Simple Elixir State Machine for Validation

At some point, we will have an entity which the status can be transitioned to many possibilities and we want to have a rule to enforce an integrity of the data.

For example, this is a human emotion state change diagram.

And some transitions are okay, while some are not.

# 👌 OK
iex> update(%Human{emotion: :hungry}, %{emotion: :full})
{:ok, %Human{emotion: :full}}

# 🙅‍♂️ No, get something to eat first!
iex> update(%Human{emotion: :hungry}, %{emotion: :sleepy})
{:error, "invalid status change"}

🤖 Simple validation with pattern matching

For elixir developer, you can guess the pattern matching is pretty good for solving state change problem like this.

def update(%Human{} = human, attrs) do
  with :ok <- validate_status(human.emotion, attrs.emotion),
  ...
end

def validate_status(current_status, new_status) do
  case {current_status, new_status} do
    # Put all the transition paths as a whitelist
    {:hungry, :full} -> :ok
    {:hungry, :angry} -> :ok
    {:angry, :angry} -> :ok
    {:angry, :full} -> :ok
    {:full, :sleepy} -> :ok

    # return error for the rest
    _ -> {:error, "invalid status change"}
  end
end

⚡️ Can we embed this rules into Ecto changeset?

If we want the rule to be strictly applied to the ecto data schema, we can also build a custom changeset validation for status change as well.

def changeset(human, attrs \\ %{}) do
  human
  |> cast(...)
  |> validate_required(...)
  |> validate_status_change() # <- custom validation
end

def validate_status_change(changeset) do
  # Get current and new field data
  current_status = changeset.data.status
  new_status = get_field(changeset, :status)

  case {current_status, new_status} do
    # do nothing if ok
    {:hungry, :full} -> changeset
    {:hungry, :angry} -> changeset
    {:angry, :angry} -> changeset
    {:angry, :full} -> changeset
    {:full, :sleepy} -> changeset
    {nil, _} -> changeset # any new status is ok

    # add error to ecto changeset errors
    _ -> add_error(changeset, :status, "invalid status change")
  end
end

That's it! Hope this could give you some idea in your next task. Feel free to discuss if you have any comments! 😊