Rating Movies

The Request Lifecycle

Before we start, let’s take a look at the request lifecycle when saving a review. If you prefer, you can skip to Save the Rating in a Write Transaction.

On every Movie page, the user is invited to rate a movie on scale of 1 to 5. The form pictured to the right gives the user the ability to select a rating between 1 and 5 and click submit to save the rating.

When the form is submitted, the website sends a POST request to /api/account/ratings/{movieId} and the following will happen:

A rating is represented in the graph as a relationship going from a :User to a :Movie node with the type :RATED. The relationship has two properties; the rating (an integer) and a timestamp to represent when the relationship was created.

After the data is saved, the UI expects the movie details to be returned, with an additional property called rating, which will be the rating that the current user has given for the movie.

Let’s take a look at the existing method in the RatingService.

public Map<String,Object> add(String userId, String movieId, int rating) {
    // TODO: Convert the native integer into a Neo4j Integer
    // TODO: Save the rating in the database
    // TODO: Return movie details and a rating

    var copy = new HashMap<>(pulpfiction);
    copy.put("rating",rating);
    return copy;
}

Your challenge is to replace the TODO comments in this method with working code.

// Save the rating in the database

// Open a new session
try (var session = this.driver.session()) {

    // Run the cypher query
    var movie = session.executeWrite(tx -> {
        String query = """
                MATCH (u:User {userId: $userId})
                MATCH (m:Movie {tmdbId: $movieId})

                MERGE (u)-[r:RATED]->(m)
                SET r.rating = $rating, r.timestamp = timestamp()

                RETURN m { .*, rating: r.rating } AS movie
                """;
        var res = tx.run(query, Values.parameters("userId", userId, "movieId", movieId, "rating", rating));
        return res.single().get("movie").asMap();
    });

} catch(NoSuchRecordException e) {
    throw new ValidationException("Movie or user not found to add rating", Map.of("movie", movieId, "user", userId));
}

// Return movie details with rating
return movie;

Working Solution

public Map<String,Object> add(String userId, String movieId, int rating) {
    // Save the rating in the database

    // Open a new session
    try (var session = this.driver.session()) {

        // Run the cypher query
        var movie = session.executeWrite(tx -> {
            String query = """
                    MATCH (u:User {userId: $userId})
                    MATCH (m:Movie {tmdbId: $movieId})

                    MERGE (u)-[r:RATED]->(m)
                    SET r.rating = $rating, r.timestamp = timestamp()

                    RETURN m { .*, rating: r.rating } AS movie
                    """;
            var res = tx.run(query, Values.parameters("userId", userId, "movieId", movieId, "rating", rating));
            return res.single().get("movie").asMap();
        });

        // Return movie details with rating
        return movie;
    } catch(NoSuchRecordException e) {
        throw new ValidationException("Movie or user not found to add rating", Map.of("movie", movieId, "user", userId));
    }
}

Testing

To test that this functionality has been correctly implemented, run the following code in a new terminal session:

mvn test -Dtest=neoflix._06_RatingMoviesTest#writeMovieRatingAsInt

The test file is located at src/test/java/neoflix/_06_RatingMoviesTest.java.

git checkout 06-rating-movies