Liquid syntax validator

application.rb

# config/application.rb
# ...

module AppName
  # ...
  config.autoload_paths << Rails.root.join("validators")
  # ...
end

liquid_validator.rb

# app/validators/liquid_validator.rb
class LiquidValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    Liquid::Template.parse(value)
  rescue Liquid::SyntaxError => e
    record.errors.add(attribute, e.message)
  end
end

message.rb

# app/models/message.rb
class Message < ActiveRecord::Base
  validates :content, liquid: true
end

message_spec.rb

# spec/models/message_spec.rb
require 'rails_helper'

RSpec.describe Message, type: :model do
  describe 'Liquid validation' do
    subject { build :message }
    
    it 'allows valid content' do
      subject.content = 'Hi {{ firstname }}!'
      subject.valid?
      expect(subject).to be_valid
    end

    it 'adds an error for invalid content' do
      subject.content = '{{ firstname, how are you today?'
      subject.valid?
      expect(subject).not_to be_valid
      expect(subject.errors).to have_key(:content)
    end
  end
end