Trailblazer use case: listing items

readme.md

I've tried to user Trailblazer to fetch some data and render it as json. Here described several problems:

• It is imposible to validate just params, without model.
• Operation#validate returns instance of operation, not value of the block. So I should use intermediate variable to return value from #process method.

Compare:

# returns `Operation`
def process(params)
  validate(params, model) do |f|
    model
  end
end

# returns model
def process(params)
  model
end

This behaviour is very confusing.

• If I return data from #process method could not access operation' errors from else block in controller
• In the code base result of running Operation#run always refered as operation, but it is not Operation, it is whatever returns #process

pagination.rb

module Pagination 
  extend ActiveSupport::Concern
  
  included do 
    contract do 
      property :limit, validates: { numericality: { greater_than_or_equal_to: 0, less_than_or_equal_to: 50 } }
      property :offset, validates: { numericality: { greater_than_or_equal_to: 0 } }
    end
  end
end

purchases.rb

class User < ActiveRecord::Base
  class Purchases < Trailblazer::Operation
    include Pagination
    
    contract do 
      property :ids, type: List
    end
    
    def process(params)
      validate(params) do |f|
        purchases = 
          if f.ids
            f.user.purchases.where(id: f.ids)
          else 
            f.user.purchases
          end
        purchases.paginate(params.slice(:limit, :offset))
      end
    end
  end
end

purchases_controller.rb

class PurchasesController < ActionController::Base
  def index
    run User::Purchases params.merge(user: current_user) do |purchases|
      render json: purchases
    end.else do |operation|
      fail BadRequestError, operation.errors.full_messages
    end
  end
end