do  
> this, but I don’t know it. ☹
> 
> -andy

Hmmm, but the flaw with that approach is that if you are calling .save  
on a child object created via the append operator to a newly created  
parent, it can only be saved when the parent is saved (the parent.id is  
not present until it actually gets into the database).

Wonder if there is some way to override the default save behaviour?

---

<div class="post-metadata">

**Author:** ![generik](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/4da419/32.png) [@generik](https://www.ruby-forum.com/u/generik)\
**Post date:** [January 18, 2006, 8:09am UTC](https://www.ruby-forum.com/t/an-object-that-initializes-multiple-objects/62235/7 "2006-01-18T08:09:00Z")

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Andrew S. wrote:

> I believe that using the above statement will handle student association  
> in  
> the new contact for you. I know this works for has\_and\_belongs\_to\_many,  
> just haven’t had the chance to use has\_many yet.
> 
> -andy

Thanks Andrew, it does work 🙂

However one issue that I have been trying to smooth out which is also  
discussed at [http://www.ruby-forum.com/topic/51957](http://www.ruby-forum.com/topic/51957), let’s extend on our  
example again and now this Contact class validates input postcodes by  
matching them with the correct town and state names.

The trouble with using \<\< is it seems like error messages that results  
from a child object failing to save does not seem to show up to the  
parent class. Is there a way to actually communicate the validation  
error messages between the different .save methods?