Decoupling client code and tests from complex structures

I have written a description of one way to allow complex structures to change without breaking a lot of client code and tests. It describes the use of one package you can add to Mix dependencies, plus two modules you can copy and tweak. I show the beginning of the writeup below. The whole thing is at Stubbing Complex Structures. Around 300 lines, including code samples.

Sometimes, despite what you might want, you end up with a complex structure – sometimes called a “God object” – that’s used in many places in your code. If that client code contains text like user.privileges[:author].read, you have two problems:

1. Any attempt to change the “shape” of the complex structure becomes hard because there are so many places to change. That locks you into complexity because it’s too painful to undo it by, for example, breaking the single structure into several.

2. Tests have to construct sample data. In a dynamically typed language, they don’t have to create a complete God object; they need only supply the fields the code under test actually uses. But, once again, changes to the structure can require a lot of changes to tests.

Here, I’ll show how to avoid such coupling, using the code in MockeryExtras.Getters and MockeryExtras.Given. The emphasis is on both simplifying change and avoiding busywork.

Look in the example directory for working code to adapt.

TL;DR

Make getters

defmodule Example.RunningExample do
  import MockeryExtras.Getters
  
  defstruct [:example, :history]

  getters :example, [
    eens: [], field_checks: %{}
  ]
  getters :example, :metadata, [
    :name, :workflow_name, :repo, :module_under_test
  ]

Use getters - tersely and stubbably - in client code:

defmodule Example.Steps do
  use Example.From

  def assert_valid_changeset(running, which_changeset) do 
    from(running, use: [:name, :workflow_name])                 # <<<<
    from_history(running, changeset: which_changeset)           # <<<<
    # `name = RunningExample.name(running)`, etc. is too wordy

    do_something_with(name, workflow_name, changeset)
  end

The above requires copying and slightly tweaking code in Example.From.

Don’t build structures, stub getters

defmodule Example.StepsTest do
  ... 
  import Example.RunningStubs

  setup do # overridable defaults
    stub [name: :example, workflow_name: :success]
    :ok
  end

  test "..." do 
    stub(field_checks: %{name: "Bossie"}, ...)
    stub_history(inserted_value: ...)
    ...
    assert ...
end

The above requires copying and slightly tweaking code in Example.RunningStubs.