Working example

src/main/java/com/RESTAPI/RESTAPI/Controllers/UserController.java

 package com.RESTAPI.RESTAPI.Controllers;
 
-import com.RESTAPI.RESTAPI.Entities.Compte;
 import com.RESTAPI.RESTAPI.Entities.User;
+import com.RESTAPI.RESTAPI.Repositories.CompteRepository;
 import com.RESTAPI.RESTAPI.Repositories.UserRepository;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.http.HttpStatus;
@@ -14,10 +14,14 @@ import org.springframework.web.bind.annotation.*;
 public class UserController {
     @Autowired
     private UserRepository userRepository;
+    @Autowired
+    private CompteRepository compteRepository;
 
     @PutMapping(path="/")
     public @ResponseBody
-    User addNewUser (@ModelAttribute User user, @ModelAttribute Compte compte) {
+    User addNewUser (@RequestBody User user) {
+        compteRepository.save(user.getCompte());
+        user.setCompte(user.getCompte());
         userRepository.save(user);
         return user;
     }

src/main/java/com/RESTAPI/RESTAPI/Entities/Compte.java

 package com.RESTAPI.RESTAPI.Entities;
 
-import lombok.Builder;
-import lombok.Getter;
-import lombok.Setter;
+import lombok.*;
 
 import javax.persistence.Entity;
 import javax.persistence.GeneratedValue;
@@ -14,6 +12,8 @@ import java.util.Date;
 @Getter
 @Setter
 @Builder
+@NoArgsConstructor
+@AllArgsConstructor
 public class Compte {
     @Id
     @GeneratedValue(strategy= GenerationType.IDENTITY)