How to inherit from another Contract

Support dry-validation
1

Hi, I tried to find how to extend a contract using inheritance to add a few more fields on top of the parent contract.

Say I have a send_email API endpoint (it’s a contrived example, so there can be some details that may look wrong), and the following contract for that

class SendEmailContract < Dry::Validation::Contract
  params do
    required(:from).filled(:string)
    required(:to).filled(:string)
    required(:text).filled(:string)
  end
end

Then I want to support html email, and want a :html parameter. I want to do something like

class SendHTMLEmailContract < SendEmailContract
  params do
    required(:html).filled(:string)
  end  
end

instead of defining it afresh.

I’ve looked at all the related posts, but a lot of them were for version 0.x, and even in the documentation, I could only find ones involving inheriting a dry-schema from another dry-schema. I want to avoid that because it looks like I have to define both SendEmailSchema and SendEmailContract (which uses the schema to validate), and SendHTMLEmailSchema and SendHTMLEmailContract. Basically two things for every contract.

Is there a support for inheritance for contracts?

2

This is not yet supported. We’d like to add it in 2.0.0 though. There was an attempt at implementing it but it wasn’t finished.

3

Thanks! Is there any workaround? Duplicating code for hundreds of contracts will be hard to maintain and prone to error.

4

Code in your example actually works

dry-validation> SendHTMLEmailContract.new.(from: '1', to: '2', text: '3', html: '4')
=> #<Dry::Validation::Result{:html=>"4", :from=>"1", :to=>"2", :text=>"3"} errors={}>

There are still limitations, though. Inheritance works, but there’s no easy way to compose contracts like in the referenced PR.

5

@flash-gordon My example was just a made-up syntax. I’m surprised it works, but is there anything you would change in my example to make it more correct (even though it’s a workaround)?
Also, if I’m just planning on using it with inheritance, will this work well enough despite the limitations?