Cache your partials, not the other way around.

It might seem nice and tidy to do this:

# items/index.json.jbuilder
json.items @items do
  json.partial! "item", item: item
end

# items/show.json.jbuilder
json.item do
  json.partial! "item", item: @item
end

# items/_items.json.jbuilder
json.cache! item do
  json(item, :id, :name, ...)
end

The templates are DRY and things are organized.

But tucking the call to cache! into the _item partial is a performance hit. Consider:

# items/index.json.jbuilder
json.items @items do
  json.cache! item do
    json.partial! "item", item: item
  end
end

# items/show.json.jbuilder
json.item do
  json.cache! item do
    json.partial! "item", item: @item
  end
end

# items/_items.json.jbuilder
json(item, :id, :name, ...)

While slightly more repetitive, this structure saves the calls to partial! once the cache is warm. Loading a partial isn't free, and the time it takes to load one up adds up when aggregated over x items per request, and y requests per minute.