Rails: ActionDispatch::Request.parameter_parsers for multipart/form-data

Question

In my rails API, I have added an initialiser that will change the keys of the JSON input from snake-case to underscore-separated. Like so:

ActionDispatch::Reques         


        

Answer 1

When you look at the DEFAULT_PARSERS, it uses the Mime class, so whatever we end up using will likely need to be recognizable by the Mime class. So we can check Mime::Types to see what's available.

On that page, we see that content-type: multipart/form-data is mapped to :multipart_form. Indeed, while using

ActionDispatch::Request.parameter_parsers[:multipart_form] = -> (raw_post) {
  raise "Parsing Parameters: #{raw_post}"
}

and then submitting a form with a file field, I can trigger the error.

Answer 2

Although, according to Simple Lime's answer, :multipart_form is the right key for the default parser for requests with content-type: multipart/form-data, it does not work like the way it does for JSON.

This is the work around I implemented:

class ApplicationController < ActionController::API
    before_action :transform_params_if_multipart!

    private 
    def transform_params_if_multipart!
        params.deep_transform_keys!(&:underscore) if /^multipart\/form-data*/.match(request.headers['content-type'])
    end
end