# Realising a kind of virtual methods

**URL:** <https://www.ruby-forum.com/t/realising-a-kind-of-virtual-methods/160761>\
**Category:** Rails\
**Created:** [February 19, 2009, 1:08pm UTC](https://www.ruby-forum.com/t/realising-a-kind-of-virtual-methods/160761 "2009-02-19T13:08:17Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![msmsms](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/34f0e0/32.png) [@msmsms](https://www.ruby-forum.com/u/msmsms)\
**Post date:** [February 19, 2009, 1:08pm UTC](https://www.ruby-forum.com/t/realising-a-kind-of-virtual-methods/160761/1 "2009-02-19T13:08:17Z")

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Hey,

I want to realise some kind of virtual methods. I’ll explain this with  
an example:

Let’s assume I’ve got a class called “Animal”. Every “Animal” instance  
can have various attributes, which are not fix, they depend on the  
annotations associated within the “animal” database table. So I could  
create a method called “getAttribute()”:

animal = Animal.new()  
animal.getAttribute(“WEIGHT”)

If there’s an annotation called “WEIGHT”, the weight is returned,  
otherwise “nil”. The thing is: I don’t want to write “getAttribute  
(XYZ)” each time to get the data. I want to write:

animal.weight

But there is no predefined accessor method called “weight”. Some how,  
I must catch this. I’ve got two ideas, of which I’m not sure whether  
they make sense and can be implemented:

1. The accessor methods are created dynamically at runtime, so that  
they internally access the annotation data and return what they’ve  
found or “nil”.

2. A kind of “catch-all”-method which takes the name of the methode,  
which we try to call, as a parameter. Some pseudo-code:

catchAll(nameOfDesiredMethod)  
FindAnnotation(nameOfDesiredMethod) // returning attribute value or  
nil  
end

I don’t know, if there’s a known technique for this kind of problem.  
But I know, that there’s something similar, if you look at the  
ActiveRecord find() method. There you can say find\_by\_attribute, for  
example.

It’s hard to explain, I did my best.

Thank you for your suggestions!

Best wishes,  
ms

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<div class="post-metadata">

**Author:** ![msmsms](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/34f0e0/32.png) [@msmsms](https://www.ruby-forum.com/u/msmsms)\
**Post date:** [February 19, 2009, 1:17pm UTC](https://www.ruby-forum.com/t/realising-a-kind-of-virtual-methods/160761/2 "2009-02-19T13:17:06Z")

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> 1. A kind of “catch-all”-method which takes the name of the methode,  
> example.  
> ActiveRecord does exactly this for database attributes you might find  
> nosing around in there is useful. The magic method you are looking for  
> is method\_missing.

Fred

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**Author:** ![msmsms](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/34f0e0/32.png) [@msmsms](https://www.ruby-forum.com/u/msmsms)\
**Post date:** [February 20, 2009, 3:48am UTC](https://www.ruby-forum.com/t/realising-a-kind-of-virtual-methods/160761/3 "2009-02-20T03:48:30Z")

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On Feb 19, 7:07 am, ms [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> animal = Animal.new()  
> they make sense and can be implemented:

As Fred pointed out, method\_missing is what you’re thinking of; it’s  
what  
AR uses to handle dynamic finders (find\_by\_x).

That being said, however, if you’re referring to the fields in Ruby  
code and  
doing so often enough that a shorthand is good - you should really  
just have  
a DB field for it and be done.

Otherwise, you end up with a table like in this article:

> **[tblStaffDirectory](https://thedailywtf.com/articles/tblStaffDirectory)**
>
> Originally Posted At: http://weblogs.asp.net/alex\_papadimoulis/archive/2004/05/20/136018.aspx
> 
> 
> I suspect this one may be a bit less common than tblState ...

…which isn’t so good.

Finally, if you’re really just interested in attaching a few  
“annotation”-style fields  
to a model, take a look at serialized columns. Big warning: you gain  
flexibility, but  
at the cost of losing the ability to query values in SQL. I’ve used  
serialized columns,  
for example, to hold some fields that were imported by users from CSV  
files and  
then exported with the regular data fields to other CSV files. In that  
case, my code  
didn’t care _what_ the fields were, it just needed to store them.

–Matt J.