Hashes for Data

Written by Luke Morton on 23rd September 2013

This is my take on using hashes to transfer data between behaviour. You might know hashes as maps or associative arrays.

If you're up for separating data and behaviour as discussed in a previous post then you'll need some kind of container for your data whilst passing it around your application. If you're walking away from the object zoo then you might be missing the monkeys. Hopefully I can assure you the world of separating data from behaviour is a more tidy and efficient one.

Let's work with an example. We'll start with a data source. In this case, a mapper method called #find_one_by_id. When given an ID it returns a user.

Surprise surprise I'm returning a Hash here. Why is this cool? Because it's already independent of the data layer it just came from. It's not a Sequel dataset, or MongoDB cursor. It's a hash representing a record. Sequel and MongoDB even have nice ways of exporting records as hashes. And when you're testing, you can just make up hashes as you go along like my example above!

So let's talk about another layer. Data modelling. That's the transformative step that uses mappers and data to build up a model of data required by the application. The following example uses the mapper defined above and also a user ID to produce a data model.

Who'da thunk it? #to_hash returns a hash! You should also note it takes a hash to begin with. Versatile little things aren't they?

Let's add another tier. A view model.

So we pass in another hash to ProfileViewModel#to_hash. This time with one key, :profile_user which in fact is the hash produced by UserDataModel#to_hash.

At this point you might have your object pirate hat on. You're shouting at me, "Y U NO OBJECT?!"

Well an object has implementation details. They have methods. They can also have side effects.

So then you say, "SEW DOES HASH IN REWBEE!"

Well yes it does. But a Hash is a core object provided by the ruby language. It doesn't belong to a library like ActiveRecord or DataMapper. It is a common data type that has a bunch of useful methods that everyone knows about. All you need to care about are the key names.

So now you're getting smarter. You're saying, "Why can't I extend Hash and add custom behaviour?"

"In ruby we can make objects quack like a Hash."

Well yah. But you still might have side effects. Passing a sequel query from your data mapper into your view model to be executed at some point in the future might error at that point in the future. At least if you transform your data provided by a mapper into a hash in the data model layer you can contain data errors where they are relevant.

You can mock out with ease too. Anywhere. If all your objects do is speak to each other in hashes then they all automatically speak the same language.

It's true we pass mapper objects into the data model. So there are exceptions. However data mappers are directly related to data models. Mappers are used by models to get data to be modelled. In fact because mappers are called inside modellers, their many methods are never exposed to anyone but the modeller. This means your application logic only knows to pass in mapper objects to a single entry point on a data model object. It's brilliant I tell you!

Speak in hashes my friends. Speak with a Hash.

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Feel free to read some more thoughts or go back to the introduction.