@ManyToOne relationship in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Spring Boot

import jakarta.persistence.*;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @[1]
    private Customer customer;

    // getters and setters
}

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

AManyToMany

BManyToOne

COneToMany

DOneToOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using @OneToMany instead of @ManyToOne

Forgetting to import the annotation

Confusing the direction of the relationship

The @ManyToOne annotation defines a many-to-one relationship from Order to Customer.

2fill in blank

medium

Complete the code to specify the foreign key column name for the many-to-one relationship.

Spring Boot

import jakarta.persistence.*;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "[1]")
    private Customer customer;

    // getters and setters
}

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

Aorder_id

BcustomerName

Cid

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name that doesn't exist

Confusing the primary key column with the foreign key column

The @JoinColumn annotation specifies the foreign key column name in the Order table that references the Customer entity.

3fill in blank

hard

Fix the error in the code to correctly map the many-to-one relationship with lazy loading.

Spring Boot

import jakarta.persistence.*;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.[1])
    @JoinColumn(name = "customer_id")
    private Customer customer;

    // getters and setters
}

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

ALAZY

BIMMEDIATE

CDEFERRED

DEAGER

Attempts:

3 left

💡 Hint

Common Mistakes

Using EAGER when lazy loading is intended

Using invalid fetch type values

Using FetchType.LAZY delays loading the related Customer until it is accessed, improving performance.

4fill in blank

hard

Fill both blanks to complete the code for a bidirectional many-to-one relationship.

Spring Boot

import jakarta.persistence.*;
import java.util.List;

@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @[1](mappedBy = "customer")
    private List<Order> [2];

    // getters and setters
}

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

AOneToMany

BManyToOne

Corders

DcustomerOrders

Attempts:

3 left

💡 Hint

Common Mistakes

Using @ManyToOne on the 'one' side

Using a singular field name for a list

The @OneToMany annotation on Customer defines the list of orders. The field name orders holds these related Order objects.

5fill in blank

hard

Fill all three blanks to complete the code for an Order entity with a many-to-one relationship and cascade persist.

Spring Boot

import jakarta.persistence.*;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(cascade = CascadeType.[1], fetch = FetchType.[2])
    @JoinColumn(name = "[3]")
    private Customer customer;

    // getters and setters
}

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

APERSIST

BLAZY

Ccustomer_id

DREMOVE

Attempts:

3 left

💡 Hint

Common Mistakes

Using REMOVE cascade when not intended

Using EAGER fetch when lazy is better for performance

Wrong join column name

The cascade type PERSIST ensures saving the Customer when saving the Order. The fetch type LAZY delays loading the customer until needed. The join column customer_id is the foreign key.

Practice

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

easy

A. A many-to-one relationship where many entities link to one entity

B. A one-to-many relationship where one entity links to many entities

C. A one-to-one relationship between two entities

D. A many-to-many relationship between two entities

Solution

1. Step 1: Understand relationship types in JPA

   @ManyToOne means many instances of an entity relate to one instance of another entity.

2. Step 2: Match the description to the annotation

   The annotation @ManyToOne specifically defines many entities pointing to one entity.

3. Final Answer:

   A many-to-one relationship where many entities link to one entity -> Option A

4. Quick Check:

   @ManyToOne = many entities to one entity [OK]

Hint: Remember: Many objects point to one with @ManyToOne [OK]

Common Mistakes:

• Confusing @ManyToOne with @OneToMany
• Thinking it means one-to-one
• Mixing it up with many-to-many

2. Which of the following is the correct way to declare a @ManyToOne relationship with a join column named category_id?

easy

A. @ManyToOne
@JoinColumn(name = "category_id")
private Category category;

B. @OneToMany
@JoinColumn(name = "category_id")
private Category category;

C. @ManyToOne
@JoinColumn(columnName = "category_id")
private Category category;

D. @ManyToOne
@JoinColumn(name = category_id)
private Category category;

Solution

1. Step 1: Check correct annotation usage

   The relationship is many-to-one, so @ManyToOne is correct.

2. Step 2: Verify @JoinColumn syntax

   The attribute to specify column name is name and the value must be a string in quotes.

3. Final Answer:

   @ManyToOne @JoinColumn(name = "category_id") private Category category; -> Option A

4. Quick Check:

   @JoinColumn uses name="column_name" [OK]

Hint: Use @JoinColumn(name = "column_name") with quotes [OK]

Common Mistakes:

• Using @OneToMany instead of @ManyToOne
• Using columnName instead of name in @JoinColumn
• Omitting quotes around column name

3. Given the entities below, what will be the output of System.out.println(order.getCustomer().getName()); if the order is linked to a customer named "Alice"?

public class Order {
  @ManyToOne
  @JoinColumn(name = "customer_id")
  private Customer customer;

  public Customer getCustomer() { return customer; }
}

public class Customer {
  private String name;
  public String getName() { return name; }
}

medium

A. Compilation error

B. customer_id

C. null

D. Alice

Solution

1. Step 1: Understand the relationship and method calls

   The order has a customer linked via @ManyToOne, so calling getCustomer() returns the Customer object.

2. Step 2: Access the customer's name

   Calling getName() on the Customer returns the customer's name, which is "Alice".

3. Final Answer:

   Alice -> Option D

4. Quick Check:

   order.getCustomer().getName() = "Alice" [OK]

Hint: Follow the chain: order -> customer -> name [OK]

Common Mistakes:

• Expecting the join column name instead of customer name
• Assuming null if not initialized
• Thinking it causes a compile error

4. Identify the error in the following code snippet that uses @ManyToOne:

@Entity
public class Book {
  @Id
  private Long id;
  @ManyToOne
  @JoinColumn(name = "author_id")
  private Author author;

  public Author getAuthor() { return author; }
  public void setAuthor(Author author) { this.author = author; }
}

@Entity
public class Author {
  private String name;
  public String getName() { return name; }
}

medium

A. Author class is missing @Entity annotation

B. Book class should use @OneToMany instead of @ManyToOne

C. Missing @Id annotation in Author class

D. Join column name should be "authorId" not "author_id"

Solution

1. Step 1: Check entity requirements

   Every JPA entity must have a primary key annotated with @Id. The Author class lacks this.

2. Step 2: Verify other annotations

   Author has @Entity but no @Id, which will cause runtime errors.

3. Final Answer:

   Missing @Id annotation in Author class -> Option C

4. Quick Check:

   Entity requires @Id field [OK]

Hint: Always add @Id to every entity class [OK]

Common Mistakes:

• Confusing relationship annotations
• Ignoring missing primary key
• Assuming join column naming causes error

5. You have two entities: Order and Customer. Each order belongs to one customer, but a customer can have many orders. You want to fetch all orders with their customers efficiently. Which approach correctly uses @ManyToOne for eager loading?

hard

A. Use @ManyToOne(fetch = FetchType.LAZY) on Order's customer and fetch customers separately

B. Use @ManyToOne(fetch = FetchType.EAGER) on Order's customer to load customers with orders

C. Use @OneToMany(fetch = FetchType.EAGER) on Customer's orders and no mapping on Order

D. Use @ManyToOne without fetch type and manually query customers for each order

Solution

1. Step 1: Understand fetch types in @ManyToOne

   By default, @ManyToOne uses FetchType.EAGER, loading the related customer eagerly with the order.

2. Step 2: Match the best approach

   Explicitly using fetch = FetchType.EAGER on the Order's customer field ensures efficient eager loading for this many-to-one relationship.

3. Final Answer:

   Use @ManyToOne(fetch = FetchType.EAGER) on Order's customer to load customers with orders -> Option B

4. Quick Check:

   @ManyToOne = EAGER by default [OK]

Hint: EAGER fetch on @ManyToOne loads related data together [OK]

Common Mistakes:

• Using LAZY fetch without proper query optimization
• Using @OneToMany on Customer without Order mapping
• Manually querying related entities inefficiently