JPA OneToOne Mapping doesn't update child but instead inserts a new one

RuleEntity

@Entity
@Table(name = "rules")
public class RuleEntity {
	@Id
	@GeneratedValue(strategy=GenerationType.IDENTITY)
	private Integer RuleId;
	
	@Column(name = "rule_expression")
	private String RuleExpression;
	
	@Column(name = "rule_frequncy")
	private Integer RuleFrequency;

	@OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
	@JoinColumn(name="alert_id")
	private AlertEntity alertEntity;
}

AlertEntity

@Entity
@Table(name = "alerts")
public class AlertEntity {
	@Id
	@GeneratedValue(strategy=GenerationType.IDENTITY)
	@Column(name = "alert_id")
	private Integer alertId;

	@Column(name = "action")
	private String alertAction;
}

RuleRepository

@Repository
@Transactional
public interface RuleRepository extends JpaRepository<RuleEntity, Integer>{}

Code

RuleEntity ruleEntity= new RuleEntity();
AlertEntity alertEntity = new AlertEntity();
alertEntity.setAlertAction(data.getAlertAction());
ruleEntity.setAlertEntity(alertEntity);
if(newRule) ruleEntity.setRuleId(data.getRuleId());
ruleEntity.setRuleExpression(data.getRuleExpression());
ruleEntity.setRuleFrequency(data.getRuleFrequency());
ruleRepository.save(ruleEntity);

When I use ruleRepository.save(ruleEntity) and don't provide ruleEntity.RuleId a new rule and alert are inserted and alert primary key is provided in alert_id column of the rules table.

If I provide ruleEntity.RuleId and save ruleEntity then data associated with RuleId gets updated but new row gets inserted in alerts table.

What I want is to update existing alert by using id from rules table. How to achieve this?

Instead of this logic:

if(newRule) ruleEntity.setRuleId(data.getRuleId());

you should try to fetch the possibly existing rule, something like:

RuleEntity ruleEntity;
if(newRule) {
    ruleEntity = new RuleEntity();
} else {
    ruleEntity = ruleRepository.findById(data.getRuleId())
}

Now depending on if ruleEntity has alertEntity already you just getAlertEntity() and update it before saving. If not you just create a new one and then setAlertEntity(aNewAlertEntity) with values from data before saving.

Note that this is simplified example and you need to take care of possible Optional chacks and so on but the basic idea should be clear.