Thoughts on gemified apps

[dariocravero]

Hey guys,

A few questions popped up on how to use models inside a gemified app and while drafting an answer to it I think we should review how we've implemented this feature.

Here's the repo holding the example app plus the README that's on the question and that I'm pasting below. Would love to hear your thoughts about it.

Gemified apps in Padrino

This repo intends to answer How to access Padrino model and database in a “standalon” (bin/) script? and How to access a gemified Padrino Apps Model from other gem that requires that App.

The issue

In short, there are two issues of the similar nature, both related to models defined in the gemified app:

• they need to be accessed from another gems/projects;
• they need to be accessed from the gemified app's bin, doing something else other than starting the Padrino server.

  The example

First there's gemified-app. That's a Padrino app that is gemified. It also contains a model called SomeModel that has one field called property.

Then there's access-gemified-app-without-padrino; a ruby script that loads the gemified app to access the model.

Finally, there's another-app which a regular Padrino app that just loads gemified-app to use its model.

Problems with the current Padrino setup

Creating an app with padrino g project gemified-app --orm sequel --gem --tiny will give you the following gemspec:

# -*- encoding: utf-8 -*-
require File.expand_path('../lib/gemified-app/version', __FILE__)

Gem::Specification.new do |gem|
  gem.authors       = ["Darío Javier Cravero"]
  gem.email         = ["dario@uxtemple.com"]
  gem.description   = %q{Padrino gemified app example}
  gem.summary       = %q{Padrino gemified app example}
  gem.homepage      = ""

  gem.files         = `git ls-files`.split($\)
  gem.executables   = gem.files.grep(%r{^bin/}).map{ |f| File.basename(f) }
  gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
  gem.name          = "gemified-app"
  gem.require_paths = ["lib", "app"]
  gem.version       = GemifiedApp::VERSION

  gem.add_dependency 'padrino-core'
end

The key points are gem.require_paths = ["lib", "app"] and gem.add_dependency 'padrino-core'.

gem.require_paths = ["lib", "app"] explains why models/some_model.rb isn't available when we load the gem somewhere else. It simple isn't added to $LOAD_PATH :(.

gem.add_dependency 'padrino-core' hints us that something might be missing later on. What happens with dependencies like the ORM or the renderer? Should we load those? I reckon that it's a matter of what you want to achieve but I'd say that most times yes.

Our gemified app dependencies are still listed in our Gemfile which will only be added in the current scope and not in any gems requiring our gemified-app gem.

A first attempt at solving this

For this to work there are two things we should do:

Add 'models' to gem.require_paths = ["lib", "app"] so that it becomes: gem.require_paths = ["lib", "app", "models"]. That will make sure that anything inside the gemified-app/models directory is included in your gem.

To make it easier to test this, we'll use bundler and in our access-gemified-app-without-padrino test script we'll add a Gemfile that looks like this:

source 'https://rubygems.org'

gem 'gemified-app', path: '../gemified-app'
gem 'pry'

Now in your new app, go to the REPL bundle exec pry and try to require 'gemified-app'. Then try SomeModel.all. It will fail. Why? Because you didn't require 'some_model'.

It will still not work if you do that though. Why? Because none of the model's dependencies, i.e. sequel and sqlite3 (not a direct dependency but it is through the connection) are loaded.

Here you have two choices: you load them manually on your Gemfile or you define them as dependencies on gemified-app.gemspec. I regard the latter one as a better choice since you're already including the model and you're expecting its dependencies to come with it. It would like this:

# gemified-app/gemified-app.gemspec

  # ...

  gem.add_dependency 'padrino-core'
  gem.add_dependency 'padrino-helpers'
  gem.add_dependency 'slim'
  gem.add_dependency 'sqlite3'
  gem.add_dependency 'sequel'
  gem.add_development_dependency 'rake'

  # ...

# gemified-app/Gemfile
source 'https://rubygems.org'

# Distribute your app as a gem
gemspec

You would have to explicitly include all the gems you will need. This may seem cumbersome but in all fairness it gives you a greater understanding of what your app needs. Eventually you will realise you don't even need bundler and the Gemfile :).

Alright, so, go ahead launch your REPL and type require 'gemified-app' and require 'some_model'. Then try SomeModel.all. And... It will fail :(. Why? Because Sequel::Base isn't defined. Now you might be wondering: what happened to the reference to sequel I put in my gemified-app.gemspec? Well, it's just that: a reference and it won't require the gem for you. This won't happen with Padrino either because we're using

require 'rubygems' unless defined?(Gem)
require 'bundler/setup'
Bundler.require(:default, RACK_ENV)

in our config/boot.rb and that only loads required gems on our Gemfile.

So the question is... Should we load that manually? And if so, where?

Well, since this is a gem itself, I believe that the best place to do so would be in lib/gemified-app.rb. Loading all the gems needed will make this file look like:

require 'padrino-core'
require 'padrino-helpers'
require 'slim'
require 'sqlite3'
require 'sequel'

module GemifiedApp
  extend Padrino::Module
  gem! "gemified-app"
end

Alright, so we're all set... Back to the REPL, do your requires

require 'gemified-app'
require 'some_model'

and try SomeModel.all. And... It will fail :(. Again! :/ Why? Because there's no connection to the database. Padrino was loading this for us through config/database.rb.

Another question arises... Should we include config/database.rb in the gem too? The way I see it, we shouldn't. The way I see it, the database connection is something every app should locally define as it may contain specific credentials to access it or stuff like that. Our sample, access-gemified-app-without-padrino/do-somethin.rb script will then look like this:

require 'gemified-app'

Sequel::Model.plugin(:schema)
Sequel::Model.raise_on_save_failure = false # Do not throw exceptions on failure
Sequel::Model.db = Sequel.connect("sqlite:///" + File.expand_path('../../gemified-app/db/gemified_app_development.db', __FILE__), :loggers => [logger])

require 'some_model'

SomeModel.all.each do |model|
  puts %Q[#{model.id}: #{model.property}]
end

Yes, the connection code is pretty much the same than our Padrino app and we're reusing its database for this example.

That was some ride :) but we finally made it. See the sample apps in the repo for some working examples.

require some_model :/

I don't know you but I don't like that at all. Having to do something like that means that I really have to pick my models' names very carefully not to clash with anything I may want to use in the future. I reckon that modules are the answer to it but that's the current state of affairs. See the conclusion for more on this.

An alternative approach

Separate your model layer into its own gem and require it from your (gemified or not) Padrino app. This might probably be the cleanest as you can isolate tests for your models and even create different models for different situations that may or may not use the same database underneath.

It could also encapsulate all of the connection details.

Conclusion

I think we should review Padrino's approach to gemified apps.

Should we use the gemspec instead of the Gemfile for hard dependencies?

Should we namespace the models (I know we had some issues in the past with this)?

Should we teach users to do explicit requires in their gems or to inspect the dependecies and require them for them?

Should we teach our users how to load their dependencies and be more reponsible about it? At the end of the day, if they went the gemified app route they are clearly much more proficient in Ruby and should be aware of this kind of stuff.

[fwolfst]

In every case, some pointers could be given by the README.md created by padrino gen project --gem ..... It explains how to mount the App. It is the place where I would expect to read how to use its model. And thanks again Dario for picking this up.

[namusyaka]

@dariocravero Thank you for bringing this.

Should we use the gemspec instead of the Gemfile for hard dependencies?

Yeah I've also been thinking about the gemspec support.

Should we namespace the models (I know we had some issues in the past with this)?

Yes, I think so too. The models of gemified apps shouldn't contaminate global scope

[dariocravero]

@namusyaka on the models: it was because of some ORMs that we weren't namespacing the models, am I correct?

[namusyaka]

What does "some ORMs" mean? The reason is this?

[dariocravero]

@namusyaka see #1011