Allow more control over factory definitions

[germsvel]

Closes: #309, #272, #271

Description

Currently build(:user) will grab the struct/map defined in user_factory/0 and merge any attributes passed into it via struct!/2 or Map.merge/2.

It would be nice to allow users to have more control of factories if they want to, while maintaining the ease of use for the majority of cases. It seems this can be done by allowing users to change the user_factory/0 to a factory that takes in the attributes being passed in.

We can do so by checking if they have defined a factory with /1 arity (meaning they allow parameters to be passed).

Example

Let's take a look at a scenario where this might be wanted:

def user_factory do
  name = "John"
  email = sequence(:email, &"#{name}-#{&1}@foo.com")

  %User{
    name: name,
    email: email,
    age: 20
  }
end

build(:user)
# => %User{name: "John", email: "[email protected]", age: 20}

build(:user, name: "James")
# => %User{name: "James", email: "[email protected]", age: 20}

As we can see the issue is that since we derive the email from the name, the email has the old name in it, even when we pass "James" as the name. It would be nice to be able to get the name from the attributes before we create the email.

So we could define this instead,

def user_factory(attrs) do
  name = Map.get(attrs, :name, "John")
  email = sequence(:email, &"#{name}-#{&1}@foo.com")

  user = %User{
    name: name,
    email: email,
    age: 20
  }

  merge_attributes(user, attrs)
end

build(:user)
# => %User{name: "John", email: "[email protected]", age: 20}

build(:user, name: "James", age: 30)
# => %User{name: "James", email: "[email protected]", age: 30}

Note the inclusion of merge_attributes/2 in our example above. We need that so that age gets properly overridden from 20 to 30.

The merge_attributes/2 function is a new function we expose to make it easier for users to merge attributes regardless of whether it is a struct or a map. It essentially delegates to struct!/2 or Map.merge/2.

Non-map factories

Because it provides full control over the factory, we can also now define non-map factories.

def room_number_factory(_) do
  sequence(:room_number, &"50#{&1}")
end

build(:room_number)
# => "500"

[sheharyarn]

🎉🎉🎉

[composerinteralia]

I don't know Elixir that well, but this seems like a reasonable approach and the code seems fairly easy to follow. 👍