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Exercise 03: One-to-One Relationship

Learning Objective​

Learn how to implement a bidirectional @OneToOne relationship between Movie and MovieDetails entities.

Background​

A movie has exactly one set of details (plot, budget, runtime, production company), and those details belong to exactly one movie. This is a one-to-one relationship.

In a bidirectional relationship, both entities reference each other. One side must be marked as the owner of the relationship (the side with the foreign key).

Your Task​

Create a bidirectional one-to-one relationship between Movie and MovieDetails, then add an endpoint to attach details to a movie.

Step 1: Add Relationship to Movie Entity​

Modify Movie.java to reference MovieDetails:

@Entity
public class Movie {
    // ... existing fields (id, title, releaseYear, genre, rating, actors)

    // TODO: Add @OneToOne relationship
    // Use @JoinColumn(name = "movie_details_id")
    // This makes Movie the owner of the relationship
    
    // TODO: Add getter and setter for movieDetails
}

Step 2: Add Reverse Relationship to MovieDetails Entity​

Modify MovieDetails.java to reference back to Movie:

@Entity
public class MovieDetails {
    // ... existing fields (id, plot, budget, runtime, productionCompany)

    // TODO: Add @OneToOne(mappedBy = "movieDetails")
    // mappedBy indicates this is NOT the owner side
    
    // TODO: Add getter and setter for movie
}

Step 3: Add Method to MovieService​

Add a method to attach details to a movie:

public Movie addDetailsToMovie(Long movieId, MovieDetails details) {
    // TODO: Find the movie by ID
    // TODO: Save the details first using MovieDetailsRepository
    // TODO: Set the details on the movie
    // TODO: Set the movie reference on details (for bidirectional consistency)
    // TODO: Save and return the updated movie
}

Hint: You'll need to inject MovieDetailsRepository into the service.

Step 4: Add Controller Endpoint​

Add an endpoint to MovieController.java:

@PostMapping("/{id}/details")
public ResponseEntity<Movie> addDetailsToMovie(
        @PathVariable Long id,
        @RequestBody MovieDetails details) {
    // TODO: Call the service method
    // TODO: Return the updated movie
}

Step 4: Use @JsonManagedReference and @JsonBackReference​

To prevent infinite recursion during JSON serialization, modify the Movie and MovieDetails entities, adding @JsonManagedReference and @JsonBackReference annotations.

Testing Your Implementation​

  1. Restart your application

  2. Create a movie (using .http in IntelliJ):

    ### ADD MOVIE
    POST http://localhost:8080/api/movies
    Content-Type: application/json

    {
      "title": "The Dark Knight",
      "releaseYear": 2008,
      "genre": "Action",
      "rating": {"score": 9.0, "voteCount": 2500000}
    }

  3. Add details to the movie:

    ### ADD MOVIE DETAILS
    POST http://localhost:8080/api/movies/1/details
    Content-Type: application/json

    {
      "plot": "Batman faces the Joker in a battle for Gotham soul.",
      "budget": 185000000,
      "runtime": 152,
      "productionCompany": "Warner Bros"
    }

  4. Get the movie and verify it includes the details:

    ### VERIFY MOVIE WITH DETAILS
    GET http://localhost:8080/api/movies/1

  5. Check the H2 console:

    • Verify the MOVIE table has a movie_details_id foreign key column
    • Verify the MOVIE_DETAILS table does NOT have a foreign key to MOVIE

Expected Result​

The movie JSON should include the movieDetails object. The foreign key should be in the MOVIE table (owner side), not in the MOVIE_DETAILS table.

Questions to Consider​

  1. Which entity is the owner of the relationship? How can you tell?
  2. Why do we need mappedBy in the MovieDetails entity?
  3. What happens if you try to save a movie with details without saving the details first?

Continue to Exercise 04 to learn about cascading operations!