Add webhook templating

app/controllers/admin/webhooks_controller.rb

@@ -71,7 +71,7 @@ def set_webhook
     end
 
     def resource_params
-      params.require(:webhook).permit(:url, events: [])
+      params.require(:webhook).permit(:url, :template, events: [])
     end
   end
 end

app/lib/webhooks/payload_renderer.rb

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+class Webhooks::PayloadRenderer
+  class DocumentTraverser
+    INT_REGEX = /[0-9]+/
+
+    def initialize(document)
+      @document = document.with_indifferent_access
+    end
+
+    def get(path)
+      value  = @document.dig(*parse_path(path))
+      string = Oj.dump(value)
+
+      # We want to make sure people can use the variable inside
+      # other strings, so it can't be wrapped in quotes.
+      if value.is_a?(String)
+        string[1...-1]
+      else
+        string
+      end
+    end
+
+    private
+
+    def parse_path(path)
+      path.split('.').filter_map do |segment|
+        if segment.match(INT_REGEX)
+          segment.to_i
+        else
+          segment.presence
+        end
+      end
+    end
+  end
+
+  class TemplateParser < Parslet::Parser
+    rule(:dot) { str('.') }
+    rule(:digit) { match('[0-9]') }
+    rule(:property_name) { match('[a-z_]').repeat(1) }
+    rule(:array_index) { digit.repeat(1) }
+    rule(:segment) { (property_name | array_index) }
+    rule(:path) { property_name >> (dot >> segment).repeat }
+    rule(:variable) { (str('}}').absent? >> path).repeat.as(:variable) }
+    rule(:expression) { str('{{') >> variable >> str('}}') }
+    rule(:text) { (str('{{').absent? >> any).repeat(1) }
+    rule(:text_with_expressions) { (text.as(:text) | expression).repeat.as(:text) }
+    root(:text_with_expressions)
+  end
+
+  EXPRESSION_REGEXP = /
+    \{\{
+      [a-z_]+
+      (\.
+        ([a-z_]+|[0-9]+)
+      )*
+    \}\}
+  /iox
+
+  def initialize(json)
+    @document = DocumentTraverser.new(Oj.load(json))
+  end
+
+  def render(template)
+    template.gsub(EXPRESSION_REGEXP) { |match| @document.get(match[2...-2]) }
+  end
+end

app/models/webhook.rb

@@ -11,6 +11,7 @@
 #  enabled    :boolean          default(TRUE), not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
+#  template   :text
 #
 
 class Webhook < ApplicationRecord
@@ -30,6 +31,7 @@ class Webhook < ApplicationRecord
   validates :events, presence: true
 
   validate :validate_events
+  validate :validate_template
 
   before_validation :strip_events
   before_validation :generate_secret
@@ -49,7 +51,18 @@ def disable!
   private
 
   def validate_events
-    errors.add(:events, :invalid) if events.any? { |e| !EVENTS.include?(e) }
+    errors.add(:events, :invalid) if events.any? { |e| EVENTS.exclude?(e) }
+  end
+
+  def validate_template
+    return if template.blank?
+
+    begin
+      parser = Webhooks::PayloadRenderer::TemplateParser.new
+      parser.parse(template)
+    rescue Parslet::ParseFailed
+      errors.add(:template, :invalid)
+    end
   end
 
   def strip_events

app/views/admin/webhooks/_form.html.haml

@@ -7,5 +7,8 @@
   .fields-group
     = f.input :events, collection: Webhook::EVENTS, wrapper: :with_block_label, include_blank: false, as: :check_boxes, collection_wrapper_tag: 'ul', item_wrapper_tag: 'li'
 
+  .fields-group
+    = f.input :template, wrapper: :with_block_label, input_html: { placeholder: '{ "content": "Hello {{object.username}}" }' }
+
   .actions
     = f.button :button, @webhook.new_record? ? t('admin.webhooks.add_new') : t('generic.save_changes'), type: :submit

app/views/admin/webhooks/show.html.haml

@@ -2,14 +2,14 @@
   = t('admin.webhooks.title')
 
 - content_for :heading do
-  %h2
-    %small
-      = fa_icon 'inbox'
-      = t('admin.webhooks.webhook')
-    = @webhook.url
-
-- content_for :heading_actions do
-  = link_to t('admin.webhooks.edit'), edit_admin_webhook_path, class: 'button' if can?(:update, @webhook)
+  .content__heading__row
+    %h2
+      %small
+        = fa_icon 'inbox'
+        = t('admin.webhooks.webhook')
+      = @webhook.url
+    .content__heading__actions
+      = link_to t('admin.webhooks.edit'), edit_admin_webhook_path, class: 'button' if can?(:update, @webhook)
 
 .table-wrapper
   %table.table.horizontal-table

app/workers/webhooks/delivery_worker.rb

@@ -8,7 +8,7 @@ class Webhooks::DeliveryWorker
 
   def perform(webhook_id, body)
     @webhook   = Webhook.find(webhook_id)
-    @body      = body
+    @body      = @webhook.template.blank? ? body : Webhooks::PayloadRenderer.new(body).render(@webhook.template)
     @response  = nil
 
     perform_request

config/locales/simple_form.en.yml

@@ -131,6 +131,7 @@ en:
         position: Higher role decides conflict resolution in certain situations. Certain actions can only be performed on roles with a lower priority
       webhook:
         events: Select events to send
+        template: Compose your own JSON payload using variable interpolation. Leave blank for default JSON.
         url: Where events will be sent to
     labels:
       account:
@@ -304,6 +305,7 @@ en:
         position: Priority
       webhook:
         events: Enabled events
+        template: Payload template
         url: Endpoint URL
     'no': 'No'
     not_recommended: Not recommended

db/migrate/20230129023109_add_template_to_webhooks.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddTemplateToWebhooks < ActiveRecord::Migration[6.1]
+  def change
+    add_column :webhooks, :template, :text
+  end
+end

db/schema.rb

@@ -1136,6 +1136,7 @@
     t.boolean "enabled", default: true, null: false
     t.datetime "created_at", precision: 6, null: false
     t.datetime "updated_at", precision: 6, null: false
+    t.text "template"
     t.index ["url"], name: "index_webhooks_on_url", unique: true
   end
 

spec/lib/webhooks/payload_renderer_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Webhooks::PayloadRenderer do
+  subject(:renderer) { described_class.new(json) }
+
+  let(:event)   { Webhooks::EventPresenter.new(type, object) }
+  let(:payload) { ActiveModelSerializers::SerializableResource.new(event, serializer: REST::Admin::WebhookEventSerializer, scope: nil, scope_name: :current_user).as_json }
+  let(:json)    { Oj.dump(payload) }
+
+  describe '#render' do
+    context 'when event is account.approved' do
+      let(:type)   { 'account.approved' }
+      let(:object) { Fabricate(:account, display_name: 'Foo"') }
+
+      it 'renders event-related variables into template' do
+        expect(renderer.render('foo={{event}}')).to eq 'foo=account.approved'
+      end
+
+      it 'renders event-specific variables into template' do
+        expect(renderer.render('foo={{object.username}}')).to eq "foo=#{object.username}"
+      end
+
+      it 'escapes values for use in JSON' do
+        expect(renderer.render('foo={{object.account.display_name}}')).to eq 'foo=Foo\\"'
+      end
+    end
+  end
+end