Jpa failed to use update() when encountered existing record to save - spring

Question

Mysql database table structure:
database table structure
Customer.java class:
#JsonIgnore
#OneToMany(mappedBy = "customer")
private Set<OrderLine> orderLine = new HashSet<>();
Orderline.java class:
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name = "id")
private Long id;
#ManyToOne(cascade=CascadeType.ALL)
#JoinColumns({
#JoinColumn(name = "customer_id", referencedColumnName = "customer_id")})
private Customer customer;
Saving data through OrderLineController class:
#PostMapping("/orderLines")
public ResponseEntity<OrderLine> createOrderLine(#RequestBody OrderLine orderLine) {
OrderLine _orderLine = orderLineRepository.save(orderLine);
return new ResponseEntity<>(_orderLine, HttpStatus.CREATED);
}
Here is my Test file:
OrderLine newOL = new OrderLine(order, product);
newOL.setQty(1);
newOL.setCustomer(customer);
ResponseEntity<OrderLine> respOL = restTemplate.postForEntity("/api/orderLines", newOL, OrderLine.class);
assertEquals(HttpStatus.CREATED, respOL.getStatusCode());
I have previously populated an existing data in the Customer table, and now I tried to create new purchase order based on the existing customer.
In my Test file, when the save() function is executed, Jpa sees the "customer" as a new customer, but it was actually returned from the previous method:
ResponseEntity<Customer> getCustomersByCustomerId(String customerId)
Therefore trying to add a record with the same customer_id produced error, but indeed I was hoping the JpaRepository could figured out the customer_id is the same as the existing customer record, but it doesn't. The Jpa repository tried to insert this record into the customer table, thus resulting Duplicate entry error: I have been searching for possible solution for the past 5 days, but the issue still persist. Your help is much appreciated.
2022-10-06 23:36:23.722 DEBUG 16525 --- [o-auto-1-exec-3] org.hibernate.SQL :
insert
into
customer
(email_address, first_name, last_name, phone, customer_id)
values
(?, ?, ?, ?, ?)
Hibernate:
insert
into
customer
(email_address, first_name, last_name, phone, customer_id)
values
(?, ?, ?, ?, ?)
2022-10-06 23:36:23.726 TRACE 16525 --- [o-auto-1-exec-3] o.h.type.descriptor.sql.BasicBinder : binding parameter [1] as [VARCHAR] - [sammsonwonder#gmail.com]
2022-10-06 23:36:23.726 TRACE 16525 --- [o-auto-1-exec-3] o.h.type.descriptor.sql.BasicBinder : binding parameter [2] as [VARCHAR] - [sammson]
2022-10-06 23:36:23.726 TRACE 16525 --- [o-auto-1-exec-3] o.h.type.descriptor.sql.BasicBinder : binding parameter [3] as [VARCHAR] - [wonder]
2022-10-06 23:36:23.726 TRACE 16525 --- [o-auto-1-exec-3] o.h.type.descriptor.sql.BasicBinder : binding parameter [4] as [VARCHAR] - [0433233333]
2022-10-06 23:36:23.726 TRACE 16525 --- [o-auto-1-exec-3] o.h.type.descriptor.sql.BasicBinder : binding parameter [5] as [VARCHAR] - [0433233333]
2022-10-06 23:36:23.738 WARN 16525 --- [o-auto-1-exec-3] o.h.engine.jdbc.spi.SqlExceptionHelper : SQL Error: 1062, SQLState: 23000
2022-10-06 23:36:23.739 ERROR 16525 --- [o-auto-1-exec-3] o.h.engine.jdbc.spi.SqlExceptionHelper : Duplicate entry '0433233333' for key 'customer.PRIMARY'
2022-10-06 23:36:23.744 INFO 16525 --- [o-auto-1-exec-3] o.h.e.j.b.internal.AbstractBatchImpl : HHH000010: On release of batch it still contained JDBC statements
[ERROR] Tests run: 6, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 2.886 s <<< FAILURE! - in dorinca.mm.rest.SavingsTest
[ERROR] create_1st_order_and_1x_product_for_existing_customer Time elapsed: 0.723 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <201 CREATED> but was: <500 INTERNAL_SERVER_ERROR>
at dorinca.mm.rest.SavingsTest.create_1st_order_and_1x_product_for_existing_customer(SavingsTest.java:179)