Hibernate throws this exception during SessionFactory creation:

&gt; org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

This is my test case:

**Parent.java**

    @Entity
    public Parent {
     
     @Id
     @GeneratedValue(strategy=GenerationType.IDENTITY)
     private Long id;
    
     @OneToMany(mappedBy=&quot;parent&quot;, fetch=FetchType.EAGER)
     // @IndexColumn(name=&quot;INDEX_COL&quot;) if I had this the problem solve but I retrieve more children than I have, one child is null.
     private List&lt;Child&gt; children;
    
    }
    

**Child.java**

    @Entity
    public Child {
     
     @Id
     @GeneratedValue(strategy=GenerationType.IDENTITY)
     private Long id;
    
     @ManyToOne
     private Parent parent;
    
    }

How about this problem? What can I do?


----------


**EDIT**

OK, the problem I have is that another &quot;parent&quot; entity is inside my parent, my real behavior is this:

**Parent.java**

    @Entity
    public Parent {
     
     @Id
     @GeneratedValue(strategy=GenerationType.IDENTITY)
     private Long id;

     @ManyToOne
     private AnotherParent anotherParent;
    
     @OneToMany(mappedBy=&quot;parent&quot;, fetch=FetchType.EAGER)
     private List&lt;Child&gt; children;
    
    }

**AnotherParent.java**

    @Entity
    public AnotherParent {
     
     @Id
     @GeneratedValue(strategy=GenerationType.IDENTITY)
     private Long id;
    
     @OneToMany(mappedBy=&quot;parent&quot;, fetch=FetchType.EAGER)
     private List&lt;AnotherChild&gt; anotherChildren;
    
    }

Hibernate doesn&#39;t like two collections with `FetchType.EAGER`, but this seems to be a bug, I&#39;m not doing unusual things...

Removing `FetchType.EAGER` from `Parent` or `AnotherParent` solves the problem, but I need it, so real solution is to use `@LazyCollection(LazyCollectionOption.FALSE)` instead of `FetchType` (thanks to [Bozho][1] for the solution).


  [1]: https://stackoverflow.com/users/203907/bozho

Hibernate throws MultipleBagFetchException - cannot simultaneously fetch multiple bags

I think I misunderstood the meaning of cascading in the context of a `@ManyToOne` relationship.

The case:

    public class User {
    
       @OneToMany(fetch = FetchType.EAGER)
       protected Set&lt;Address&gt; userAddresses;
    
    }
    
    public class Address {
    
       @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
       protected User addressOwner;
    
    }

What is the meaning of the `cascade = CascadeType.ALL`? For example, if I delete a certain address from the database, how does the fact that I added the `cascade = CascadeType.ALL` affect my data (the `User`, I guess)?

What is the meaning of the CascadeType.ALL for a @ManyToOne JPA association

I&#39;m defining my Django models right now and I realized that there wasn&#39;t a `OneToManyField` in the model field types. I&#39;m sure there&#39;s a way to do this, so I&#39;m not sure what I&#39;m missing. I essentially have something like this:

    class Dude(models.Model):
        numbers = models.OneToManyField(&#39;PhoneNumber&#39;)
    
    class PhoneNumber(models.Model):
        number = models.CharField()

In this case, each `Dude` can have multiple `PhoneNumber`s, but the relationship should be unidirectional, in that I don&#39;t need to know from the `PhoneNumber` which `Dude` owns it, per se, as I might have many different objects that own `PhoneNumber` instances, such as a `Business` for example:

    class Business(models.Model):
        numbers = models.OneToManyField(&#39;PhoneNumber&#39;)

What would I replace `OneToManyField` (which doesn&#39;t exist) with in the model to represent this kind of relationship? I&#39;m coming from Hibernate/JPA where declaring a one-to-many relationship was as easy as:

    @OneToMany
    private List&lt;PhoneNumber&gt; phoneNumbers;

How can I express this in Django?  

How to express a One-To-Many relationship in Django?

Ok so this is probably a trivial question but I&#39;m having trouble visualizing and understanding the differences and when to use each. I&#39;m also a little unclear as to how concepts like uni-directional and bi-directional mappings affect the one-to-many/many-to-many relationships. I&#39;m using Hibernate right now so any explanation that&#39;s ORM related will be helpful. 

As an example let&#39;s say I have the following set-up:

    public class Person{
        private Long personId;
        private Set&lt;Skill&gt; skills;
        //Getters and setters
    }
    
    public class Skill{
        private Long skillId;
        private String skillName;
        //Getters and setters
    }

So in this case what kind of mapping would I have? Answers to this specific example are definitely appreciated but I would also really like an overview of when to use either one-to-many and many-to-many and when to use a join table versus a join column and unidirectional versus bidirectional.

Difference Between One-to-Many, Many-to-One and Many-to-Many?

In the example section of the [`@OneToMany` JPA annotation reference][1]:

**Example 1-59 @OneToMany - Customer Class With Generics**
    
    @Entity
    public class Customer implements Serializable {
        ...
        @OneToMany(cascade=ALL, mappedBy=&quot;customer&quot;)
        public Set&lt;Order&gt; getOrders() { 
            return orders; 
        }
        ...
    }
    
**Example 1-60 @ManyToOne - Order Class With Generics**
    
    @Entity
    public class Order implements Serializable {
        ...
        @ManyToOne
        @JoinColumn(name=&quot;CUST_ID&quot;, nullable=false)
        public Customer getCustomer() { 
            return customer; 
        }
        ...
    }

It seems to me that the `Customer` entity is the owner of the association. However, in the explanation for the `mappedBy` attribute in the same document, it is written that:

&gt; if the relationship is bidirectional,
&gt; then set the mappedBy element on the
&gt; inverse (non-owning) side of the
&gt; association to the name of the field
&gt; or property that owns the relationship
&gt; as Example 1-60 shows.

However, if I am not wrong, it looks like in the example, the `mappedBy` is actually specified on the owning side of the association, rather than the non-owning side.

So my question is basically:

 1. In a bidirectional (one-to-many/many-to-one) association, which of the entities is the owner? How can we designate the One side as the owner? How can we designate the Many side as the owner?

 2. What is meant by &quot;the inverse side of the association&quot;? How can we designate the One side as the inverse? How can we designate the Many side as the inverse?

  [1]: https://docs.oracle.com/javaee/7/api/javax/persistence/OneToMany.html

What is “the inverse side of the association” in a bidirectional JPA OneToMany/ManyToOne association?

What is the real difference between one-to-many and many-to-one relationship? It is only reversed, kind of?

I can&#39;t find any &#39;good-and-easy-to-understand&#39; tutorial about this topic other than this one: [SQL for Beginners: Part 3 - Database Relationships][1]


  [1]: http://net.tutsplus.com/tutorials/databases/sql-for-beginners-part-3-database-relationships/

Difference between one-to-many and many-to-one relationship

There&#39;s an Entity Class &quot;A&quot;. Class A might have children of the same type &quot;A&quot;. Also &quot;A&quot; should hold it&#39;s parent if it is a child.  

Is this possible? If so how should I map the relations in the Entity class?
[&quot;A&quot; has an id column.]

JPA: How to have one-to-many relation of the same Entity type

# The error
Cannot delete or update a parent row: a foreign key constraint fails.

# The classes

    class Teacher {

        /**
         *@ORM\OneToMany(targetEntity=&quot;publication&quot;, mappedBy=&quot;teacher&quot;)
         */
        protected $publications;
    }

    class Publication {

        /**
         * @ORM\ManyToOne(targetEntity=&quot;Teacher&quot;, inversedBy=&quot;publications&quot;)
         * @ORM\JoinColumn(name=&quot;teacher_id&quot;, referencedColumnName=&quot;id&quot;)
         */
        protected $teacher;
    }

# I want

What I want is to make it that when you delete a teacher, the id_teacher is modified to NULL. I want to keep the publication but without reference to Professor.

I don&#39;t know how do that in Doctrine, Is it possible? Or always the relationship has to be with a teacher?

Doctrine 2 OneToMany Cascade SET NULL

It&#39;s my first question here on stack, so please be gentle :D

I&#39;m trying to create hibernate OneToMany relationship. When I try to fetch some data from my DB, I&#39;m getting StackOverflowError. But when i remove OneToMany part, everything goes normally. This is part of my REST Service, for now it runs on VMware vFabric Server and MySQL DB.

Fetch example:
&lt;!-- language: java --&gt;

    @Inject
    private EntityManager entityManager;
    ...
    entityManager.find(League.class, 1);
    ...
    entityManager.find(Team.class, 1);


MySQL script:
&lt;!-- language: sql --&gt;

    CREATE TABLE league (
    	id int(11) NOT NULL AUTO_INCREMENT,
    	name varchar(20) COLLATE utf8_unicode_ci NOT NULL,
    	PRIMARY KEY (id)
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
    
    CREATE TABLE team (
    	team_id int(11) NOT NULL AUTO_INCREMENT,
    	name varchar(20) COLLATE utf8_unicode_ci NOT NULL,
    	fk_leagueId int(11) NOT NULL,
    	PRIMARY KEY (team_id),
    	FOREIGN KEY (fk_leagueId) REFERENCES league(id)
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

Classes:
&lt;!-- language: java --&gt;

    @XmlRootElement
    @Entity
    @Table(name = &quot;team&quot;)
    @Data
    public class Team {
        @Id
        @GeneratedValue(generator = &quot;increment&quot;)
        @GenericGenerator(name = &quot;increment&quot;, strategy = &quot;increment&quot;)
        @Column(name = &quot;team_id&quot;)
        private int id;
        @Column(name = &quot;name&quot;)
        private String name;
        @ManyToOne(fetch = FetchType.LAZY)
        @JoinColumn(name = &quot;fk_leagueId&quot;, nullable = false)
        private League league;
    }
    
    @XmlRootElement
    @Entity
    @Table(name = &quot;league&quot;)
    @Data
    public class League {
        @Id
        @GeneratedValue(generator = &quot;increment&quot;)
        @GenericGenerator(name = &quot;increment&quot;, strategy = &quot;increment&quot;)
        @Column(name = &quot;id&quot;)
        private int id;
        @Column(name = &quot;name&quot;)
        private String name;
        //if I comment 2 lines below, there is no error, and everything works fine
        @OneToMany(fetch = FetchType.LAZY, mappedBy = &quot;league&quot;)
        private Set&lt;Team&gt; teams;
    }


Error:

    Hibernate: select league0_.id as id1_1_0_, league0_.name as name2_1_0_ from league league0_ where league0_.id=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Hibernate: select teams0_.fk_leagueId as fk3_1_1_, teams0_.team_id as team1_4_1_, teams0_.team_id as team1_4_0_, teams0_.fk_leagueId as fk3_4_0_, teams0_.name as name2_4_0_ from team teams0_ where teams0_.fk_leagueId=?
    Exception in thread &quot;tomcat-http--3&quot; java.lang.StackOverflowError
    	at org.jboss.logging.JDKLogger.translate(JDKLogger.java:73)
    	at org.jboss.logging.JDKLogger.isEnabled(JDKLogger.java:85)
    	at org.jboss.logging.JDKLogger.doLog(JDKLogger.java:41)
    	at org.jboss.logging.Logger.debug(Logger.java:406)
    	at org.hibernate.internal.CoreMessageLogger_$logger.debug(CoreMessageLogger_$logger.java:525)
    	at org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:104)
    	at org.hibernate.engine.jdbc.spi.SqlStatementLogger.logStatement(SqlStatementLogger.java:95)
    	at org.hibernate.engine.jdbc.internal.StatementPreparerImpl$StatementPreparationTemplate.prepareStatement(StatementPreparerImpl.java:180)
    	at org.hibernate.engine.jdbc.internal.StatementPreparerImpl.prepareQueryStatement(StatementPreparerImpl.java:159)
    	at org.hibernate.loader.Loader.prepareQueryStatement(Loader.java:1858)
    	at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1835)
    	at org.hibernate.loader.Loader.executeQueryStatement(Loader.java:1815)
    	at org.hibernate.loader.Loader.doQuery(Loader.java:899)
    	at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:341)
    	at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:311)
    	at org.hibernate.loader.Loader.loadCollection(Loader.java:2234)
    	at org.hibernate.loader.collection.CollectionLoader.initialize(CollectionLoader.java:65)
    	at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:674)
    	at org.hibernate.event.internal.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:83)
    	at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:1849)
    	at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:549)
    	at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:234)
    	at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:545)
    	at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:124)
    	at org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:428)
    	at com.lukaszb.motspe.webapp.model.League.hashCode(League.java:21)
    	at com.lukaszb.motspe.webapp.model.Team.hashCode(Team.java:20)
    	at java.util.HashMap.hash(HashMap.java:351)
    	at java.util.HashMap.put(HashMap.java:471)
    	at java.util.HashSet.add(HashSet.java:217)
    ...


**Edit:**

I was able to solve this out thanks to @Thihara and @KarIP. I have overriden toString() for Team and League like this:

 

    @Override
    public String toString() {
    	return &quot;League [id=&quot; + id + &quot;, name=&quot; + name + &quot;]&quot;;
    }
    
    @Override
    public String toString() {
        return &quot;Team [id=&quot; + id + &quot;, name=&quot; + name + &quot;]&quot;;
    }
and was able to fetch data from DB as wanted. But then while parsing I got JAXB error with infinite cycles. So i annotated Team and League class with `@XmlAccessorType(XmlAccessType.FIELD)`, so it won&#39;t look at methods, and Team `league` field as `@XmlTransient`, so it won&#39;t be parsed.

Here I can even remove my toString() implementations, and it still works.
I&#39;m not completely sure why.
Problem is solved, but I would like to hear more precise explanation. I don&#39;t know why JAXB stopped data fetch, even if it&#39;s after whole DB communication process(or isn&#39;t?).
To be more specific, I&#39;m using Jersey for that:

    @GET
    @Path(&quot;search/id&quot;)
    @Produces({ MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML })
    public List&lt;League&gt; searchById(@QueryParam(&quot;id&quot;) int id) {
    return Arrays.asList(leagueDAO.getById(id));
    }

Hibernate OneToMany java.lang.StackOverflowError

I have been trying to get to grips with Hibernate&#39;s inverse attribute, and it seems to be just one of those things that is conceptually difficult. 

The gist that I get is that when you have a parent entity (e.g. Parent) that has a collection of Child objects using a **one-to-many** mapping, setting inverse=true on the mapping tells Hibernate that &#39;the other side (the Child) has responsibility to update itself to maintain the foreign key reference in its table&#39;. 

Doing this appears to have 2 benefits when it comes to adding Children to the collection in your code, and then saving the Parent (with cascade-all set): [you save an unneccessary hit on the database][1] (because without inverse set, Hibernate thinks it has two places to update the FK relationship), and according to the official docs:

&gt;  If the &lt;key&gt; column of a
&gt; &lt;one-to-many&gt; association is declared
&gt; NOT NULL, NHibernate may cause
&gt; constraint violations when it creates
&gt; or updates the association. To prevent
&gt; this problem, you must use a
&gt; bidirectional association with the
&gt; many valued end (the set or bag)
&gt; marked as inverse=&quot;true&quot;.

This all seems to make sense so far. What I don&#39;t get is this: when would you **NOT** want to use inverse=true on a one-to-many relationship?

  [1]: http://nhprof.com/Learn/Alert?name=SuperfluousManyToOneUpdate

When to use inverse=false on NHibernate / Hibernate OneToMany relationships?

I am reading currently the documentation of Hibernate regarding the [entity associations][1] and I come accross a little difficulty to figure out some things. It has to do in essence with the difference between `ManyToOne` and `OneToMany` associations. Although I have used them in real projects, I cannot comprehend completely the difference between them. To my understanding, if a table / an entity has a `ManyToOne` association with another, then the association should be from the other side `OneToMany`. So, how should we decide which one to choose based on a specific case and how does it affect the database/queries/results? Is there everywhere a good example? 

P.S.: I reckon it would be helpful due to its relevance to the question, if someone could besides explain what is the point of the owner of the association and the difference between Bidirectional and Unidirectional association.   


  [1]: http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html/entity.html#entity-mapping-association

Hibernate/JPA ManyToOne vs OneToMany

Maybe this is a stupid question but it&#39;s bugging me.

I have a bi-directional one to many relationship of Employee to Vehicles. When I persist an Employee in the database for the first time (i.e. it has no assigned ID) I also want its associated Vehicles to be persisted. 

This works fine for me at the moment, except that my saved Vehicle entity is not getting the associated Employee mapped automatically, and in the database the employee_id foreign key column in the Vehicle table is null.

My question is, is it possible to have the Vehicle&#39;s employee persisted at the same time the Employee itself is being persisted? I realise that the Employee would need to be saved first, then the Vehicle saved afterwards. Can JPA do this automatically for me? Or do I have to do something like the following:

    
    Vehicle vehicle1 = new Vehicle();
    Set&lt;Vehicle&gt; vehicles = new HashSet&lt;Vehicle&gt;();
    vehicles.add(vehicle1);

    Employee newEmployee = new Employee(&quot;matt&quot;);
    newEmployee.setVehicles(vehicles);
    Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

    vehicle1.setAssociatedEmployee(savedEmployee);
    vehicleDao.persistOrMerge(vehicle1);

Thanks!

Edit: As requested, here&#39;s my mappings (without all the other methods etc.)

    @Entity
    public class Employee {
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        @Column(name=&quot;employee_id&quot;)
        private Long id;
    
        @OneToMany(mappedBy=&quot;associatedEmployee&quot;, cascade=CascadeType.ALL)
        private Set&lt;Vehicle&gt; vehicles;
      
        ...
    
    }
    
    @Entity 
    public class Vehicle {
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        @Column(name=&quot;vehicle_id&quot;)
        private Long id;
    
        @ManyToOne
        @JoinColumn(name=&quot;employee_id&quot;)
        private Employee associatedEmployee;
      
        ...
    }

I just realised I should have had the following method defined on my Employee class:

    public void addVehicle(Vehicle vehicle) {
    	vehicle.setAssociatedEmployee(this);
    	vehicles.add(vehicle);
    }


Now the code above will look like this:

    Vehicle vehicle1 = new Vehicle();

    Employee newEmployee = new Employee(&quot;matt&quot;);
    newEmployee.addVehicle(vehicle1);
    Employee savedEmployee = employeeDao.persistOrMerge(newEmployee);

Much simpler and cleaner. Thanks for your help everyone!


JPA - Persisting a One to Many relationship

I&#39;m working on forum template using Flask. When I attempt creating a new thread in the browser using forms, SQLAlchemy throws an AttributeError. The problem showed up when I tried implementing a one-to-many relationship with Forum-to-Thread and a one-to-many relationship with Thread-to-User.

models.py

    class User(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      username = db.Column(db.String(32), index=True, unique=True)
      password = db.Column(db.String(32), index=True)
      email = db.Column(db.String(120), index=True, unique=True)
      role = db.Column(db.SmallInteger, default=ROLE_USER)
    
      posts = db.relationship(&#39;Post&#39;, backref=&#39;author&#39;, lazy=&#39;dynamic&#39;)

    class Forum(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      title = db.Column(db.String(128))
      description = db.Column(db.Text)
    
      threads = db.relationship(&#39;Thread&#39;, backref=&#39;forum&#39;, lazy=&#39;dynamic&#39;)
    	  
    class Thread(db.Model):
      
      id = db.Column(db.Integer, primary_key=True)
      title = db.Column(db.String(128))
      author= db.Column(db.String(32))
      timestamp = db.Column(db.DateTime)
      forum_id = db.Column(db.Integer, db.ForeignKey(&#39;forum.id&#39;))
    
      posts = db.relationship(&#39;Post&#39;, backref=&#39;thread&#39;, lazy=&#39;dynamic&#39;)
    	
    class Post(db.Model):
      
      id = db.Column(db.Integer, primary_key=True)
      body = db.Column(db.Text)
      timestamp = db.Column(db.DateTime)
      thread_id = db.Column(db.Integer, db.ForeignKey(&#39;thread.id&#39;))
      user_id = db.Column(db.Integer, db.ForeignKey(&#39;user.id&#39;))

All the new posts/threads and handled within views.py

views.py

    @app.route(&#39;/forum/id=&lt;id&gt;/submit&#39;, methods=[&#39;GET&#39;,&#39;POST&#39;])
    @login_required
    def new_thread(id):
      form = ThreadForm()
      forum = Forum.query.filter_by(id=id).first()
      if form.validate_on_submit():
        thread = Thread(title=form.title.data,
                        author=g.user.username,
                        timestamp=datetime.utcnow())
        db.session.add(thread)
        db.session.flush()
        post = Post(body=form.body.data,
                    timestamp=datetime.utcnow(),
                    thread=thread.id,
                    author=g.user.id)
        db.session.add(post)
        db.session.commit()
        flash(&#39;Post successful.&#39;)
        return redirect(url_for(&#39;forum_index&#39;, id=id))
      return render_template(&#39;forum/thread_submit.html&#39;, title=forum.title, form=form) 

AttributeError: &#39;int&#39; object has no attribute &#39;_sa_instance_state&#39;

I have the following two entities:

**1- PlayList:**

    @OneToMany(fetch = FetchType.EAGER, mappedBy = &quot;playlist&quot;, orphanRemoval = true, cascade =   CascadeType.ALL)
    @OrderBy(&quot;adOrder&quot;)
    private Set&lt;PlaylistadMap&gt; PlaylistadMaps = new HashSet&lt;PlaylistadMap&gt;(0);

- **CascadeType.ALL**: is needed for save and update on the PlaylistadMap collection when saving or updating the playlist entity.
- **orphanRemoval = true**: is needed when deleting the playlist entity, the PlaylistadMap references should be deleteed too.


**2- PlaylistadMap:**

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = &quot;fk_playlist&quot;, referencedColumnName = &quot;pkid&quot;, nullable = false)
    private Playlist playlist;

when deleting a playlist by using `getCurrentSession().delete();` i am getting the following exception:

    org.springframework.dao.InvalidDataAccessApiUsageException: deleted object would be re-saved by cascade (remove deleted object from associations): [com.xeno.advertisingsuite.web.domain.PlaylistadMap#6]; nested exception is org.hibernate.ObjectDeletedException: deleted object would be re-saved by cascade (remove deleted object from associations): [com.xeno.advertisingsuite.web.domain.PlaylistadMap#6]
    	at org.springframework.orm.hibernate3.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:657)
    	at org.springframework.orm.hibernate3.HibernateTransactionManager.convertHibernateAccessException(HibernateTransactionManager.java:793)
    	at org.springframework.orm.hibernate3.HibernateTransactionManager.doCommit(HibernateTransactionManager.java:664)
    	at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:754)
    	at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:723)
    	at org.springframework.transaction.interceptor.TransactionAspectSupport.commitTransactionAfterReturning(TransactionAspectSupport.java:393)
    	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:120)
    	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
    	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202)
    	at $Proxy54.deletePlayList(Unknown Source)
    	at com.xeno.advertisingsuite.web.beans.PlayListBean.deletePlaylist(PlayListBean.java:282)
    	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    	at java.lang.reflect.Method.invoke(Method.java:597)
    	at org.apache.el.parser.AstValue.invoke(AstValue.java:262)
    	at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:278)
    	at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:105)
    	at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
    	... 103 more
    Caused by: org.hibernate.ObjectDeletedException: deleted object would be re-saved by cascade (remove deleted object from associations): [com.xeno.advertisingsuite.web.domain.PlaylistadMap#6]
    	at org.hibernate.impl.SessionImpl.forceFlush(SessionImpl.java:1220)
    	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:188)
    	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:117)
    	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93)
    	at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:677)
    	at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:669)
    	at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:252)
    	at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:392)
    	at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:335)
    	at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:204)
    	at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:425)
    	at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:362)
    	at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:338)
    	at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:204)
    	at org.hibernate.engine.Cascade.cascade(Cascade.java:161)
    	at org.hibernate.event.def.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:154)
    	at org.hibernate.event.def.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:145)
    	at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:88)
    	at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
    	at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1206)
    	at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:375)
    	at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)
    	at org.springframework.orm.hibernate3.HibernateTransactionManager.doCommit(HibernateTransactionManager.java:656)

please advise how to fix this exception.

Deleted object would be re-saved by cascade (remove deleted object from associations)

I have 2 classes in a one-to-many relationship and a HQL query that is a bit strange. Even if I have read some questions already posted, it does not seem clear to me.

    Class Department{
       @OneToMany(fetch=FetchType.EAGER, mappedBy=&quot;department&quot;)
       Set&lt;Employee&gt; employees;
    }
    Class Employee{
       @ManyToOne
       @JoinColumn(name=&quot;id_department&quot;)
       Department department;
    }

When I use the following query I get duplicates Department objects: 

`session.createQuery(&quot;select dep from Department as dep left join dep.employees&quot;);`

Thus, I have to use distinct:

`session.createQuery(&quot;select distinct dep from Department as dep left join dep.employees&quot;);`

Is this behaviour an expected one? I consider this unusual, as comparing it with SQL.

One-To-Many relationship gets duplicate objects without using &quot;distinct&quot;. Why?

I have three classes one of the names is User and this user has other classes instances. Like this;

    public class User{
        @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL)
    	public List&lt;APost&gt; aPosts;
    
    	@OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL)
    	public List&lt;BPost&gt; bPosts;
    }


 

       public class BPost extends Post {
        
        @ManyToOne(fetch=FetchType.LAZY) 	
        public User user;
     }
        
        public class APost extends Post {
        
         @ManyToOne(fetch=FetchType.LAZY) 
         public User user;
     }

it&#39;s working like this but generates empty tables in DB. Which have to contain foreign keys. When I tried to use `mappedBy` and `JoinColumn` annotations I got failed. How can I resolve this?

**Extra information:**

When I changed with;

     @ManyToOne(fetch=FetchType.LAZY)
     @JoinColumn(name=&quot;id&quot;)
     public User user;

and

     @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy=&quot;id&quot;)
     public List&lt;APost&gt; aPosts;

I&#39;m getting

A JPA error occurred (Unable to build EntityManagerFactory): Repeated column in mapping for entity: models.post.APost column: id (should be mapped with insert=&quot;false&quot; update=&quot;false&quot;)

**Final Edit:** Finally, I was totally wrong about JPA annotations. :( When I change

    @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy=&quot;id&quot;)
    
to

    @OneToMany(fetch=FetchType.LAZY, cascade = CascadeType.ALL, mappedBy=&quot;user&quot;)
    
and

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name=&quot;user_id&quot;)

everything works ok. :)

JPA OneToMany and ManyToOne throw: Repeated column in mapping for entity column (should be mapped with insert=&quot;false&quot; update=&quot;false&quot;)

I have users, and users belong to a dealership.  

Upon user registration, I&#39;m trying to save a new user, and a new dealership. 

User database has a `dealership_id` column, which I want to be populated with the ID of the newly created dealership. 

This is my current code in the `UserController` store method.

	public function store()
	{
		$user = new User();
		$user-&gt;email = Input::get(&#39;email&#39;);
		$user-&gt;password = Input::get(&#39;password&#39;);


		$dealership = new Dealership();
		$dealership-&gt;name = Input::get(&#39;dealership_name&#39;);

		$user-&gt;push();
		return &quot;User Saved&quot;;

	}


Trying to use `$user-&gt;push();` User data gets updated, but dealership is not created or updated. 

All One to-Many Solutions on Gang of Coders

Total of 17 One to-Many Solutions