Add ExMachina.with_attrs/1

[paulcsmith]

Recently I had to create a lot of events that all had the same user_id

user_id = 123
create(:event, name: "cancelled", user_id: user_id)
create(:event, name: "started exercise", user_id: user_id)
create(:event, name: "finished exercise", user_id: user_id)

I think it would be nice to have a function that allows you do to something like this

ExMachina.with(user_id: user_id) do
    create(:event, name: "cancelled")
    create(:event, name: "started exercise")
end

I'm not exactly sure how this would work, or if this is the syntax/naming we want, but I think we could come up with something

[jsteiner]

Is the user an Ecto model, such that this would be better treated as a has_many? If not, could you just use a function instead?

[paulcsmith]

No the user is not an Ecto model. It is just an integer in this case. It is working with an external data source. Calling a function on each create would work, but would probably not be any better than what I'm doing now. I think it would be nice to be able to avoid that repetition if possible. I also think it would work well if user was an Ecto model

user = create(:user)
ExMachina.with(user: user) do
  create(:event, name: "whatever")
  create(:event, name: "foo")
end

Maybe you have another idea though? I'd love to use a regular function and not a macro if possible

[henrik]

Sketched out one idea of how this could be done without macros, just for fun. I guess this isn't currying in a strict sense, so it might not be a good idea to keep that name.

defmodule ExMachina do
  def example do
    user_event = curry(:create, :event, user: 123)

    # Instead of…
    create(:event, name: "started", user: 123)
    create(:event, name: "finished", user: 123)

    # Try…
    user_event.(name: "started")
    user_event.(name: "finished")
  end

  def curry(action, model, defaults) do
    fn (attrs) ->
      full_attrs = Keyword.merge(defaults, attrs)
      apply(__MODULE__, action, [model, full_attrs])
    end
  end

  def create(model, attrs) do
    IO.puts "create #{model} with #{inspect attrs}"
  end
end

ExMachina.example

[henrik]

If it's a common use case for a single attribute to vary, one could even do this:

  def example do
    user_event = curry(:create, :event, :name, user: 123)

    user_event.("started")
    user_event.("finished")
  end

  def curry(action, model, attribute_name, defaults) do
    fn (attribute_value) ->
      full_attrs = Keyword.merge(defaults, [{attribute_name, attribute_value}])
      apply(__MODULE__, action, [model, full_attrs])
    end
  end

[henrik]

One minor issue with this solution is that you can't do user_event.(), should you like to, because
anonymous functions can't have variable numbers of arguments.

Then again, user_event.([]) works fine.

[henrik]

For the fun of it, here's a slightly evil version abusing Records: https://gist.github.com/henrik/bff879a97f7df44a8830

[paulcsmith]

@henrik Those are some interesting ideas. I'm not sure if/which option we will go with but this definitely made me think :) I would like to avoid a macro if possible, but also have a nice clean API.

Another option might be to just allow another parameter in create that gets merged into the passed in atrributes, but honestly I'm not sure I like it much :S

attrs = [user_id: 123]
create(:event, attrs, name: "cancelled")

If you have any other ideas feel free to post here them here :)

[paulcsmith]

Something like this might work too, but you'd have to put the . before the parens which is less than ideal :S

w = ExMachina.with(MyApp.Factory, user_id: 123)
w.create.(:event, name: "cancelled)

defmodule ExMachina.with do
  def with(module, default_attrs) do
     default_attrs = Enum.into(%{})
    %{
      create: fn(factroy, attrs) ->
        ExMachina.create(module, Map.merge(default_attrs, attrs))
      end),
      build: fn(factory, attrs) -> ExMachina.build...
    }
  end
end

[henrik]

On 13 Oct 2015, at 18:41, Paul Smith notifications@github.com wrote:
Another option might be to just allow another parameter in create that gets merged into the passed in atrributes, but honestly I'm not sure I like it much :S

attrs = [user_id: 123]
create(:event, attrs, name: "cancelled")
Oh yeah, I considered that also but forgot to mention it. I think it's got a few things going for it. It's super simple, for one. It's also a bit reminiscent of FactoryGirl traits.

[henrik]

Another simple one:

with_user = [user: 123]
ExMachina.create(:event, with_user ++ [name: "started"])

Don't like how you need to add in the square brackets, though. And it creates duplicate keys if they're in both lists, which I'm not sure how ExMachina handles.

[paulcsmith]

I believe we could do something very similar to what's done with Ecto migrations. See Henrik's example here: https://gist.github.com/henrik/25516815e6680e1c7a82

I believe this would allow us to have the syntax:

MyApp.Factory.with_attrs(user_id: 1) do
  create(:event)
end

The problem with this is that I don't think you could pipe other functions because I don't think they would be available inside this scope. I'd have to try it out

[paulcsmith]

I'm going to close this and revisit it later. Right now it's not a priority right now.