TIL Rack defaults to URL encoding

Tweak Rack's test helpers to test a JSON API

Author: seanpdoyle

rails/empty-arrays-in-json-request.md

@@ -0,0 +1,75 @@
+`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
+
+```json
+{ "tags": ["foo", "bar"] }
+```
+
+would be
+
+```
+?tags[]=foo&tags[]=bar
+```
+
+This works as expected for non-empty arrays:
+
+```ruby
+put "/api/v1/post/1", {
+  post: { tags: ["foo"] }
+}
+```
+
+However, it isn't possible to represent `[]` as a URL encoded string.
+
+```ruby
+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][rack-test]
+* append `.json` to the URL
+* explicitly set the `"Content-Type"` header to `"application/json"`
+
+```ruby
+put "/api/v1/post/1.json", {
+  post: { tags: [] }
+}.to_json, { "Content-Type" => "application.json" }
+```
+
+This pattern can be extracted to a helper:
+
+```ruby
+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.
+
+[rack-test]: https://github.com/brynary/rack-test/blob/acdbee66fc765f15c2d3a1a372c368fe8ee0a49c/lib/rack/test.rb#L218-L228