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Differences between JPA predicate using Entity and property

Say I have the following Entity classes:
#Entity public class MyEntity {
#Id private String id;
#ManyToOne private MyOtherEntity myOtherEntity;
}
#Entity public class MyOtherEntity {
#Id private String id;
#Column private String name;
}
Now I want to do a query to get all the MyEntitys linked to a certain MyOtherEntity, I wonder the difference between the following 3 predicates:
cb.equal(root.get(MyEntity_.myOtherEntity), myOtherEntity);
cb.equal(root.get(MyEntity_.myOtherEntity).get(MyOtherEntity_.id), myOtherEntity.getId());
cb.equal(root.get(MyEntity_.myOtherEntity).get(MyOtherEntity_.name), myOtherEntity.getName());
How would the generated SQLs look like in each case? And which one is most efficient?

For a start I suggest to take the trouble and enable SQL logging in Hibernate while developing - see here. Knowing the exact statements Hibernate creates for your JPA queries is invaluable, e.g. you have a chance to spot N+1 query problems, excessive joins etc.
Having said that, in your case the statements should look like as follows:
cb.equal(root.get(MyEntity_.myOtherEntity), myOtherEntity) → SELECT ... FROM MyEntity WHERE MyEntity.myOtherEntity_id = ?. In cases like this, Hibernate usually knows to optimize and avoid the unnecessary join.
cb.equal(root.get(MyEntity_.myOtherEntity).get(MyOtherEntity_.id), myOtherEntity.getId()) → Should be like above; again Hibernate should know that the .get(MyOtherEntity_.id) is already in the table and avoid the unnecessay join.
I have seen Hibernate working the way I describe for the cases above. Definitely enable SQL logging to verify, there may be details for your own use case that make it behave in a different way!
cb.equal(root.get(MyEntity_.myOtherEntity).get(MyOtherEntity_.name), myOtherEntity.getName()) → Will definitely create a join because it cannot find myOtherEntity.name in the MyEntity table: SELECT ... FROM MyEntity e JOIN MyOtherEntity oe ON ... WHERE oe.name = ?

EclipseLink native query and FetchType behaviour

I'm trying to understand EclipseLink behaviour in case if I use native query. So I have Entity like this:
class Entity {
#OneToOne(fetch = FetchType.LAZY)
#JoinColumn(name="other_entity_id")
private OtherEntity otherEntity;
#Column(name = "name")
private String name;
//gets ... sets ...
}
and corresponding table looks like:
**ENTITY**
INTEGER ID;
VARCHAR NAME;
OTHER_ENTITY_ID;
And then I run native query
Query query = getEntityManager().runNativeQuery("select * from ENTITY", Entity.class);
query.getResultList()
Within Entity I have declared OtherEntity otherEntity which is annotated with FetchType.LAZY, however my query selects (*) - all of the columns, including OTHER_ENTITY_ID. The question is - if I run native query that fetches all columns, will fields annotated with FetchType.LAZY populated as if they were FetchType.EAGER or not? I've never worked with EclipseLink before and tyring to decide is it worth using it or not so I would really appreciate any help
Thanks, Cheers

My first advice is to turn on EclipseLink's SQL logging, and execute the equivalent JPQL to load what you are looking for and see the SQL EclipseLink generates to accomplish that to get an understanding of what is required to build objects in your native queries based on your current mappings.
Relationships generally loaded with a secondary query using the values read in from the foreign keys, so eager or lazy fetching is not affected by the native query to read in "Entity" - the query requires the other_entity_id value regardless of the fetch type. When required based on eager/lazy loading, EclipseLink will issue the query required by the mapping.
You can change this though by marking that the relationship is to use joining. In this case, EclipseLink will expect not only the Entity values to be in the query, but the referenced OtherEntity values as well.

#GeneratedValue polymorphic abstract superclass over MySQL

In a Spring MVC application using Hibernate and MySQL, I have an abstract superclass BaseEntity that manages the values of the IDs for all the other entities in the model. The id field uses #GeneratedValue. I am encountering a problem whenever my code tries to save any of the subclasses that extend BaseEntity. The problem comes with the choice of GenerationType for the #GeneratedValue.
At every place in my code where a subclass of BaseEntity tries to save to the underlying MySQL database, I get the following error:
ERROR SqlExceptionHelper - Table 'docbd.hibernate_sequences' doesn't exist
I have read many postings about this on SO and on google, but they either deal with other databases (not MySQL) or they do not deal with abstract superclasses. I cannot solve the problem by using GenerationType.IDENTITY because I am using an abstract superclass to manage id fields for all entities in the model. Similarly, I cannot use GenerationType.SEQUENCE because MySQL does not support sequences.
So how do I solve this problem?
Here is the code for BaseEntity.java:
#Entity
#Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class BaseEntity {
#Id
#GeneratedValue(strategy = GenerationType.TABLE)
protected Integer id;
public void setId(Integer id) {this.id = id;}
public Integer getId() {return id;}
public boolean isNew() {return (this.id == null);}
}
Here is an example of the code for one of the entities that extends BaseEntity:
#Entity
#Table(name = "ccd")
public class CCD extends BaseEntity{
//other stuff
}
Here is the DDL:
CREATE TABLE IF NOT EXISTS ccd(
id int(11) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
#other stuff
)engine=InnoDB;SHOW WARNINGS;
Here is the JPQL code in the DAO:
#Override
#Transactional
public void saveCCD(CCD ccd) {
if (ccd.getId() == null) {
System.out.println("[[[[[[[[[[[[ about to persist CCD ]]]]]]]]]]]]]]]]]]]]");
this.em.persist(ccd);
this.em.flush();
}
else {
System.out.println("]]]]]]]]]]]]]]]]]] about to merge CCD [[[[[[[[[[[[[[[[[[[[[");
this.em.merge(ccd);
this.em.flush();
}
}
EDIT:
The reason I cannot use #MappedSuperClass in this situation is that I need to have ManyToOne relationships that allow for multiple subtypes to be used interchangeably. Look at the AccessLog class below as an example. It has an actor_entity and a target_entity. There can be many types of actor entities and many types of target entities, but they all inherit from BaseEntity. This inheritance enables the underlying accesslogs data table in MySQL to just have one actor_entity_id field and just one target_entity_id field instead of having to have several fields for each. When I change #Entity above BaseEntity to #MappedSuperClass, a different error gets thrown indicating that AccessLog cannot find BaseEntity. BaseEntity needs #Entity annotation in order for AccessLog to have polymorphic properties.
#Entity
#Table(name = "accesslogs")
public class AccessLog extends BaseEntity{
#ManyToOne
#JoinColumn(name = "actorentity_id")
private BaseEntity actor_entity;
#ManyToOne
#JoinColumn(name = "targetentity_id")
private BaseEntity target_entity;
#Column(name="action_code")
private String action;
//getters, setters, & other stuff
}
SECOND EDIT:
As per JBNizet's suggestion, I created a hibernate_sequences table as follows:
CREATE TABLE IF NOT EXISTS hibernate_sequences(
sequence_next_hi_value int(11) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY
)engine=InnoDB;SHOW WARNINGS;
But now I am getting the following error:
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'sequence_name' in 'where clause'
Here is the hibernate sql causing the error, followed by the next 2 lines of the stack trace:
Hibernate: select sequence_next_hi_value from hibernate_sequences where sequence_name = 'BaseEntity' for update
ERROR MultipleHiLoPerTableGenerator - HHH000351: Could not read or init a hi value
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'sequence_name' in 'where clause'
How do I resolve this?

What a mess... AUTO_INCREMENT is MySQL's hidden sequence. The radical problem is that MySQL can not insert and return the PK at the same time, but Hibernate need this while INSERTing a new Entity.
The Problems you run into:
If Hibernate save a new Entity, he try to immerdentelly set the id to the new EntityBean. Therefore hibernate must read what ID will the Database use before hibernate save the new Tuple to the Table.
If you have multiple Servers who access the database, you shall let hibernate's session-factory decide to use the built-in sequence(AUTO-INCREMENT) or let hibernate decide (GenerationType.AUTO/GenerationType.IDENTITY) how large the open range of reserved PK's is (Job of a DB-Architect). (We have about 20 servers to one Database, so on a good-used table we use a PK-distance of +100). If only one server have access to the database GenerationType.TABLE shall be correct.
Hibernate must calculate the next id by yourself using max(*)+1 but:
What if two requests ask for max(*)+1 at the same time/with the same result? Right: The last try to insert will fail.
So you need to have a Table LAST_IDS in the database who stores the last Table-PK's. If you like to add one, you must do this steps:
Start read-optimistic transaction.
SELECT MAX(address_id) FROM LAST_IDS
store the maximum in a java-variable i.e.: $OldID.
$NewID = $OldID + 1. (+100 in pessimistic-lock)
UPDATE LAST_IDS SET address_id= $newID WHERE address_id= $oldID?
commit the read-optimistic transaction.
if commit was successfull, store $newID to setID() in the HibernateBean you like to save.
Finally let Hibernate call the insert.
This is the only way i know.
BTW: Hibernate-Entitys shall only use inheritance if the Database support inheritance between tables like PostgreSQL or Oracle.

Because you use the TABLE identifier generator you need to have that table created. If you are not using the enhanced identifier generators, chances are you are going to use the MultipleHiLoPerTableGenerator.
The MultipleHiLoPerTableGenerator can use one table for all table identifier generators.
My suggestion is to grab the table ddl from your integration tests, in case you use hbmddl to build the test schema. If you use flyway or liquibase for testing, you can add a maven plugin to generate the ddl schema.
Once you have the schema, you need to take the exact create table command and make add it to your MySQL database.

Hibernate mapping a specific field to be loaded by a table join

Is there a way I can map a field in an hibernate object to be loaded with a table query?
As an example lets say Table_Message has fields id(int),message_key(varchar),message_content(Clob),language(varchar). This table will hold messages in different languages(locale).
And another table thats mapped to an entity using hibernate. Comments with fields id(int),comment_message_id(varchar),created_date(datetime). comment_message_id refers to Table_Message's message_key column.
EDIT: Table_Message is NOT a mapped Entity in hibernate
Assuming my comment class is
public class Comment
{
int id;
String message;
Date createdDate;
}
Is there a way to tell hibernate to load message by joining Comment table and Table_Message table by message_key with a default locale (for example 'en').
Basically is there a way to tell hibernate to load a field by running a specific query? And if so what is that way?
I know about writing a Custom SQL query for loading the entity. But since I'm using XDoclet there doesn't seem to be a way to do that. Also it will be very convenient if there's a way to do that for a single field.

I guess ResultTransformer may help you in this. Please check
http://docs.jboss.org/hibernate/orm/3.3/api/org/hibernate/transform/ResultTransformer.html
http://stackoverflow.com/questions/6423948/resulttransformer-in-hibernate-return-null

You must join the tables by comment_message_id with message_key and further filter the result by language. I assume the message_key is unique.
As a side notice: you should use integer keys to have better performance.
You can try to write a database view in SQL and create an entity to opaque the view:
CREATE VIEW Comment_Table_Message AS
SELECT c.id, c.comment_message_id, c.created_date, m.id AS mid,
m.message_content, m.language
FROM Comment c, Table_Message m
WHERE c.comment_message_id = t.message_key;
Now you can create an entity CommentTableMessage and use JPQL to filter results by language:
SELECT x FROM CommentTableMessage x WHERE x.language=?1
If Table_Message was a Hibernate entity you would write (in JPA terms):
#Entity
public class Comment
{
int id;
#ManyToOne()
#JoinColumn(name="comment_message_id")
TableMessage tableMessage;
String message;
Date createdDate;
}
#Entity
public class TableMessage {
int id;
#Id
String messageKey;
bytes[] messageContent; //I don't know how you want to deal with Blobs?
String language;
}
Having that you can write a simple JPA Query: (Can you use JPA ? - next assumption)
SELECT c FROM Comment c WHERE c.tableMessage.language=?1

JPA Query For Exists In

I have the following Entities (reduced and renamed for this example)
#Entity
public class Case {
#Id
private Long id;
#ManyToOne(optional=false)
private CourtConfiguration courtConfiguration;
#ElementCollection(fetch=FetchType.EAGER)
private List<String> caseNumbers;
}
Second Entity
#Entity
public class CourtConfiguration {
#Id
private Long id;
String countyId;
String referenceId;
....
}
I am trying to search using JPQL for all Cases that have a certain courtConfiguration countyId and have caseNumbers containing all of a provided set of important caseNumbers.
So my query needs the countyId and set of caseNumbers as parameters. Called countyId and importantCaseNumbers respectively.
I have tried and failed to get it to work.
My query looks like this
String query = "SELECT case FROM Case case JOIN case.caseNumbers caseNumbers WHERE ";
query += "case.caseConfiguration.countyId = :countyId ";
The bit above works until I add my caseNumber conditions.
I have tried a foreach importantNumbers to extend the query and as soon as the list of important numbers goes above one it doesn't work. No values get returned.
for (String importantCaseNum : importantCaseNumbers) {
query += " AND '"+importantCaseNum+"' in (caseNumbers)";
}
Any suggestions/pointers appreciated. I guess what I am looking for is a case.caseNumbers contains (importantNumbers) clause.
Update I have reverted to native SQL for my query as I didn't want to tie myself into hibernate by using HQL. Thanks to #soulcheck and #mikko for helping me out. I'll post up when the hibernate JPA fix is available.
Thanks
Paul

Syntactically correct way to build this JPQL query is with MEMBER OF. But because of problem reported in HHH-5209 it doesn't work with old Hibernate versions (fix version 4.1.8, 4.3.0.Beta1). According bug report HQL version of this query works, so your options includes at least:
Using JPQL query and switching to some other JPA implementation
Using HQL instead and sticking with Hibernate:
query += " AND '"+importantCaseNum+"' in elements(caseNumbers)";