Jurgen Jurgen - 1 year ago 75
Ruby Question

How to extend a model in a Rails engine from another Rails engine?

I think the simplest way to explain this is with a contrived example. I am using Rails 2.3.8 with Ruby 1.8.7 and ActiveRecord to MySQL db.

I have the following (contrived) model in one engine (installed in vendor/plugins directory of main app):

# contrived_app/vendor/plugins/concerts_engine/app/models/
class Concert < ActiveRecord::Base
has_many :artists
belongs_to :venue

And the following (contrived) model in another engine:

# contrived_app/vendor/plugins/tickets_engine/app/models/
class Ticket < ActiveRecord::Base
belongs_to :concert

How can I include/extend

has_many :tickets

in the Concert model?

Also, is the load order important, and if so what happens if the Concert model hasn't been defined/loaded yet?

Is it possible to only include

belongs_to :concert

if Concert been defined?

Answer Source

Turns out I can do the following in the tickets_engine:

# contrived_app/vendor/plugins/tickets_engine/config/initializers/concert_extensions.rb
# or
# contrived_app/config/initializers/concert_extensions.rb
Rails.logger.info "\n~~~ Loading extensions to the Concert model from #{__FILE__}\n"

Concert.class_eval do
  has_many :tickets

Personally, my preferred method is to extend the concert model from the tickets_engine, but load order and dependencies are important. i.e. tickets_engine would need to add a dependency on the concerts_engine in it's gemspec, and concert_engine would need to be loaded before tickets_engine.

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