ex_machina + stream_data

[ibarchenkov]

Hello ex_machina maintainers.

I've discovered that data for Ecto factories could be generated using StreamData library. Here's an example. The most compelling thing to me in this approach is that I can define one set of functions and use them simultaneously as Ecto factories in unit/controller tests and as generators in property-based testing.
I have a working solution and almost ready to share it with the community.
Please, consider the following questions:

• Do you want me to contribute such functionality to your project?
• If not, do you mind if I borrow some code in params_for* functions?

Thank you.

[germsvel]

Hi @ibarchenkov, thanks for reaching out! I haven't had the chance to use StreamData yet, but property-based testing is something near the top of my list to devote some time into. Given that I'm not familiar with that, I'm not sure I can judge whether what you're talking about would be a good addition to ExMachina or not. So unfortunately I don't think contributing that functionality to the project would be a good idea yet before I can give that some more serious research and thinking.

To your other question,

If not, do you mind if I borrow some code in params_for* functions?

Absolutely don't mind! If you find something useful in the codebase, feel free to copy it.

And maybe once I get more well versed with property-based testing and StreamData, I'll come back here and ask you how it's going!

Either way, thank you for reaching out and asking!

[Munksgaard]

I know it's been a while, but I'm curious about whether this issue has been given any thought? I would love to start using ex_machina, but at the same time I want to keep my stream generators. I feel like the two would be a natural marriage, but I'm likely overlooking some deeper complexity.

Any thoughts?

[doomspork]

Howdy @Munksgaard! This library has been moved from Thoughtbot to the BEAM Community organization so we're getting up to speed on some of these older issues.

Are you looking to contribute a PR to bring the stream_data functionality to ex_machina? If so I'm happy to review and test but perhaps we could first start by discussing how you'd envision it working?

[Munksgaard]

Hi @doomspork, thanks for chiming in!

I'm not actively using ex_machina yet, so I would probably need some pointers on how to do it properly, but with a bit of help I'd be delighted to try bringing stream_data to ex_machina.

First of all, I see that @ibarchenkov has created his own library that kind of goes in the direction I'm thinking: ecto_factory_stream. However, it lacks some of the niceties from ex_machina, but perhaps we can be inspired by some of their ideas.

As for ex_machina, what I'd like to see, is a way to specify the factory in terms of generators. I think my ideal user interface would be something like this:

# When defining factories/generators
def user_generator do
  gen all name <- StreamData.string(),
               ...
               do
    %User{name: name, ...}
  end
end

# In my tests:
# create a user for a specific test
test "something" do
  user = build(:user, name: "John") # Uses the user_generator from above, but just takes the first element
  ...
end

# Add a property test that relies on the user
property "some property for users hold" do
  gen all user <- generate(:user, name: "John") do # Uses the generator from above
    ...
  end
end

# In my priv/repo/seeds.exs:
# Insert some seeds
insert_list(10, :user) # Uses the generator from above

Of course, this is different from how ex_machina works now, where it relies on e.g. a user_factory function. If we want to keep that interface, perhaps the handling of user_factory could be extended to match on whether the return value is a StreamData struct? Then generate(:user) only works if the factory returns a stream data generator.

[Munksgaard]

I've tried my hand at a PR here: #497