Ruby metaprogramming to generate ActiveRecord associations on a given model?

Question

I'm extracting part of my app into a Gem, which involves creating some new ActiveRecord models. At the moment these models look something like this:

Class Wordcount < ActiveRecord::Base
  belongs_to :keyword
  belongs_to :article
end

Class Keyword < ActiveRecord::Base
  has_many :wordcounts
  has_many :articles, :through => :wordcounts
end

The gem will hook into any ActiveRecord model with String or Text fields, not just my defined Article model in the existing Rails app this code comes from.

What do I need to do in order to generate the relevant AR associations based on whichever model the gem's functionality is being applied to? I'm hoping to end up with:

has_my_gem_functionality :on => [:field1, :field2, ...]

And for the rest to be safely encapsulated within the gem.

I'm assuming I'll also have to metaprogram the Migrations as well. I think what I'm trying to do is relatively simple, it's just slightly too far out of my comfort zone.

Can you be more specific on what migrations might be needed for this gem to function? — PinnyM
– PinnyM, Commented Jul 30, 2012 at 19:10
So the migrations would for the most part act indepedently: adding columns to the wordcounts and keywords tables. They will also need foreign keys for the tables they are hooking into, namely :article at present. I'm actually not certain about this, sorry: when you have a belongs_to or a has_many association, which tables (or both?) need foreign keys? — smallsense
– smallsense, Commented Jul 30, 2012 at 19:20

iblue · Accepted Answer · 2012-07-30 19:20:34Z

1

Use the source, luke! :)

Just take a look at the source of some other gems that do similar things. For example:

acts-as-taggable-on (includes a migration)
activerecord-embedding (a very small gem that does some associations)

answered Jul 30, 2012 at 19:20

iblue

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smallsense Over a year ago

Thanks for this. I had a look at a few other gems but just got confused; these examples are a little clearer. So if I understand correctly, I need to do class_eval do ;end, and use that to add methods to the class. I'll look into that method more now.

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