What is the difference between ActiveResource and ActiveModel? - ruby-on-rails-3

Question

As a preface to this question: I am brand new to Rails development (and web-development in general), and some of my concerns may be unfounded, so any feedback would be very helpful.
I am attempting to make a Rails application that plugs into a RESTful API. I have been trying to determine the best way to go about this, and from what I understand, its narrowed down to making my own model from the ground up, utilizing ActiveModel, or utilizing ActiveResource.
It is unclear to me the advantages/disadvantages of each, and to be frank, I do not yet fully understand the difference between ActiveModel and ActiveResource. Can somebody provide me with insight regarding these three options and what makes the most 'sense' in a ror context? Thanks!
The best answer wouldn't just say "Use ActiveModel", or "Use ActiveResource" with instructions on doing so, however that would be helpful as well. I would really appreciate an answer explaining why I should use that thing, etc.
A few constraints I am dealing with are that I need to be using a key when I call the API, and a good number of the API calls will contain additional parameters.

Answer 1

So the key to choosing which package to use here is whether:
You are RETRIEVING data from the web API and want to
store/manipulate on YOUR server, or
You are MANIPULATING data via the web API and don't intend on storing anything on your server.
If #1, you'll need ActiveRecord, as it's Rails' package for manipulation and storage of data on your Postgres/MySQL/etc database.
If #2, you can use ActiveResource exclusively, which will let you retrieve data from the web API, work with it at runtime, and then make changes by posting back to the web API.
Many applications though, will often use both of these packages. ActiveResource to grab data very easily, and then applying it to ActiveRecord models (like User, or Location, etc) which you can use locally without having to grab data from the API over and over again.
To give you an example, for a service I was working on I grabbed Geolocation data from a public source (looking up coordinates for zipcodes), I then saved that data to local Location objects using ActiveRecord so I could look them up repeatedly without the delay of the Web API call. (if you're smart, you'll refresh this data from the web API from time to time)
Determining whether ActiveResource will work for you
Do the service requests conform to the documentation protocol? Look at the Expects a response of block in the Find method, for example. If so, you might be good to go without extra work.
Note: The documentation is a little out of sync with the changelog — as of Rails 3.1:
The default format has been changed to JSON for all requests. If you want to continue to use XML you will need to set self.format = :xml in the class. eg.
Also, ActiveResource has been removed entirely from the Rails 4.0 branch, so if you're looking forward to starting a new rails application and want the latest and greatest, this isn't an option at all — but all hope is not lost, there are plenty of gems that make interacting with RESTful interfaces simpler, like Faraday (full disclosure: I have not used faraday myself so can really comment on its efficacy, but I located it here, and there are a number of other options.
Note (from the same link above): Active Resource is built on a standard XML format for requesting and submitting resources over HTTP. It mirrors the RESTful routing built into Action Controller but will also work with any other REST service that properly implements the protocol. REST uses HTTP, but unlike “typical” web applications, it makes use of all the verbs available in the HTTP specification.
If the answer to the above is no (it does not conform) you'll need to write a wrapper class, see Facebooker for an example of how this is done in an actively maintained gem.
References:
Great tutorial on ActiveResource
Getting started with ActiveRecord
Keep in mind if you're just starting web development, you'll need to understand database and model basics too — you have your work cut out for you. :)

Answer 2

I will give it a try, and hopefully others will come and correct (or add) to it, so a better picture will exist ...
I see the following main differences between the two:
ActiceResource provides an interface to a resource that is (normally) accessible remotely by a (Rails) RESTful API. It is not stored kept locally, but read, updated, created and deleted by the API only. As Ryan Bates states it: "ActiveResource allows you to easily communicate between multiple Rails applications."
ActiveRecord (or nowadays ActiveModel) stores its record (or model) in a database locally, and allows others to access it remotely by a web interface for
Showing pages
Returning JSON maps or
Returning XML structures
Advantages and Disadvantages
To use ActiveResource, you have to check that your local Rails app can speak to the remote application, so that the RESTful API is compatible.
If both options are free to choose, here are some arguments:
ActiveResource will normally more expensive (calling a remote system costs at least more time) than ActiveModel.
ActiveModel will cost your own resources (setup of database at least), but this is normally not a burden.
So at the end, it depends on if you want to store something (ActiveModel) or only retrieve something (ActiveResource), which means you will use ActiveResource and (perhaps) ActiveMiodel.