Why not use a Review object directly with gson instead of a Map while writing tests?

TLDR: it helps to clarify your test code.

We've designed the POST request of the API so that it is expecting two attributes/values, and it also pulls the course_id from the url params. Using gson.toJson(review) includes all fields from that object.

    @Test
    public void exampleReviewJsonOutputMapVsGson() {
        //Map
        Map<String, Object> values = new HashMap<>();
        values.put("rating", 3);
        values.put("comment", "it was okay");
        System.out.println(gson.toJson(values));

        //Gson with Review.class
        Review review = new Review(course.getId(), 3, "it was okay");
        System.out.println(gson.toJson(review));
    }

/* console output:
{"rating":3,"comment":"it was okay"}
{"id":0,"course_id":1,"rating":3,"comment":"it was okay"}
*/

When using the API, and specifically when POSTing reviews, there actually doesn't seem to be any harm in using Gson how you've described it.

course_id is overwritten before persisting because we're manually setting it with .setCourseId() in the post route after we deserialize from json. The id is also ignored, because that's a value generated by Sql2o when it persists to the database, and it's not even in our ReviewDao implementation. If it was, I believe it would cause a SQL error if we tried to set it manually.

Very important to consider is how clients will be submitting data to your API in their requests. Using a map is more representational of how the data will be submitted—attriutes supplied in key/value pairs. Hopefully, if this was a production system, you'd have developer facing documentation describing what types of requests can be made (POST/GET), which URLs they can be submitted to (our defined routes), the necessary headers (Content-Type of "application/json"), and which key/value attributes they need to include in their json requests.