liquid_syntax_validator.rb

# Class validates if passed record's attribute value has proper Liquid template syntax.
class LiquidSyntaxValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, template)
    Liquid::Template.parse(template)
  rescue Liquid::SyntaxError
    record.errors.add(attribute, :liquid_syntax_error)
  end

end

liquid_syntax_validator_spec.rb

require 'spec_helper'

describe LiquidSyntaxValidator do

  let(:email_template) { build(:email_template) }
  let(:attribute_to_valid) { :body }
  let(:validator_options) { { attributes: attribute_to_valid } }
  let(:validator) { LiquidSyntaxValidator.new(validator_options) }

  describe "#validate_each" do
    context "and record is valid" do
      before { validator.validate_each(email_template, attribute_to_valid, email_template.body) }

      it("doesn't add errors on :body field") { email_template.errors[:body].should be_blank }
    end

    context "and record isn't valid" do
      before do
        email_template.body = "Bad syntax template {{ username"
        validator.validate_each(email_template, attribute_to_valid, email_template.body)
      end

      it("adds error on :body field") { email_template.errors[:body].should be_present }
    end
  end

end