Ruby on Rails - Alternatives to STI?

Question

I have many different models (close to 20) that share some common attributes but also differ to some degree in others. STI seems attractive at first, but I have no idea how the various models will evolve over time with rapid product development.

A good parallel to our application that comes to mind is Yelp. How would Yelp manage something in Rails? All of the postings have some common attributes like "address". Yet, they differ quite a lot on others. For example, you have a reservation option for restaurants and maybe not for others. Restaurants also have a ton of other attributes like "Alcohol allowed" that don't apply to others. Doing this with STI will get out of hand pretty quickly.

So whats the next best option? HStore with Postgres? I am not comfortable using HStore for anything but small things. HStore solves some problems while introduces others like lack of data types, lack of referential integrity checks etc. I'd like a solid relational database as the foundation to build upon. So in the Yelp case, probably, a restaurant model is where I am going. I've taken a look at suggestions like here - http://mediumexposure.com/multiple-table-inheritance-active-record/, but I am not happy to do so much monkey patching to get something so common going.

So I am wondering what other alternatives exist (if any) or should I just bite the bullet, grind my teeth and copy those common attributes into the 20 models? I am thinking my problems would come from the migration files rather than the code itself. For example, if I setup my migrations to loop through tables and set those attributes on the tables, then would I have mitigated the extent of the problem with having different models?

Am I overlooking something critical that might cause a ton of problems down the road with a separate models?

Answer 1

I see a few options here:

1. Bite the bullet and create your 20 different models with a lot of the same attributes. It's possible that these models will drift over time - adding new fields to one specific type - and you'll create a 200 column table with STI. Maybe you don't - the future is hard to see, especially with exploratory/agile software.

2. Store non referential fields in a NoSQL (document) database. Use your relational database for parts of the record that are relational (a user has many reviews and a review has one business), but keep the type specific stuff in a NoSQL database. Keep an external_document_id in your Rails models and external_record_id / external_record_type in your NoSQL document schema so you can still query all bars that allow smoking using whatever NoSQL ORM you end up using.

3. Create an Attributes model. An attribute belongs_to :parent_object, polymorphic: true with a key and value field. With this approach you might have a base Business model and each business can has_many :attributes. Certain (non-relational?) attributes of the business (allows_smoking) are one Attribute record. An Attribute's key could be a string or could be a numeral you have Ruby constants for. You're essentially using the Attribute entities to create a SQL version of option #2. It might be a good option, and I've used this myself for User or Profile models. (Although there are some performance hits to be aware of with this approach).

I'd really worry about having that many (independent) models for something that sounds subclass-ey. It's possible you might be able to DRY up common behavior/methods by using Concerns (syntactic sugar over the mixin concept, see an awesome SO answer on concerns in Rails 4). You still have your (initial) migration problem, of course.

Answer 2

Adding another option here: Serialized LOB (272). ActiveRecord allows you to do this to an object using serialize:

class User < ActiveRecord::Base serialize :preferences end

user = User.create(preferences: { "background" => "black", "display" => large }) User.find(user.id).preferences # => { "background" => "black", "display" => large }

(Example code from ActiveRecord::Base docs.)

The important consequence to understand is that attributes stored in a Serialized LOB will not be indexable and certainly not searchable in any performant manner. If you later discover that a column needs to be available as an index you'll have to write [most likely] a Ruby program to perform the transformation (though by default serialization is in Yaml so any Yaml parser will suffice).

The advantage is that you don't have to make any technology changes to your stack in order to apply this pattern. Its easy to moderate - based on the amount of data you have collected - to migrate away from this pattern.