fix: Preserve locals when rendering inline partial for object

[moberegger]

Ran into an odd bug. Given a template like

# locals (json:, foo:, my_local: 'default')

json.extract! foo, :id, :name
json.myLocal my_local

Rendering to a collection inline like

json.foos, @foos, partial: 'foos/foo', as: :foo, my_local: 'custom'

Would correctly render with the value of my_local with the value of 'custom'

{
  "foos": [
    "id": 1,
    "name": "Test",
    "myLocal": "custom"
  ]
}

But rendering a single object wasn't picking up the value for the local

json.foo, @foo, partial: 'foos/foo', as: :foo, my_local: 'custom'

{
  "foo": {
    "id": 1,
    "name": "Test",
    "myLocal": "default"
  }
}

If the partial didn't have a default value set for strict locals

# locals (json:, foo:, my_local:)

json.extract! foo, :id, :name
json.myLocal my_local

The following

json.foo, @foo, partial: 'foos/foo', as: :foo, my_local: 'custom'

would raise

ActionView::Template::Error: missing local: :my_local for app/views/foos/_foo.json.jbuilder...

The reason is because this render path in _set_inline_partial would set a key for locals before the render is done in _render_partial_with_options. _render_partial_with_options attempts to pluck key args for locals, but since the key is already there, they are effectively ignored.

This may have been a regression introduced by #591 which made it no longer reverse_merge! on the locals key. I haven't verified this, though.

The fix was to not have _set_inline_partial set locals, and instead defer that logic to _render_partial_with_options.

I added some extra tests cases to cover this.

• A test for rendering an object to an inline partial (there were no tests for this)
• A test for rendering an object to an inline partial with locals
• A test for rendering a collection to an inline partial with locals

I also updated the docs to include an example for rendering an object to an inline partial. This is supported by the library, but was seemingly undocumented. Also documented rendering to partials inline with other locals, which was also supported by the library but seemingly undocumented.

As an aside: With some small refactoring, I think we can drop the need for the intermediary _set_inline_partial method. It doesn't do too much and I think the coordination of locals can all be handled within _render_partial_with_options. This would save a bit on indirection and also align everything to the same render path. Outside of the scope of this PR, but perhaps something I can try later.

[moberegger]

This is the crux of the fix.

[moberegger]

Since this only had one call site...

[moberegger]

... I just inlined it here.