Add automatic support for Grape entities

[andyw8]

I'm trying to cache the output generated from a custom entity:

get '/products/:id' do
  cache do
    product = Product.find params[:id]
    present product, with: Entities::Product
  end
end

But this fails with:

no _dump_data is defined for class StringIO

I guess the stream somehow needs to be converted to a String first?

[fancyremarker]

Hi @andyw8, yes, converting to a String would be one way to work around this. At the core of this issue is the fact that ActiveSupport::Cache::Store doesn't support serializing StringIO objects. Off the top of my head, I think the only way we would solve this in Garner would be to detect cache objects of type StringIO, and then:

1. Call read on the object before storing.
2. Annotate the cache object to indicate that it was originally of type StringIO (something we don't do currently).
3. Re-wrap the cached string in a StringIO object on fetching.

I couldn't find much on the topic, but the fact that ActiveSupport::Cache::Store doesn't do this by default leads me to believe it might not be prudent.

An alternative is to only cache the product = Product.find params[:id], and to present the product outside the cache block. But this would incur the cost of re-presenting on each request.

[dblock]

We could also build support for StringIO when defined in Garner proper within the Grape extension. So this issue can be renamed to "support Grape Entity presenters". I think it would be as simple as checking that the type is StringIO and calling to_s on it.

[fancyremarker]

Going to close and mark this one wontfix, since Garner v0.4.0+ defers this sort of decision to the underlying cache/marshaling implementation.

[andoriyu]

So what is the work around this?

[dblock]

@andoriyu It depends on your scenario. If you have a StringIO in the garner block, add a to_s to it, for example.

[andoriyu]

I figured it out. Maybe makes sense to add this to documentation:

            garner.bind(@song) do
              song = present @song, with: ::AdminSongPresenter
              song.as_json
            end

[dblock]

I'd welcome any README change that makes sense :)