Using Jbuilder outside jbuilder templates. See https://buttercms.com/blog/json-serialization-in-rails-a-complete-guide

jbuilder_serializer.rb

require 'jbuilder'

Post = Struct.new(:id, :title, :content, :comments)
Comment = Struct.new(:id, :author, :body)

class PostWithCommentsSerializer
  attr_reader :object

  def initialize(object)
    @object = object
  end

  def jbuild(*args, &block)
    Jbuilder.new(*args, &block).attributes!
  end

  def to_json
    jbuild do |json|
      json.id object.id
      json.title object.title
      json.content object.content

      json.comments(object.comments) do |comment|
        json.id comment.id
        json.author comment.author
        json.body comment.body
      end
    end
  end
end


post = Post.new('first', 'First Post', 'Welcome to our site', [
  Comment.new(1, 'Marek', 'Great!'),
  Comment.new(2, 'Riki', ':grumpycat:'),
])

puts PostWithCommentsSerializer.new(post).to_json
# {"id"=>"first", "title"=>"First Post", "content"=>"Welcome to our site", "comments"=>[{"id"=>1, "author"=>"Marek", "body"=>"Great!"}, {"id"=>2, "author"=>"Riki", "body"=>":grumpycat:"}]}