Промежуточное сохранение. Надеюсь, что будет работать

spring_online_calculator/src/main/java/premium_store/model/GameClient.java

@@ -12,12 +12,13 @@ public class GameClient {
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
 
-    @Column(nullable = false, length = 255)
+    @Column(name = "nickName", nullable = false, length = 255)
     private String nickName;
 
-    @Column(nullable = false, length = 255)
+    @Column(name = "email", nullable = false, length = 255)
     private String email;
 
+    @Column(name = "balance")
     private Integer balance;
 
     @ManyToMany(mappedBy = "gameClients", fetch= FetchType.EAGER)

spring_online_calculator/src/main/java/premium_store/model/Nation.java

@@ -1,8 +1,7 @@
 package premium_store.model;
 
 import javax.persistence.*;
-import java.util.Objects;
-import java.util.Set;
+import java.util.*;
 
 @Entity
 @Table(name = "NATIONS")
@@ -11,8 +10,12 @@ public class Nation {
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
 
+    @Column(name = "nation")
     private String nation;
 
+    @OneToMany(mappedBy ="nation", cascade = CascadeType.MERGE,fetch = FetchType.EAGER)
+    private List<Tank> tanks = new ArrayList<>();
+
     public Nation() {
     }
 
@@ -34,6 +37,15 @@ public class Nation {
         this.nation = nation;
     }
 
+    public List<Tank> getTanks(){
+        return tanks;
+    }
+
+    public void addNewTank(Tank tank) {
+        tanks.add(tank);
+        tank.setNation(this);
+    }
+
     //метод для сравнения
     @Override
     public boolean equals(Object o) {

spring_online_calculator/src/main/java/premium_store/model/Tank.java

@@ -13,16 +13,16 @@ public class Tank {
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
 
+    @Column(name = "name")
     private String name;
 
-    @ManyToOne
-    @JoinColumn(name = "nation")
+    @ManyToOne(fetch = FetchType.EAGER, cascade = CascadeType.MERGE)
     private Nation nation;
 
-    @ManyToOne
-    @JoinColumn(name = "tankLevel")
+    @ManyToOne(fetch = FetchType.EAGER, cascade = CascadeType.MERGE)
     private TankLevel tankLevel;
 
+    @Column(name = "cost")
     private int cost;
 
     //реализация двунаправленной связи многие-ко-многим

spring_online_calculator/src/main/java/premium_store/model/TankLevel.java

@@ -1,8 +1,7 @@
 package premium_store.model;
 
 import javax.persistence.*;
-import java.util.Objects;
-import java.util.Set;
+import java.util.*;
 
 @Entity
 @Table(name = "TANKLEVELS")
@@ -11,9 +10,12 @@ public class TankLevel {
     @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
 
-    @Column(nullable = false)
+    @Column(name = "level", nullable = false)
     private int level;
 
+    @OneToMany(mappedBy = "tankLevel", cascade = CascadeType.MERGE, fetch = FetchType.EAGER)
+    private List<Tank> tanksOfThisLevel = new ArrayList<>();
+
     public TankLevel() {
     }
 
@@ -35,6 +37,15 @@ public class TankLevel {
         this.level = level;
     }
 
+    public List<Tank> getTanks(){
+        return tanksOfThisLevel;
+    }
+
+    public void addNewTank(Tank tank) {
+        tanksOfThisLevel.add(tank);
+        tank.setLevel(this);
+    }
+
     //метод для сравнения
     @Override
     public boolean equals(Object o) {

spring_online_calculator/src/main/resources/application.properties

@@ -8,4 +8,4 @@ spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
 spring.jpa.hibernate.ddl-auto=update
 spring.h2.console.enabled=true
 spring.h2.console.settings.trace=false
-spring.h2.console.settings.web-allow-others=false
+spring.h2.console.settings.web-allow-others=false