How to dynamically add properties to a ROAR representer? - ruby-on-rails

Question

I am using ROAR to implement an API for a rails application. This application deals with tickets that can have attributes like a subject and a description, but also have user defined attributes. For simplicity lets assume a ticket looks like:
class Ticket
attr_accessor :subject, :description
def custom_attributes
# in reality these attributes depend on the current ticket instance
# they are not hard-coded into the class
[['priority', 'high'], ['Operating System', 'Ubuntu']]
end
end
The desired JSON output for such a ticket looks as follows:
{
"subject": "Foo",
"description": "Bar",
"customField1": "high",
"customField2": "Ubuntu"
}
Now you might already see the problem. All properties are immediate children of the root object, this means I can't write that up as representer:
class TicketRepresenter
property :subject
property :description
# Need to iterate over instance members on the class level here...
end
Is there some mechanic that ROAR offers to accomplish that? E.g. a callback that is executed in the context of an actual instance, e.g.
def call_me_on_write
represented.custom_attributes.each do |attribute|
add_property('customField1', attribute[1])
end
end
Is there something like this in ROAR that I have overlooked to accomplish this?
I looked in both the docs for ROAR and the docs for representable, but could not find anything.
Disclaimer
I tried to simplify the actual circumstances to make the question more readable. If you think that important information are missing, please tell me. I will thankfully provide more details.
Out of scope
Please do not discuss whether the chosen JSON format is a good/bad idea, I want to evaluate whether ROAR would support it.

Answer 1

I believe the best approach for the problem would be to use Roar's writer:. It completely turns control over the output to you by passing a handful of values it calls options to a provided lambda.
For example:
property :desired_property_name, writer: -> (represented:, doc:, **) do
doc[:desired_key] = represented.desired_value
end
There are a lot of uses not covered by the github readme but which are documented on the Trailblazer website. This one in particular can be found at http://trailblazer.to/gems/representable/3.0/function-api.html#writer.
Cheers!

Answer 2

I ended up dynamically creating classes from my basic representer:
class TicketRepresenter
property :subject
property :description
def self.create(ticket, context = {})
klass = Class.new(TicketRepresenter) # create a subclass of my representer
ticket.custom_attributes.each do |attribute|
# for each custom attribute in the actual instance insert a property into the created class
property "customField#{attribute.id}".to_sym
getter: -> (*) { attribute.value }
end
# return an instance of the class created above
klass.new(ticket, context)
end
end
Basically that means the actual representer class used to create the JSON is a different one for each Ticket.
If you wanted to read a Ticket back from JSON, it is neccessary to correctly initialize the representer so that the created representer class knows about your custom fields and also define setters.
You will now need to conventionally call the new create method instead of new.
If you need your representer to be created by ROAR (e.g. for a collection), you can use the Polymorphic Object Creation mechanism of ROAR.
Note: The code above does not exactly fit the example of custom attributes posted in my question, but you should get the idea (in the example an attribute did not have members like id and value, but was list consisting of key and value).