TIL.rb

require 'pp'

class Model
  def initialize(args={})
    set(args)
  end

  def set(attributes={})
    return if attributes.nil? || attributes.empty? || !attributes.is_a?(Hash)
    attributes.each do |k, v|
      setter = "#{k}="
      self.public_send(setter, v) if self.respond_to?(setter)
    end
  end
end

class Foo < Model
  attr_accessor :id, :bar
end

class Qux < Model
  attr_accessor :id, :baz
end

foos = [
  Foo.new(id: 1, bar: 'abc'),
  Foo.new(id: 2, bar: 'def')
]

quxs = [
  Qux.new(id: 9, baz: '123'),
  Qux.new(id: 8, baz: '456')
]

builder = Jbuilder.new do |json|
  json.set!(:data) do
    json.set!(:type, 'jbuilder-tests')
    json.set!(:attributes) do
      json.set!(:title, 'this is a test')
    end
  end

  json.included do
    json.array!(foos) do |foo|
      json.set!(:id, foo.id.to_s)
      json.set!(:bar, foo.bar)
    end

    json.array!(quxs) do |qux|
      json.set!(:id, qux.id.to_s)
      json.set!(:baz, qux.baz.to_s)
    end
  end
end

pp builder.target!


# {
#   "data":{
#     "type":"jbuilder-tests",
#     "attributes":{
#       "title":"this is a test"
#     }
#   },
#   "included":[
#     {"id":"1","bar":"abc"},
#     {"id":"2","bar":"def"},
#     {"id":"9","baz":"123"},
#     {"id":"8","baz":"456"}
# }