I'm using Spring Boot 2.6.4 and Java 17. And I previously had an Entity called BlogPostComment but recently decided that just Comment is more concise. I don't have a data.sql file to explicitly create tables and let Hibernate handle all the database operations for me. So I'm expecting that the table previously named blog_post_comment would be renamed as comment. However, when I rerun my application after renaming the entity, Hibernate creates two tables blog_post_comment and comment instead of just the latter.
Before renaming:
#Entity
public class BlogPostComment { ... }
After renaming:
#Entity
public class Comment { ... }
I've tried adding #Table(name = "comment") annotation to this entity, but Hibernate created the table with the old name all the same. And I've also tried invalidating IntelliJ IDEA caches, still did not solve this problem. Please help me identify the cause of this error, thank you.
It is possible that your hibernate.hbm2ddl.auto property in application.properties is set to none . What none does is that no action is performed. The schema will not be generated. Hence your changes will appear as a new table in your database. What you should do then is to set the property to update and then run the application. What update does is that the database schema will be updated by comparing the existing database schema with the entity mappings.
PS: If no property is defined, the default property is none. You should add the property and set to update
I strongly doubt that Hibernate creates a blog_post_comment table after you renamed the entity. I suspect this is just still around from the previous run.
Hibernate (or any other JPA implementation) does not know about you renaming the entity. It has no knowledge what so ever about the entities present during the last start of the application. Therefore it doesn't know that there is a relationship between the existing blog_post_comment table in the database and the not yet present comment table it is about to create.
When Hibernate "updates" a schema it checks if a required table already exists and if so it modifies it to match what is required by the entities. But even then it won't rename columns, it would just create new ones.
In generally you should use Hibernates schema creation feature only during development and never for actually deploying a schema into production or even worse, updating a schema in production. For this you should use specialised tools like Flyway or Liquibase which exist for exactly this purpose.
Related
I am not sure about question since i am not familiar concept of migration exactly. I have just known this is used for updating database without deleting tables manually from database console. Since I have known this as I mentioned, I think like that, If I set this property to "create-drop", I can achieve migration. Am I correct? Can anyone explain it to me or advice any reference?
For the record, the spring.jpa.hibernate.ddl-auto property is Spring Data JPA specific and is their way to specify a value that will eventually be passed to Hibernate under the property it knows, hibernate.hbm2ddl.auto.
The values create, create-drop, validate, and update basically influence how the schema tool management will manipulate the database schema at startup.
For example, the update operation will query the JDBC driver's API to get the database metadata and then Hibernate compares the object model it creates based on reading your annotated classes or HBM XML mappings and will attempt to adjust the schema on-the-fly.
The update operation for example will attempt to add new columns, constraints, etc but will never remove a column or constraint that may have existed previously but no longer does as part of the object model from a prior run.
Typically in test case scenarios, you'll likely use create-drop so that you create your schema, your test case adds some mock data, you run your tests, and then during the test case cleanup, the schema objects are dropped, leaving an empty database.
In development, it's often common to see developers use update to automatically modify the schema to add new additions upon restart. But again understand, this does not remove a column or constraint that may exist from previous executions that is no longer necessary.
In production, it's often highly recommended you use none or simply don't specify this property. That is because it's common practice for DBAs to review migration scripts for database changes, particularly if your database is shared across multiple services and applications.
The possible values for the “spring.jpa.hibernate.ddl-auto” configuration property are the following ones:
none - No action is performed. The schema will not be generated.
create-only - The database schema will be generated.
drop - The database schema will be dropped.
create - The database schema will be dropped and created afterward.
create-drop - The database schema will be dropped and created afterward. Upon closing the SessionFactory, the database schema will be dropped.
validate - The database schema will be validated using the entity mappings.
update - The database schema will be updated by comparing the existing database schema with the entity mappings.
These are some of the basic things to be known,
validate: validate the schema, makes no changes to the database.
update: update the schema.
create: creates the schema, destroying previous data.
create-drop: drop the schema when the SessionFactory is closed explicitly, typically when the application is stopped.
none: does nothing with the schema, makes no changes to the database
These options seem intended to be developers tools and not to facilitate any production level databases.
Let's assume that we have java entities already implemented and annotated with Jpa annotations.
We also have an existing database slightly different to the schema described by said entities.
How I can link the data base with my entities without the code?
Otherwise, how can i proceed from the begining when implementing my entities to make this stuff configurable ( give the user the possiblity of specifying the names of the columns corresponding to the fields of each entity in an externalized configuration file)?
NB: I use hibernate as an ORM.
I believe this is what you are looking for
I have a small schema consisting of ~10 classes mapped by jpa2 with hibernate as provider. All of the classes are build in the same basic way (#Entity annotation for the class, id with #Id and #GeneratedValue). All classes have default constructors and default getter/setters.
All but one of the classes are in a relation to one another (via associations or inheritance). And this one class does not get a database table created in the database during the schema generation process.
I've tried the following settings with hbm2ddl.auto:
create: All tables but the one are created. When the entity is accessed for the first time an exception is thrown: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table '...' doesn't exist (or the oracle equivalent if the code is run on Oracle DB).
validate: hibernate complains about the missing table during schema validation.
update: The table does not get created and an exception (like with create) is thrown the first time an entity of that class is accessed.
Has anyone any idea on this?
Try the following: modify the class of that Entity to only have the id field. Make sure the class is annotated with #Entity at the id field is annotated with #Id. Make sure this entity is declared in the list of mapped-classes (or is in a package that is declared amongst the mapped packages). Add "TRACE" level logging to "org.hibernate" via log4j.xml. Start a hibernate session (start your app), and look in the log for the statements Hibernate generates for the creation of the schema. If the table is missing from there, then you're not configuring the bean correctly and hibernate is not considering it as being part of the model. If the sql for the creation of the corresponding table is there, but there's an error when it's executed, see what that SQL actually contains, this might help you find out what hibernate doesn't like. When the "empty" bean is finally created, start adding the properties/associations to it one at a time and redo the same check as before. At some point you'll get an error when you add a certain something, and you need to look for a solution from there.
Stupid, stupid me. I have used an sql-keyword (update) as a name for a member-variable of the class.
I assumed the ORA-01747 message wanted to tell me about a failing select, but instead a closer look revealed that it actually pointed out as to why the table was not created at all.
I use JPA 2 with Hibernate Entity Manager 3.6.4. Once I have marked my entities with various annotations (#Entity, #MappedSuperClass etc), I put in my persistence.xml file the default schema to use (hibernate.default_schema property).
I know it's possible to create automatically the objects contained in the schema.
But is it possible to create the schema itself automatically and then create the objects it contains ?
EDIT :
I use this parameter too : hibernate.hbm2ddl.auto, to tell Hibernate to create the schema if it doesn't exists yet. No luck, Hibernate doesn't create it !
I have googled a little bit and find this post : Hibernate hbm2ddl won't create schema before creating tables.
The fact that Hibernate does not create a schema before creating table is a bug. Other database suffer from this situation : H2, Postgresql etc.
This bug is planned to be fixed with 5.0.0 release of Hibernate.
So, for now, the only workaround is to create the schema by yourself, either manually or by a mean offered by your database vendor, since Hibernate can't do it itself :\
I managed to build a workaround that uses the hbm2ddl default flow.
Since it always calls the "database-object" drop statements BEFORE creating schema, you can do something like this:
<database-object>
<create></create>
<drop>DROP SCHEMA IF EXISTS myschema cascade; CREATE SCHEMA myschema</drop>
</database-object>
unfortunately the create clause is mandatory and sadly it's only executed AFTER schema creation, no matter what order you put it on cfg.xml, so I made it empty, that way you don't have errors trying to creating schema again (it was already created together with drop)
I annotated a bunch of POJO's so JPA can use them to create tables in Hibernate. It appears that all of the tables are created except one very central table called "Revision". The Revision class has an #Entity(name="RevisionT") annotation so it will be renamed to RevisionT so there is not a conflict with any reserved words in MySQL (the target database).
I delete the entire database, recreate it and basically open and close a JPA session. All the tables seem to get recreated without a problem.
Why would a single table be missing from the created schema? What instrumentation can be used to see what Hibernate is producing and which errors?
Thanks.
UPDATE: I tried to create as a Derby DB and it was successful. However, one of the fields has a a name of "index". I use #org.hibernate.annotations.IndexColumn to specify the name to something other than a reserved word. However, the column is always called "index" when it is created.
Here's a sample of the suspect annotations.
#ManyToOne
#JoinColumn(name="MasterTopID")
#IndexColumn(name="Cx3tHApe")
protected MasterTop masterTop;
Instead of creating MasterTop.Cx3tHApe as a field, it creates MasterTop.Index. Why is the name ignored?
In case this helps anybody, this happened to me today and it turned out I was using a reserved word on my entity definition:
#OneToMany(mappedBy="list")
#OrderColumn(name="order")
private List<Wish> wishes;
"order" in this case, and Hibernate just skipped over this class. So check your user defined names! :)
Cheers,
Mark
Answer to your side question (What instrumentation can be used to see what Hibernate is producing and which errors?)
You can org.hibernate.tool.hbm2ddl.SchemaExport to generate your tables.
AnnotationConfiguration conf = (new AnnotationConfiguration()).configure();
new SchemaExport(conf).create(showHql, run);
The first argument allows you to see which HQL this command generates (CREATE TABLEs etc). The second one is whether it should actually perform any modifications (ie false = dry-run).
So running it with (true, false) will show you exactly what Hibernate would do to your tables, without changing anything.
name attribute on #Entity is not what you want to use for this purpose. Use #Table annotation instead:
#Entity
#Table(name="RevisionT")
public class Revision {
It is due to column names matches with your underlying Database sql reserved words.....try by changing name of columns.....i had faced same problem by changing names it did work.
For me it was a syntax mistake in columnDefinition. Easily detectable by the debug logs.
Perhaps you're using the wrong annotation. Once I accidentally annotated an entity with #org.hibernate.annotations.Entity instead of #javax.persistence.Entity, and Hibernate just skipped it.
Put hibernate.show_sql config property to true and see if Hibernate generated the create table code. If so, copy the create statement into your database client (ui or command line) and see if your database returns an error.
For me the error was not apparent until I did so.
I used the field name 'condition' which was a reserved MySQL word.
So check your field names...