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Hibernate does not allow to save data with similar id

I'm learning Spring and few days ago i started learning Hibernate. I have my studying project where i need to create a shop with products. Here is entity class
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import javax.persistence.*;
#Entity
#Table(name = "cart")
#Data
#NoArgsConstructor
#AllArgsConstructor
public class Cart {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private int id;
#Column(name = "quantity")
private int quantity;
#Column(name = "mask")
private String mask;
#Column(name = "price")
private int price;
So, i create interface
import org.springframework.data.jpa.repository.JpaRepository;
public interface CartRepository extends JpaRepository<Cart, Integer> {
}
and create controllers
#Autowired
CartRepository cartList;
#RequestMapping("/add-black-mask")
public String addBlackMask() {
cartList.save(new Cart(1, 1, "black", 3));
return "masks/add-black-mask";
}
#RequestMapping("/add-build-mask")
public String addBuildMask() {
cartList.save(new Cart(2, 1, "build", 5));
return "masks/add-build-mask";
}
#RequestMapping(value = "/save_product_to_cart")
public ModelAndView saveProduct(#ModelAttribute(value = "cart")
Cart cart, BindingResult result) {
int index = Integer.parseInt(String.valueOf(cartList.count()));
if (cart.getId() == 0) {
cart.setId(index + 1);
cartList.save(cart);
} else {
Cart cart1 = cartList.getOne(cart.getId());
cart1.setMask(cart.getMask());
cart1.setQuantity(cart.getQuantity());
cart1.setPrice(cart.getPrice());
cartList.save(cart1);
}
return new ModelAndView("redirect:/");
}
Also, there are some other controllers for view, Thymeleaf etc, but its ok. My problem is - when i save my product 1 time - its ok, but when i save second - it didnt work( i think because i can't save 2 rows with similar ID) So it seems i have UNIQUE ID in my table and it can not be repeated. Question is - how can i delete unique id or change my code in any way? Thanks in advance!
p.s. i read some other topics here but it didnt help me.

when you use GENERATIONTYPE.IDENTITY you are asking hibernate to let the database handle Ids for you, you should not set it yourself. you are changing the value of the id, just create a new product, set all the fields and inside a transactional context, save your product. Also always use wrapped versions of primitives for serialization purposes. (Long is an object but long is a primitive.) you can google boxing and unboxing and learn more about this.

Let me answer this question:
First of all, using annotations as our configure method is just a convenient method instead of coping the endless XML configuration file.
The #Idannotation is inherited from javax.persistence.Id， indicating the member field below is the primary key of current entity. Hence your Hibernate and spring framework as well as you can do some reflect works based on this annotation. for details please check javadoc for Id
The #GeneratedValue annotation is to configure the way of increment of the specified column(field). For example when using Mysql, you may specify auto_increment in the definition of table to make it self-incremental, and then use
#GeneratedValue(strategy = GenerationType.IDENTITY)
in the Java code to denote that you also acknowledged to use this database server side strategy. Also, you may change the value in this annotation to fit different requirements.
1. Define Sequence in database
For instance, Oracle has to use sequence as increment method, say we create a sequence in Oracle:
create sequence oracle_seq;
2. Refer the database sequence
Now that we have the sequence in database, but we need to establish the relation between Java and DB, by using #SequenceGenerator:
#SequenceGenerator(name="seq",sequenceName="oracle_seq")
sequenceName is the real name of a sequence in Oracle, name is what you want to call it in Java. You need to specify sequenceName if it is different from name, otherwise just use name. I usually ignore sequenceName to save my time.
3. Use sequence in Java
Finally, it is time to make use this sequence in Java. Just add #GeneratedValue:
#GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")
The generator field refers to which sequence generator you want to use. Notice it is not the real sequence name in DB, but the name you specified in name field of SequenceGenerator.
4. Complete
So the complete version should be like this:
public class Cart
{
#Id
#SequenceGenerator(name="seq",sequenceName="oracle_seq")
#GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")
private Integer id;
}
Now start using these annotations to make your JavaWeb development easier.
On top of that I would like you to understand all 4 ways of ID generation in hibernate. You can think of reading in your free time
GenerationType.AUTO
GenerationType.IDENTITY (your case)
GenerationType.SEQUENCE
GenerationType.TABLE {Rarely used nowdays}

Using JPA to access a view without a unique id returns null

I am using JPA and have a view I would like to access. I am using a mapped entity with an embedded Id in many of my other classes to access tables with similar requirements. However here whenever there are nulls in the view that comprise the id, the whole object is returned as null. There are the right number of entities returned when i query, but they are null.
Here are the classes:
{
#Entity
#Table(name = "VW_PRODUCT")
public class VwProduct implements Serializable {
#EmbeddedId
private VwProductId id;
public VwProduct() {
}
}
{
#Embeddable
public class VwProductId implements java.io.Serializable {
#Column(name = "PROD_NAME", nullable=true)
private String prodName;
#Column(name = "PROD_CTGRY", nullable=true)
private String prodCtgry;
#Column(name = "PROD_SBCTGRY", nullable=true)
private String prodSbctgry;
}
I omitted things like getters and setters and hashcode but i think my question is clear; how do I access this view, when some of its values are null?
Thank you!

If you have a primary key column with null, and search on that column will always return no objects.
There are three possible solutions that I am aware of, in order of complexity/wonkiness. (For people not working on a read-only view, do not do any of the following. You will blow your foot off with an incredibly large shotgun.)
The easiest answer is to change your definition of the view and add something like a rowid or generated serial and then make that the primary key.
The second answer, and this is both implementation specific and hibernate specific, is to have a primary key of #ID #ROWID String id;
The last answer, is more complex, is by mapping all three of your fields to a "NullableString" UserType, and have a nullSafeGet that maps null to something non-null, like '' or something. You'll get duplicates, but since this is a read-only view, you don't really care.

How to return an entity by property if it already exists?

#Entity
public class Language {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
#Column(length = 2)
private String code; //EN, DE, US
public Language(String code) {
this.code = code;
}
}
#Entity
public class ProductText {
#OneToOne(Cascade.ALL)
private Language lang;
}
ProductText text = new ProductText();
text.setLang(new Language("en")); //what if "en" exists?
dao.save(text);
Now, when I persist the ProductText, everytime a new Language object would be generated.
Can I prevent this, and in case a language table entry with code = 'en' exists this existing entity should be linked instead.
My initial goal is to not having to repeat the countryCodeString "EN" multiple times in my product-text table, but just reference the id. But does this really make sense? Should I rather just use the plain String without an extra table? (I later want to query a list of productTexts where lang = 'de').
Is the only change executing a select like dao.findByLang("en") before?
Or is there also some hibernate feature that would support this without explicit executing a query myself?

Do you process the value "en" further or do you display it directly? If only used for displaying purposes, I would just store the string, but if you want to reduce redundancy by using foreign key IDs you have to create an Entity containing the language string en which can be persisted via entity manager and which you have to obtain out of the entity manager before persisting to reuse it.

If there is only three different possible values for the language, you can also use an enum like this :
public enum Language {
EN("EN"),
DE("DE"),
US("US");
private String code; //EN, DE, US
public Language(String code) {
this.code = code;
}
// Getter...
}
#Entity
public class ProductText {
#Enumerated(EnumType.STRING)
// Or #Enumerated(EnumType.ORDINAL)
private Language lang;
}
EnumType.STRING will store the enum in the database as a String, while EnumType.ORDINAL will store it as an int. Int is maybe a little more efficient, but the mapping could change if you insert a new value in your enum. String is more flexible since it will use the names of your enum members.
In both case, you don't have to manage a separate entity and hibernate will not create an additional table, and it's more type-safe than using a plain string.

If the only value in Language is a 2 or 3 letter string, why not just have the string as a member? This will be quicker and more efficient.

Is there any Simplest way to get Table metadata (column name list) information, in Spring Data JPA ? which could I use on universal database

I am in a situation where I want to get all table's column list using spring data jpa, my database is flexible so, the query should be work on all kind of database.

JPA specification contains the Metamodel API that allows you to query information about the managed types and their managed fields. It does not however cover the underlying database. So, there is nothing out-of-the-box in JPA yet for querying the database metadata.
The way each RDBMS stores meta information is also different so there cannot be a simple, database-agnostic solution.
What you want can however be achieved through a few hops.
Step 1: Define an entity class that will hold metadata information.
#Entity
#IdClass(TableMetadataKey.class)
#Table(name = "table_metadata")
class TableMetadata {
#Column(name = "column_name")
#Id
String columnName;
#Column(name = "table_name")
#Id
String tableName;
public static class TableMetadataKey implements Serializable {
String columnName;
String tableName;
}
}
Step 2: Add the repository for the entity.
public interface TableMetadataRepository extends JpaRepository<TableMetadata, TableMetadataKey>
{
TableMetadata findByTableName(String tableName);
}
Step 3: Define a database view named table_metadata to be mapped to the entity class. This will have to be defined using a database-specific query (because each database has a different way of storing its metadata).
Database-specific optimizations can be performed on this step, such as, using materialized views with Oracle for faster access, etc.
Alternatively, a table named table_metadata can be created with the required columns and populated periodically using a SQL script.
Now the application has full access to the required metadata.
List<TableMetadata> metadata = tableMetadataRepository.findAll()
TableMetadata metadata = tableMetadataRepository.findByTableName("myTable");
One issue to be noted is that not all tables in a schema may be mapped as JPA entities or not all columns in all tables may be mapped as entity fields. Therefore, directly querying the database metadata may give results that do not match the entity classes and fields.

You can get the Column name list using your Entity or Model. What we need is #Column, which should be used in your Entity. You will get all the details which you have specified in #Column. All the parameters are Optional, although it is good to define all.
#Column(name, columnDefinition, insertable, length, nullable,
precision, scale, table, unique, updatable)
We can get all fields declared in Entity by User.class.getDeclaredFields() ( in general ModelName.class.getDeclaredFields()). After getting all feilds we can get particular Column using field.getAnnotation(Column.class) we can also get all the details specified in #Column as below
Columns: #javax.persistence.Column(nullable=false, precision=2, unique=true, name=id, length=2, scale=1, updatable=false, columnDefinition=, table=, insertable=true)
Columns: #javax.persistence.Column(nullable=true, precision=0, unique=false, name=client_id, length=255, scale=0, updatable=true, columnDefinition=, table=, insertable=true)
Columns: #javax.persistence.Column(nullable=true, precision=0, unique=false, name=firstname, length=255, scale=0, updatable=true, columnDefinition=, table=, insertable=true)
Columns: #javax.persistence.Column(nullable=true, precision=0, unique=false, name=lastname, length=255, scale=0, updatable=true, columnDefinition=, table=, insertable=true)
create endPoint or method as per requirement
#GetMapping(value= "/columns/name")
public List<String> tableColumnsName()
{
List<String> Columns = new ArrayList<String>();
Field[] fields = User.class.getDeclaredFields();
for (Field field : fields) {
Column col = field.getAnnotation(Column.class);
if (col != null) {
Columns.add(col.name());
System.out.println("Columns: "+col);
}
}
return Columns;
}
Entity/Model
#Entity
#Table
public class User {
#Id
#GeneratedValue(strategy=GenerationType.SEQUENCE)
#Column(name="id")
public int id;
#Column(name="client_id")
private int clientId;
#Column(name="firstname")
private String firstname;
#Column(name="lastname")
private String lastname;
//AllArgConstructor-Constructor
//Getters-Setters
}
Tested via Postman

SchemaCrawler has a Java API that allows you to work with database metadata in a general way, this is, without caring about the specific database platform.
http://www.schemacrawler.com

The above solution works for "simple primary key". But for "composite primary key", the solution is mentioned below
BuidingKey --> Composite Primary key
#GetMapping("/columns")
public List<String> getColumns() {
List<String> entityColumns = Arrays.asList(Building.class.getDeclaredFields()).stream().map(Field::getName)
.collect(Collectors.toList());
List<String> entityCompositePKColumns = Arrays.asList(BuidingKey.class.getDeclaredFields()).stream().map(Field::getName)
.collect(Collectors.toList());
entityCompositePKColumns.addAll(entityColumns);
return entityCompositePKColumns;
}

JPA primary key value is always 0

I have a post class and it kind of works, but there's one problem: the primary key doesn't increase.
#Entity
#Table(name="posts")
public class Post extends GenericModel{
#Id
#Column(name="post_id")
public int id;
#Column(name="post_situation")
public String situation;
#Column(name="post_date")
public Date date;
#Column(name="post_userid")
public int userid;
#OneToMany(mappedBy="post", cascade=CascadeType.ALL)
public List<Block> blocks;
public Post addBlock(String content, int position){
Block b = new Block(this, content, position);
b.save();
this.blocks.add(b);
this.save();
return this;
}
public Post(String situation, Date date){
this.situation = situation;
this.date = date;
this.userid = 2;
}
}
When I call it the first time on an empty table, it works fine, but the second time, I'm getting PersistenceException occured : org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
The post_id column always has 0. Any idea how to fix this? I have the #Id annotation in palce..
This is how I have in my controller:
Post p = new Post("Midden in het middenoosten.", new Date()).save();
Any ideas what's causing this problem?

It seems that you want the primary key values to be auto-generated. If that is the case, if you'll need to add the #GeneratedValue annotation to the id attribute, in addition to the #Id annotation. Your code should therefore be:
#Id
#Column(name="post_id")
#GeneratedValue
public int id;
There are several strategies available to generate the Ids. You would have to read up on those to decide if you want to choose the TABLE -based, SEQUENCE -based or the IDENTITY -based strategy (which depends on what your database supports). If you choose a strategy explicitly, the define strategy will be used, instead of the default AUTO strategy. Explicit strategy decisions, are communicated in code as:
#Id
#Column(name="post_id")
#GeneratedValue(strategy=SEQUENCE, generator="POST_SEQ")
public int id;
Without generated values, the default value for integers in Java, i.e. 0 will be persisted for the post_id column. Due to the primary key constraint, you cannot have a second row with the same key, resulting in the described failure.

There are several strategies available to generate id:
GenerationType.AUTO
GenerationType.SEQUENCE
GenerationType.IDENTITY
GenerationType.TABLE
If you want the primary key values to be auto-generated, use GenerationType.AUTO, it works with MySQL.