I'm building a mountable engine.
From a basic install, I've moved my existing application into the engine namespace.
The engine starts, but I've getting error messages because there are no tables in a db for the controller to reference.
How do I get my models in a database from the engine?
How can I test this with the provided dummy app?
I looked into dm-migrations, but I've yet to use that with DataMapper, so I'm sort of in clueless, uncharted waters. Giant question marks and all that. I'll poke around, but there is nothing through a search that is useful, right now.
Here is the application I'm playing with:
http://github.com/blueblank/Ticket-Engine
The simple and straight forward answer is to create a generator that copies the models to the main app. That is it and simple, though looking at it in the long run, having the dm models in the engines app/models folder as the only prerequisite for installing models to an application from an engine (or mountable engine) would be fantastic.
Related
We want to add the routes defined by an LDAP authorization application (a pure API application) to our core rails application, under the path /ldap-auth.
We have the option to use any technology we want for the LDAP authorization application -- we could write it with rails-api, Sinatra, even pure rack -- though we lean toward rails for codebase consistency. The key point is that it's a self-contained application, with its own tests and release schedule.
What would be the most idiomatic rails technology for this use case? Reading the guides it seems that a rails Engine or a mounted Rails API app are the natural possibilities.
My questions are:
What factors should guide my choice of one over the other?
The Engine guide states that "engines and applications can be thought of as almost the same thing, just with subtle differences, as you'll see throughout this guide." However, even after reading through the guide, it's still unclear to me why one would choose to use an engine, versus mounting an ordinary rails app. I'd appreciate clarification on this point.
Finally, if we do decide to go with a mounted rails API application, we'd like to keep it in the same repo. Where is the most idiomatic place to the code? Under /lib?
I'm currently upgrading an app to rails 4. I planned to install this app for few persons but they have different needs. I decide to use this upgrading version time to put some of my models in engines. The objective is to spend less time tweaking the app for the needs of the situation. The core app will manage the basic resources and the engines will add features (The app is managing membership for small organizations).
I read a lot documentation including the "guides". I tested some engines to see the behaviour of the app. Here are my questions I couldn't answer by my searches:
1 - Naming convention:
How do you name your engines? My first attempt was to name it by their function but when I generated my first model I saw that I can't use the function name for my model.
I was think about something like: Coreappname_functionality
for exemple I want to add activities for my members the engine will be named : member_activities
2 - full vs mountable
I read a lot about this subject, lot of people seem to use mountable engines. I try both of them and I think that the full option is really fast to implement (no routing, no namespace isolation that I have to be aware of). But I also understand the risk for the class collisions. If I'm the only one writing code for this app is it a bad habit to use a full engine(it's just a laziness question). Are there others advantages of mountable engine even if I'm not planning to use them in another app?
3 - "if engine exists?"
Inside the core app i'll put the code that all the engines need. For example inside the side bar I want to display the list of the last activities but only if the activities engine is used. The objective is to put all the necessary code inside the core app but use this code depending on the present of engines or not.
During my test time I used:
if defined? Activity
#activities = Activity.all
end
and render it the views something like this:
<% if defined? Activity %>
<h3><%= #activities.first.title %></h3>
<% end %>
It's working well but I'm not sure that it is a good practice. Is there an alternative?
Do you have any advices before I jump in the engine's world?
I prefer posting my questions before instead of posting my errors after the attempt!
For anybody reading that...
I think there is a misunderstanding about what an isolated engine is. In fact it is a bit confusing. Some may think that's choosing between isolated and not isolated is a bit like a matter of preference. But that's not entirely true.
Isolated engine is just that. An isolated engine. It's not suitable for a "module" of your application. It's more like a "subapplication" of you application. The difference is critical. Modules of your application probably will share models, api or some business logic. Still, encapsulation of responsibilities is important, but there will be some connections. For example probably almost all of your modules will be using something like EventDispatcher module. While isolated engine is a whole application on it's own. Although it could use models from the host application it can't use models from different isolated engine (at least without hacks, a lot of pain in 'certain situations' and that's generally a bad idea causing bad design).
So if you try to implement your app modules using isolated engines your probably end up storing all your models in main/host application to be reusable between your - in fact - subapplications. Probably the same with views. And maybe with assets too. And if some business logic have to be shared it probably would end up in main application too. So that's basically defeats the whole purpose. That's why you should use not isolated engines for your modules. While isolated engines are good for a - totally encapsulated - subapplication. For example if you want to have an e-commerce shop alongside with your main application (spree gem does exactly that).
A more enterprise example would be an ERP system. It could have subapplications like: CRM, Resource Management, etc (isolated engines). And those subapplications could have their own modules (not isolated engines).
Just for the completeness of my answer. As a isolated engine i mean that generated using rails plugin new engine_name --mountable, while a not isolated would be generated using rails plugin new engine_name --full.
Disclaimer:
Maybe there are some cases when you would want to go different way (probably when implementing some kind of magic gem doing some magic things), but that's up to you to figure it out. By this answer I just mean it should be applicable in most applications.
In computing terms, an engine is something that is continually producing output based on input. But in Ruby, the term seems a little bit loose. The people who've created Refinery CMS have taken to calling gems that extend the functionality of their system, 'engines'.
Basically, what I want to know is, is Spree, the open source ecommerce cms written in ruby, an engine? Would calling it the 'spree engine' be correct?
As defined by Rails, an Engine is a lot like an application within an application. Spree is one of these, and there are others. Each engine has its own app folder with the same sort of structure you'd see in a top-level application.
You can combine one or more engines together into an application, then add your own functionality on top. That's what makes systems like Spree far more flexible than a fixed-puropose Rails app that you have limited control over.
Things that extend Rails are only truly engines if they are self-contained applications. Many gems add functionality that's much more specific than that, so are better termed "plug-ins" or "modules" depending on the phrasing.
It's actually pretty easy to build your own engine and can be useful for separating and re-using code across several different applications.
I think that there is quite good explanation in guide Getting Started with Engines.
Spree is actually comprised of many engines...
The overcooked version: Engines come kitted with (many of) the guts of a typical rails application, with a few bonuses: namespacing out of the box, generators for easily copying migrations, and the ability to mount it in another rails application.
From http://guides.rubyonrails.org/engines.html
1 What are engines?
Engines can be considered miniature applications that provide functionality to their host applications. A Rails application is actually just a "supercharged" engine, with the Rails::Application class inheriting a lot of its behavior from Rails::Engine.
Try to read this guide: http://guides.rubyonrails.org/engines.html and also this cast: http://railscasts.com/episodes/277-mountable-engines
Just finished watching "Railscast 277" on mountable engines in Rails, what are practical uses of mountable engines?
One of the best examples would be Devise (for authentication) which is an engine (plugin) but its mounted as a shared-engine; you also get namespaced engines and this helps to retain a sense of context.
Another example would be the Active Admin add-on, error notification plugins...and a whole lot more. In terms of 'mounting' from a purely routing point of view, you can see how — with a namespaced engine you can serve a completely isolated rails app from a route from within your app itself.
You should consider reading this as well.
Two major uses:
Reusability. Gems that expose a lot of application functionality are often distributed as engines.
Modularity. Particularly for large applications, there are often parts of the application that don't seem like they want to be mixed in with the main application, yet are too closely related to be extracted to entirely separate applications. Engines can work well for this.
Spree takes this to extremes: the core Spree framework is the main Rails application, and to customize it, you write engines. This means you're never modifying the Spree core directly.
Does Rails 3 have anything akin to slices in Merb, or areas in Django where there is a layer of organization above the controller? I heard at some point that they may allow hosting one rails app within another but I can't find any information on that in the pre-release material. Any hints on how to do something like this?
You're right there doesn't seem to be a lot of official documentation on it yet. But yes, you can have application slices -- they're called engines in Rails. Actually they've been available in Rails since 2.3. Currently, you basically make a plugin that has a complete application structure and set up your routes in there to "mount" your app against a specific URL. In Rails 3, engines are basically first-class objects in the Rails stack. I believe they can still be plugins but you will also be able keep it away from your application in a separate gem and require it in your application much more easily. You should be able to find stuff pretty easily on teh Google, but here's an oft-linked but informative reference: https://gist.github.com/af7e572c2dc973add221