Testing Random Data in Emails with Bamboo

Published on October 16, 2017|Original Publishing|3m read

Testing a scenario where an app sends an email is easy, but how do you test something random in an email, like a password reset token? When we test a function that intentionally returns random data, it’s a little tougher.

In those times, we often tackle the problem by:

  1. Testing behavior and static data, ignoring the dynamic data.
  2. Using a mock to rid the randomized data, and then test everything.

Let’s walk through how to do this with Bamboo.

Bamboo provides test helpers to help you assert behavior and data in your app. A really common email scenario is sending users password reset links. The idea behind these reset links is that they’re secure and unique, and we ensure this by generating a random token and signing it with user’s data to make it secure. How do we test this then?

There are two ways!

Use regex to cover the static text and skip dynamic text.

Here we are testing the behavior and static data, ignoring the dynamic data. Bamboo provides assert_email_delivered_with() which accepts a keyword list of parts of the email, and what those parts should match. We can match the email entirely by supplying a string like [subject: "Password reset link for MyApp"], or we can supply a regex, [text: ~r/reset_token=/), and the assertion will check if the regex matches.

Here’s a fuller integration test example:

test "customers can request a password reset link", %{session: session} do
  customer = insert(:customer)
  session =
    session
    |> visit(password_reset_path(MyApp.Endpoint, :new))
    |> fill_in(:password_reset, :email, with: customer.email)
    |> click_on("Send link")

  assert_email_delivered_with(subject: "Password reset link for MyApp")
  assert_email_delivered_with(text_body: ~r/reset_token=/)
  assert_email_delivered_with(html_body: ~r/reset_token=/)
end

Use a mock to rid the random data, and test the whole thing!

Here you can guarantee the behavior and (mocked) data, but it’s a little more setup.

Here’s an example:

# lib/mock_token_generator.ex
defmodule MyApp.MockTokenGenerator do
  @token "123"

  # This should match the interface of the real TokenGenerator
  def generate, do: @token

  # We're going to expose this in the mock so we can get the assertion
  # right
  def token, do: @token
end


# config/config.exs
config :my_app, token_generator: MyApp.TokenGenerator


# config/test.exs
config :my_app, token_generator: MyApp.MockTokenGenerator


# web/controllers/password_reset_controller.ex
defmodule MyApp.PasswordResetController do
  @generator Application.get_env(:my_app, :token_generator)

  def create(conn, params) do
    #...
    # use the @generator.generate function
    # do your email thing
    #...
  end
end


# test/features/password_reset_test.exs
# ...
alias MyApp.MockTokenGenerator

test "customers can request a password reset link", %{session: session} do
  customer = insert(:customer)
  session =
    session
    |> visit(password_reset_path(MyApp.Endpoint, :new))
    |> fill_in(:password_reset, :email, with: customer.email)
    |> click_on("Send link")

  assert_email_delivered_with(subject: "Password reset link for MyApp")
  assert_email_delivered_with(text_body: """
    Here's the entire body of the text email. You might test the entire
    text version of the email, and use regex to test the HTML version

    Here's your password reset link: https://myapp.com/password?reset_token=#{MockTokenGenerator.token}
  """)
  assert_email_delivered_with(html_body: ~r|https://myapp.com/password?reset_token=#{MockTokenGenerator.token}|)
end

See? Bamboo makes it easy. Give it a shot and let us know what you think.