I have one question that if we are creating any table in database i.e. not having any primary key(all columns values are nullable).
can't we do mapping without ID field in Hibernate O/R Mapping i.e. .hbm.xml file while working with hibernate.
The problem which i am facing is my .hbm.xml file without ID field is not getting validated.
I got the answers from u all, but i have one question that can we use transient keyword prefixed to ID variable in the entity class so that not to persist that value into database?
The only real solution is to fix your broken data model. An entity that does not have a primary key is not relational data, so you can't expect an ORM product to be able to know how to handle it.
And how Hibernate will perform update without a primary key or an uniquely identified row?
Thanks.
You cannot do that in hibernate. And you shouldn't do it anyway. Databases warn you that it is not advisable not to have a primary key.
If you really don't have a set of columns that form a unique key, you have two options:
add an auto-increment primary key (preferred)
use raw JDBC to get the data from that particular table. You can do that with hibernate's session.doWork(..) method.
Related
l have a table without a Primary Key or Unique Key column. I need to manipulate this table from the Spring application by using JPA and Hibernate.
While I was trying to map this table’s entity, it fails because there's no #Id mapping.
I tried to make a composite id from all the entity properties to ensure uniqueness, but in this case, the entity has returned an immutable object.
I need to manipulate some variables of this entity. What should I do? To alter the table by adding a unique column will bring me new challanges. Is there a way without altering table?
Let's assume you wanted to change those records with SQL. How would you know which row to update or delete if you don't have a unique column?
Unless you create a unique column, there's nothing you can do about it.
There should be a unique way to identify your records. Sometimes, you have multiple columns. In this case, you use a composed primary key. In JPA, you can implement a composed primary key in two ways: #EmbeddedId or #IdClass.
I am looking for an answer if it is possible or not in hibernate.
What I am trying to achieve is that if a particular table exists in the DB then the application should do all the regular operations with it (which exists in the code - find,save.. etc.).
Else just ignore the table (#Repository) and the fields in the #Entity class, and will skip all the related code.
I have the same question regarding ignoring particular non-existing field of an existing table and the field is annotated with #Column, is it possible to ignore the field if it does not exist in table?
I want to use save method of JPA but which can ignore that field if needed.
That's impossible because with Hibernate you map a Class to table and the table and all the mapped columns MUST exist.
What you are trying to do is dynamic SQL and there you would need to read the database dictonary to check what exsits and generate the code during runtime.
I know that we can get the primary key of the newly inserted record with object.getId() when we do hibernate session.save(object). I want to understand how is hibernate getting this id. I want to achieve the same using a plain SQL query instead of using hibernate.
DB Server is MySQL.
I agree with #JB Nizet about generation: it depends on your annotation and, in any case, the id is assigned to your bean at the commit phase not before so the persistent provider must read the new generated id from database.
Many databases offer a mechanism for doing what is required. MySQL provides a function LAST_INSERT_ID(). More details here or there
I'm working on a legacy application which uses Hibernate and MySQL. In one of my DB tables, I've found duplicate foreign key constraints. Names are like the following:
FK3EBE45E8C4027E24
FK3EBE45E8F5ADD75E
Now I want to drop one index and rename another one from database only. Will there be any impact on hibernate functionalities?
No
There will not be any impact on the Hibernate code. Only when you make changes to the structure of the table - add/remove/rename a column, change the datatype, then there will be an impact as you will have to make changes to the DTO. MySQL Indexes are abstractions for Hibernate. Hibernate doesn't care whether there's an index or not. It will create a query and send to the database.
Renaming a constraint will be impact only on automatic schema update (create). Hibernate will try to delete constraint by name and generate an exception. It is not a problem (for Hibernate 5, don't know about other versions), a schema update will not stop.
If you don't use automatic schema update, you will not have any problems.
I have come across a hibernate hbm configuration where id attribute is mapped to a column in table which is not the primary key. The mapping is just used for read and not update. Though this works for most of the cases, I have found some cases where , when queried using get() on this mapping , it throws org.hibernate.HibernateException: More than one row with the given identifier was found
So my question is :
If I define a hbm with id mapped to non-primary key in table, can it allow more than two records with same id ?
At what layer is the primary key constraint checked, hibernate or database ?
Thanks,
Gaurav.
It will work if the column has all the features of a PK:
unique among all the rows of the table
immutable
The constraint is enforced at the database level. But Hibernate needs its ID to be unique. So it will choke if two rows have the same ID in database and are loaded in the same session.
I can't see any good reason to use a non-PK column as the ID of an entity, except if you're forced to map a legacy table, which can't change, and doesn't have any PK defined, even if one column or set of columns could be defined as the PK.