Allow setting first-time-slug

[Tuxified]

Hi,

Thanks for this package, it has been helpful for us. For our use case we found one issue, this PR is meant as a starting point for discussion.

In our application we allow users to set the slug. This works fine for content that already exists (is persisted to DB), but when creating new content the slug generator will override the given slug resulting in duplicates.

If you want to, I can also add tests to demonstrate what it fixes?

Best,
Tonći ( @Tuxified )

[sobolevn]

If you want to, I can also add tests to demonstrate what it fixes?

Yes, please

[coveralls]

Pull Request Test Coverage Report for Build 526a9920e83446ab385ed7c91fd6944c1cd21449-PR-283

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-2.3%) to 93.182%

---

Files with Coverage Reduction	New Missed Lines	%
lib/ecto_autoslug_field/slug_generator.ex	1	96.55%

Totals
Change from base Build 1d19dd7a46c87f2890ebe7829e49458484e7bfe1:	-2.3%
Covered Lines:	41
Relevant Lines:	44

---

💛 - Coveralls

[Tuxified]

Hi @sobolevn,

I've run into a little trouble with the current test setup. I feel the test cases are not cleanly separated at the moment.
I hope I can clarify what I mean:

In the setup block of (for example of SlugGeneratorTest) we setup 1 fixture:

setup do
  {:ok,
    %{
      user: User.changeset(%User{}, @valid_attrs),
      opts: [
        to: :simple_slug,
        slug_builder: &build_slug/2
      ]
    }}
end

and then we have a unit test to test 1 function (SlugGenerator.maybe_generate_slug/3)

test "maybe_generate_slug with multiple sources", fixture do
  changeset =
    maybe_generate_slug(fixture.user, [:name, :company], fixture.opts)

  assert changeset.changes.simple_slug == "nikita-sobolev-wemake-services"
end

However, the User.changeset/2 function in our setup block calls the function under test (SlugGenerator.maybe_generate_slug/3) several times already, tainting our test case. In my opinion the unit test doesn't describe the correct behaviour as the slugs are already generated by the time our unit test runs.

WDYT ? If you agree with me, is it ok if I propose a change in a different PR?

[sobolevn]

I have a very little memory of this code base (I wrote it 8 years ago), so, please feel free to improve it :)