Use to_json when rendering json response

[alexspeller]

I was trying to debug why an inertia json response was 5x slower than the initial html response.

After some investigation, it turns out that render json: foo does not actually do the same thing as foo.to_json does. It does a whole lot of rails magic instead, which is both a lot slower and inconsistent.

Changing this to use page.to_json ensures consistency with the props output in inertia.html.erb, avoiding weird inconsistencies between props renders and full page renders, and is also a lot faster (especially if you are using a library like Oj to optimise json generation).

[BrandonShar]

This is awesome, thanks @alexspeller !

Out of curiosity did you do any profiling or metrics for this? It would be cool to see the difference. No need to create something if you didn't.

[alexspeller]

I made a controller that renders this data structure (from memory, so not including the time to generate it):

    data = (1..1_000_000).map {
      {
        a: rand,
        b: rand,
        c: rand,
        d: rand,
        h: {
          a: rand,
          b: rand,
          h: {
            a: rand,
            b: rand,
          },
        },
      }
    }

With Oj.optimize_rails

Initia page load rendering html: 2655ms
Inertia json render without to_json: 6150ms
Inertia json render with to_json: 2131ms

Without Oj.optimize_rails

Initia page load rendering html: 9634ms
Inertia json render without to_json: 14824ms
Inertia json render with to_json: 8752ms

I believe this gets worse with the complexity of the data structure, as render json: foo calls as_json recursively and also calls object.dup a bunch, so uses a lot more time and memory, whereas my understanding is that to_json calls to_json recursively.

[BrandonShar]

Wow, that's a big difference! I'm source diving a bit in Rails to understand this better and it looks like this mimics the logic here

Am I looking in the wrong place or is there something happening elsewhere first?

[bknoles]

Love this and love the helpful writeup! If I know @skryukov I suspect he did a backflip of joy when he saw "5x faster"!

Any reason not to merge this @BrandonShar ?

[BrandonShar]

@bknoles Only because I'm genuinely not clear on why it's working based on that link I posted above. It looks like the same logic is happening just in different places so I'm hoping to understand a bit more about what's happening here (or be told I'm looking in the wrong place in rails source, it would not be the first time 😄)

[bknoles]

ah ok, i didn't realize that was why you were source diving