@OneToMany relationship in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a one-to-many relationship in a Spring Boot entity.

Spring Boot

import jakarta.persistence.*;

@Entity
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @[1]
    private List<Employee> employees;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

AOneToOne

BOneToMany

CManyToOne

DManyToMany

Attempts:

3 left

💡 Hint

Common Mistakes

Using @ManyToOne instead of @OneToMany

Using @OneToOne which is for one-to-one relationships

The @OneToMany annotation is used to define a one-to-many relationship from the Department entity to the Employee entity.

2fill in blank

medium

Complete the code to specify the mappedBy attribute in the @OneToMany annotation.

Spring Boot

@OneToMany(mappedBy = "[1]")
private List<Order> orders;

Drag options to blanks, or click blank then click option'

Acustomer

Border

Corders

Dcustomers

Attempts:

3 left

💡 Hint

Common Mistakes

Using the plural form 'orders' instead of the singular 'customer'

Confusing the owning side field name

The mappedBy attribute refers to the field name in the child entity that owns the relationship. Here, it should be the field in Order that points back to Customer.

3fill in blank

hard

Fix the error in the @OneToMany annotation to enable cascade operations.

Spring Boot

@OneToMany(cascade = CascadeType.[1])
private Set<Item> items;

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ADETACH

BPERSIST

CREFRESH

DALL

Attempts:

3 left

💡 Hint

Common Mistakes

Using only PERSIST or DETACH which limits cascade behavior

Forgetting to add cascade attribute

Using CascadeType.ALL ensures that all cascade operations (persist, merge, remove, refresh, detach) are applied to the related entities.

4fill in blank

hard

Fill both blanks to complete the bidirectional @OneToMany and @ManyToOne relationship.

Spring Boot

public class Parent {

    @OneToMany(mappedBy = "[1]", cascade = CascadeType.ALL)
    private List<Child> children;
}

public class Child {

    @[2]
    private Parent parent;
}

Drag options to blanks, or click blank then click option'

Aparent

BOneToMany

CManyToOne

Dchildren

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the annotations between Parent and Child

Using wrong field names in mappedBy

The mappedBy in Parent points to the parent field in Child. The Child side uses @ManyToOne to link back to Parent.

5fill in blank

hard

Fill all three blanks to complete the @OneToMany relationship with fetch type and orphan removal.

Spring Boot

@OneToMany(mappedBy = "[1]", fetch = FetchType.[2], orphanRemoval = [3])
private List<Comment> comments;

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Apost

BLAZY

Ctrue

DEAGER

Attempts:

3 left

💡 Hint

Common Mistakes

Using EAGER fetch which loads all data immediately

Setting orphanRemoval to false or missing it

The mappedBy is the field in Comment that points to Post. FetchType.LAZY delays loading until needed. Orphan removal true deletes comments removed from the list.

Practice

1. What does the @OneToMany annotation represent in Spring Boot JPA?

easy

A. A relationship where one entity is linked to many entities

B. A relationship where many entities are linked to one entity

C. A way to delete entities automatically

D. A method to fetch data lazily

Solution

1. Step 1: Understand the meaning of @OneToMany

   The annotation defines a connection where one object relates to multiple objects, like one author having many books.

2. Step 2: Differentiate from other relationships

   @ManyToOne is the opposite, linking many entities to one. @OneToMany specifically means one to many.

3. Final Answer:

   A relationship where one entity is linked to many entities -> Option A

4. Quick Check:

   @OneToMany = one to many link [OK]

Hint: Think 'one' object owns 'many' related objects [OK]

Common Mistakes:

• Confusing @OneToMany with @ManyToOne
• Thinking it deletes entities automatically
• Assuming it controls fetch type only

2. Which of the following is the correct way to declare a @OneToMany relationship in an entity class?

easy

A. @OneToMany(mappedBy = "parent") private List<Child> children;

B. @OneToMany private Child child;

C. @OneToMany(mappedBy = "children") private Child parent;

D. @OneToMany private Map childrenMap;

Solution

1. Step 1: Check the collection type for @OneToMany

   @OneToMany requires a collection like List or Set to hold multiple related entities, so List<Child> is correct.

2. Step 2: Verify the mappedBy attribute usage

   mappedBy should point to the field name in the Child entity that owns the relationship, here "parent" is correct.

3. Final Answer:

   @OneToMany(mappedBy = "parent") private List<Child> children; -> Option A

4. Quick Check:

   Use collection + mappedBy for correct syntax [OK]

Hint: Use a collection and mappedBy to link entities [OK]

Common Mistakes:

• Using single object instead of collection
• Wrong mappedBy value
• Using Map instead of List or Set

3. Given the following code snippet, what will be the output when fetching a Department entity?

@Entity
public class Department {
  @Id
  private Long id;

  @OneToMany(mappedBy = "department", fetch = FetchType.EAGER)
  private List<Employee> employees;

  // getters and setters
}

Assuming the department has 3 employees, what happens when you load the department?

medium

A. Only one employee is loaded due to default limit

B. The department loads without employees until accessed

C. The department loads with all 3 employees immediately

D. An error occurs because fetch type is invalid

Solution

1. Step 1: Understand fetch type EAGER

   FetchType.EAGER means related entities are loaded immediately with the main entity.

2. Step 2: Apply to the employees list

   Since employees are marked EAGER, all 3 employees will be loaded when the department is fetched.

3. Final Answer:

   The department loads with all 3 employees immediately -> Option C

4. Quick Check:

   FetchType.EAGER loads related entities immediately [OK]

Hint: EAGER fetch loads all related data immediately [OK]

Common Mistakes:

• Confusing EAGER with LAZY fetch
• Assuming default fetch loads lazily
• Expecting errors from fetch type

4. Identify the error in this @OneToMany mapping:

@Entity
public class Order {
  @Id
  private Long id;

  @OneToMany
  private List<Item> items;

  // getters and setters
}

Why might this cause issues when saving an Order with Items?

medium

A. List<Item> should be Set<Item> for @OneToMany

B. Missing mappedBy causes owning side confusion

C. The @Id annotation is missing

D. Items should be annotated with @ManyToMany

Solution

1. Step 1: Check ownership in bidirectional @OneToMany

   Without mappedBy, JPA doesn't know which side owns the relationship, causing extra join tables or errors.

2. Step 2: Understand impact on saving

   Without ownership, saving Order and Items may not link properly, causing data inconsistency.

3. Final Answer:

   Missing mappedBy causes owning side confusion -> Option B

4. Quick Check:

   mappedBy defines owner, missing it causes issues [OK]

Hint: Always set mappedBy on non-owning side [OK]

Common Mistakes:

• Omitting mappedBy in bidirectional relationships
• Confusing collection types for @OneToMany
• Misusing @ManyToMany instead of @OneToMany

5. You want to delete a Category and all its related Product entities automatically. Which @OneToMany configuration achieves this behavior?

hard

A. @OneToMany(cascade = CascadeType.PERSIST) private List<Product> products;

B. @OneToMany(mappedBy = "category") private List<Product> products;

C. @OneToMany(fetch = FetchType.LAZY) private List<Product> products;

D. @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true) private List<Product> products;

Solution

1. Step 1: Understand cascade and orphanRemoval

   CascadeType.ALL applies all operations including delete to related entities. orphanRemoval=true removes child entities if removed from parent.

2. Step 2: Apply to deleting Category

   With cascade ALL and orphanRemoval, deleting Category deletes all linked Products automatically.

3. Final Answer:

   @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true) private List<Product> products; -> Option D

4. Quick Check:

   Use cascade ALL + orphanRemoval for auto-delete [OK]

Hint: Cascade ALL + orphanRemoval deletes children automatically [OK]

Common Mistakes:

• Forgetting cascade causes children to remain
• Using only cascade PERSIST won't delete children
• Ignoring orphanRemoval for child removal