Realising a kind of virtual methods

rubyonrails-talk
1

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

2

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.

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

3

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:

...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 Jones