Rack::Test defaults to URL Encoding Parameters

By default, Rack’s test helpers set the Content-Type header to application/x-www-form-urlencoded.

The URL encoded representation of

{ "tags": ["foo", "bar"] }

would be

?tags[]=foo&tags[]=bar

This works as expected for non-empty arrays:

put "/api/v1/post/1", {
  post: { tags: ["foo"] }
}

However, it isn’t possible to represent [] as a URL encoded string.

put "/api/v1/post/1", {
  post: { tags: [] }
}

would be invalidly encoded as:

?tags[]=

As a result, Rack ignores the tags key, which would make the post key point to an empty hash, which Rack also ignores.

To work around this issue:

• transform the params hash to a JSON string
• append .json to the URL
• explicitly set the "Content-Type" header to "application/json"

put "/api/v1/post/1.json", {
  post: { tags: [] }
}.to_json, { "Content-Type" => "application.json" }

This pattern can be extracted to a helper:

def json_put(path, params, headers_or_env = {})
  json_path = "#{path}.json"
  params_without_format = params.except(:format)
  headers = headers_or_env.reverse_merge("Content-Type" => "application/json")

  put(json_path, params_without_format.to_json, headers)
end


json_put "/api/v1/post/1", {
  post: { tags: [] }
}

Coincidentally, Basecamp has a similar set of test helpers.