Unable to save ManyToMany relationship in Hibernate

Question

This is really puzzling me.

I have the following classes:

Ingredient.java

@Entity
@Table(name = "INGREDIENTS", uniqueConstraints = 
{@UniqueConstraint(columnNames = "NAME")})
public class Ingredient implements Serializable{

@Id
@GeneratedValue
@Column(name = "ingredient_id")
private Long id;

@Column(nullable = false, name = "name")
private String name;

@Column(nullable = false, name = "name")
private Long calories;

public Ingredient() {
}

public Ingredient(String name, Long calories) {
    this.name = name;
    this.calories = calories;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public Long getCalories() {
    return calories;
}

public void setCalories(Long calories) {
    this.calories = calories;
}

public Long getId(){
    return this.id;
}

public void setId(Long id) {
    this.id = id;
}

}

Meal.java

@Entity
@Table(name = "MEALS")
public class Meal implements Serializable {

@ManyToOne
private User user;

@Id
@GeneratedValue
@Column(name = "meal_id")
private Long id;

@ElementCollection(targetClass = Ingredient.class)
private Set<Ingredient> ingredients = new HashSet<>(0);

//More fields and constructors

@ManyToMany(targetEntity = Ingredient.class, cascade = CascadeType.ALL)
@JoinTable(name = "ingredient_meal", joinColumns = {@JoinColumn(name = "meal_id")}, inverseJoinColumns = {@JoinColumn(name = "ingredient_id")})
public List<Ingredient> getIngredients() {
    return ingredients;
}

public void setIngredients(List<Ingredient> ingredients) {
    this.ingredients = ingredients;
}


public Long getId(){
    return this.id;
}

public void setId(Long id) {
    this.id = id;
}

This is the code where I persist the entities:

SessionFactory sessionFactory = 
entityManagerFactory.unwrap(SessionFactory.class);

Session session = sessionFactory.openSession();
session.beginTransaction();

 /***/
Meal coolMeal = new Meal(user, new Date(115, 0, 8), new Time(19, 0, 0), "8 -  
Moules Frites", 1000L);
coolMeal.getIngredients().add(new Ingredient("Fish2", 100L));
        session.persist(coolMeal);
    try{
        session.getTransaction().commit();
    }catch(Throwable e){
        e.printStackTrace();
        throw e;
    }
    session.close();

But I keep getting this exception:

org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: calories.tracker.app.model.Ingredient
at org.hibernate.engine.internal.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:294)
at org.hibernate.type.EntityType.getIdentifier(EntityType.java:537)
at org.hibernate.type.ManyToOneType.nullSafeSet(ManyToOneType.java:165)
at org.hibernate.persister.collection.AbstractCollectionPersister.writeElement(AbstractCollectionPersister.java:899)
at org.hibernate.persister.collection.AbstractCollectionPersister.recreate(AbstractCollectionPersister.java:1308)
at org.hibernate.action.internal.CollectionRecreateAction.execute(CollectionRecreateAction.java:67)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:463)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:349)
at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:350)
at 

However if I persist the objects in this way:

    Ingredient i1 = new Ingredient("Potatoes2", 100L);
    session.persist(i1);
    Meal coolMeal = new Meal(user, new Date(115, 0, 8), new Time(19, 0, 0), 
    "8 -  Moules Frites", 1000L);
    coolMeal.getIngredients().add(i1);

It works fine, why? Everything is exactly the same as here.

Answer 1

You declare two mappings for the ingredients field.
One in the field and another in the getter :

@ElementCollection(targetClass = Ingredient.class)
private Set<Ingredient> ingredients = new HashSet<>(0);    
 ....        
@ManyToMany(targetEntity = Ingredient.class, cascade = CascadeType.ALL)
@JoinTable(name = "ingredient_meal", joinColumns = {@JoinColumn(name = "meal_id")}, inverseJoinColumns = {@JoinColumn(name = "ingredient_id")})
public List<Ingredient> getIngredients() {
    return ingredients;
}

You should remove the annotation for the field :

@ElementCollection(targetClass = Ingredient.class)

@ElementCollection is designed to map non-entity classes while Ingredient is an Entity :

According to the Javadoc :

ElementCollection

Defines a collection of instances of a basic type or embeddable class