Steve Garing

Bill Gorder wrote:If I understand you correctly you are on the right track with what you said in your first point. Objects are uniquely defined in the persistence context by their persistent identifier (the one you annotate with @Id). If you are adding countries with a null id then it will see it as a new entity and persist it. If you were to put a unique constraint on country name you are correct that it will throw a constraint violation. You have 2 options.

1. Issue a query to find the country by name and add the result of that query to your user.
2. Since the country table is purely a look-up table don't set a generation strategy on the country object and use its name as the @Id. Of course if you have other information about the country stored, you will still have to do a findById to get the fully populated country object but you can then use the name as the id.

You can set the insertable and updateable attributes to false on your join column if this table is pre-populated and you don't want the application to be able to change them.

Thanks Bill, you did understand correctly. My apologies if the explanation was a little confusing. I was hoping not to have to do an initial call to the database to get the lookup record but it looks like I may need too. I may just do a full lookup of the countries and put them into a set rather than looking up each country by the name as I encounter each entry in the file. This way it should be one call for <200 records compared to hundreds to thousands of calls for single countries. Thanks for confirming what I thought.

Hi all,

First post on coderanch for me! I am very new (say about 8hrs) to Hibernate so I am hoping I can explain this properly.

I have a situation where I need to record the countries a user has logged in from. This information is read in from an xml file and persisted to the database. It is possible that users can have a history of logging in from many countries if they travel. When I read in the country for each user I only have the name of the country and not the primary key that the country is associated with in the DB. So I am currently creating a Country object, setting the name (without and id) and then adding that object to the countries list in the User object.

To implement this I have 3 tables, User, Country (purely a reference table) and User_Country (to handle the many-to-many situation). In my User @Entity class I am using the @JoinTable on my getCountries() method to tell hibernate about the many-to-many relationship. When I persist the User object it it is successfully persisting the User and User_Country entries but it is also creating new duplicate entries in the Country table and using these entries as the country_id in the User_Countries table. I could mark the countryName field as unique in the @Column but I suspect that won't fix my problem as it will attempt to insert a duplicate and fail.

I have 3 thoughts as to why this is happening:
1. Because I am adding countries to the User without and ID, hibernate thinks these are new countries and adding them to the database before finally updating the User_Countries table. Should I be looking up the Country by the country name and adding the complete object to the User class (can you look up record in Hibernate purely based on a unique field??)
2. I have done something wrong with the @JoinTable and I am missing something that tells Hibernate not to update the Countries table and to only update the User and User_Countries table
3. I am way off and it's another problem all together

Is anyone able to help out? Hopefully that makes sense. Thanks in advance!!

My cut down User class and Country class Code is below.

User.java
@Entity public class User implements Serializable { private static final long serialVersionUID = 2284488937952510797L; private Long id; private Collection<Country> countries; // Default Constructor public User(){ } @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator="USR_SEQ_GEN") @Column(name = "id", unique = true, nullable = false) public Long getId() { return id; } public void setId(Long id) { this.id = id; } @ManyToMany(fetch=FetchType.EAGER) @JoinTable( name = "User_Countries", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "country_id") ) public Collection<Country> getCountries() { return countries; } public void setCountries(Collection<Country> countries) { this.countrie = countries; } }

Country.java
@Entity public class Country implements Serializable { private static final long serialVersionUID = 1L; private Long id; private String countryName; private Collection<User> users; // Default Constructor public Country(){ } @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator="CON_SEQ_GEN") @Column(name = "id", unique = true, nullable = false) public Long getId() { return id; } public void setId(Long id) { this.id = id; } @Column(name="country_name", unique=false ,nullable=false) public String getCountryName() { return this.countryName; } public void setCountryName(String countryName) { this.countryName = countryName; } @ManyToMany(mappedBy="countries") public Collection<User> getUsers() { return videos; } public void setUsers(Collection<Users> users) { this.videos = videos; } }