Spring Data JPA, Hibernate: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint “toy_paths_pkey”

I have two entities: Dog and Toy. A Toy is associated with a String path.

toys_paths table:

dog_id
toy_path
toy_id

The (dog_id and toy_id) should be a unique pair.

Inside Dog entities it’s stored as Map<Toy, String>:

@Entity(name = "dogs")
public class Dog {
    @Id
    @SequenceGenerator(name = "dog_sequence",
        sequenceName = "dog_sequence",
        allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE,
        generator = "dog_sequence")
    private Long id;

    @ElementCollection
    @CollectionTable(name = "toy_paths",
        joinColumns = {@JoinColumn(name = "dog_id")})
    @MapKeyJoinColumn(name = "toy_id")
    @Column(name = "path")
    private Map<Toy, String> toyPaths;
}

@Entity(name = "toys")
publi class Toy {
    @Id
    @SequenceGenerator(name = "toy_sequence",
        sequenceName = "toy_sequence",
        allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE,
        generator = "toy_sequence")
    private Long id;
...

Inside DogService:

  public boolean createDog(DogDto dogDto) {
     ...
     Toy toy = toyRepository.findByName("A");
     dog.getToyPaths().put(toy, "/some/path");  // if this is removed, no exception

     Dog dog = copyData(dogDto);  
     dogRepository.save(dog); // exception here
     return true;
}

When I try to create a new Dog, I get the exception

org.postgresql.util.PSQLException: ERROR: 
duplicate key value violates 
unique constraint "toy_paths_pkey"

Detail: Key (dog_id, toy_id)=(67, 2) 
already exists.

It only happens when it tries to persist the Map<Toy, String>. If that map is empty, the Dog is saved in the database without errors.

What could be causing this exception? The same code used to work fine, but somehow how this is happening.

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