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To save a customer DTO with multiple phone numbers in a one-to-many relationship using a POST request, you can follow these steps:

Define the Customer DTO with the necessary fields, including an ID, name, email, and a list of Phone DTOs.
java
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public class CustomerDto {
   private Long id;
   private String name;
   private String email;
   private List<PhoneDto> phones;
   // getters and setters
}

public class PhoneDto {
   private Long id;
   private String number;
   // getters and setters
}
Create a RESTful API endpoint that accepts a POST request with the Customer DTO in the request body. This endpoint should have a URL that uniquely identifies it, for example, /customers.
java
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@PostMapping("/customers")
public ResponseEntity<CustomerDto> createCustomer(@RequestBody CustomerDto customerDto) {
   // Implementation details
}
In the implementation of the POST endpoint, you can validate the input data and save the Customer entity to your database or another data store. If the customer has multiple phone numbers, you can save them as separate Phone entities and associate them with the customer.
java
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@PostMapping("/customers")
public ResponseEntity<CustomerDto> createCustomer(@RequestBody CustomerDto customerDto) {
   // Validate input data
   if (customerDto == null || customerDto.getName() == null || customerDto.getEmail() == null) {
       return ResponseEntity.badRequest().build();
   }

   // Create Customer entity
   Customer customer = new Customer();
   customer.setName(customerDto.getName());
   customer.setEmail(customerDto.getEmail());

   // Create Phone entities
   List<Phone> phones = new ArrayList<>();
   for (PhoneDto phoneDto : customerDto.getPhones()) {
       Phone phone = new Phone();
       phone.setNumber(phoneDto.getNumber());
       phones.add(phone);
   }

   // Associate Phones with Customer
   customer.setPhones(phones);

   // Save Customer to database or data store
   customer = customerService.save(customer);

   // Convert Customer entity back to DTO and return it in the response
   CustomerDto savedCustomerDto = new CustomerDto();
   savedCustomerDto.setId(customer.getId());
   savedCustomerDto.setName(customer.getName());
   savedCustomerDto.setEmail(customer.getEmail());
   savedCustomerDto.setPhones(customer.getPhones().stream()
           .map(phone -> {
               PhoneDto phoneDto = new PhoneDto();
               phoneDto.setId(phone.getId());
               phoneDto.setNumber(phone.getNumber());
               return phoneDto;
           }).collect(Collectors.toList()));

   return ResponseEntity.ok(savedCustomerDto);
}
Finally, you can test the POST endpoint using a tool like Postman or cURL, sending a request with the JSON representation of the Customer DTO in the request body.
json
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{
   "name": "John Doe",
   "email": "johndoe@example.com",
   "phones": [
       {"number": "123-456-7890"},
       {"number": "234-567-8901"}
   ]
}
Note that in this example, the Customer and Phone entities are assumed to have their own ID fields generated by the database or data store, so we don't set them explicitly in the DTO or the implementation.