How To Fix Actioncontroller::unknown Format When Using Turbo Stream With Rails And Turbo

28 May 2024 by jean

When you follow the hotrails's turbo stream tutorial you get the impression that the following should work. Inside index.html.erb file:

<%= link_to "Edit", edit_quote_path(quote) %>

Inside quotes controller:

def edit
  respond_to do |format|
    format.turbo_stream
  end
end

When you open the page and click on the Edit link what happens is that you get the following error: ActionController::UnknownFormat.

I had to dig a bit deeper to realise that this message is shown depending on what you send under the Accept header. In general that's controller by Rails, and by default for the Edit link above the default Accept header is text/html, application/xhtml+xml.

However, in the quotes controller the only format edit responds to is turbo_stream, not text/html. Turbo stream requires Accept header to be text/vnd.turbo-stream.html. So if our controller only reponds to format turbo_stream, we need to instruct Rails to accept text/vnd.turbo-stream.html. Rails is going to use this instruction to respond with a turbo_stream.

Therefore, to fix the ActionController::UnknownFormat error, index.html.erb should be changed to:

<%= link_to "Edit", edit_quote_path(quote), data: { turbo_stream: true } %>

With this instruction the Accept header becomes text/vnd.turbo-stream.html, text/html, application/xhtml+xml which means that Rails is now instructed to accept turbo_stream and pass it as a valid format to the caller.