@OneToOne relationship in Spring Boot - Mini Project: Build & Apply

Create a @OneToOne Relationship in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage users and their profiles. Each user has exactly one profile, and each profile belongs to exactly one user.

🎯 Goal: Build two entity classes User and UserProfile with a @OneToOne relationship between them using Spring Boot and JPA annotations.

📋 What You'll Learn

Create a User entity with fields id and username

Create a UserProfile entity with fields id and bio

Add a @OneToOne relationship from User to UserProfile

Use @JoinColumn to specify the foreign key column in User

Ensure the code compiles and follows Spring Boot JPA entity conventions

💡 Why This Matters

🌍 Real World

One-to-one relationships are common in applications where two entities have a direct and exclusive link, such as a user and their profile, or a person and their passport.

💼 Career

Understanding how to model and implement @OneToOne relationships is essential for backend developers working with Spring Boot and JPA to build relational data models.

1

Create the User entity

Create a class called User annotated with @Entity. Add private fields Long id annotated with @Id and @GeneratedValue, and String username. Add public getters and setters for both fields.

Spring Boot

@Entity
public class User {
    // Your code here
}

Hint

Use @Entity on the class. Use @Id and @GeneratedValue(strategy = GenerationType.IDENTITY) on the id field.

2

Create the UserProfile entity

Create a class called UserProfile annotated with @Entity. Add private fields Long id annotated with @Id and @GeneratedValue, and String bio. Add public getters and setters for both fields.

Spring Boot

import jakarta.persistence.Entity;
import jakarta.persistence.Id;
import jakarta.persistence.GeneratedValue;
import jakarta.persistence.GenerationType;

@Entity
public class UserProfile {
    // Your code here
}

Hint

Use @Entity on the class. Use @Id and @GeneratedValue(strategy = GenerationType.IDENTITY) on the id field.

3

Add @OneToOne relationship in User

In the User class, add a private field UserProfile profile annotated with @OneToOne and @JoinColumn(name = "profile_id"). Add public getter and setter for profile.

Spring Boot

import jakarta.persistence.OneToOne;
import jakarta.persistence.JoinColumn;

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

    // Your code here
}

Hint

Use @OneToOne and @JoinColumn(name = "profile_id") on the profile field in User.

4

Complete the @OneToOne relationship setup

Ensure both User and UserProfile classes are in the same package and properly imported. Add @Entity annotations and all necessary imports. Confirm the code compiles without errors.

Spring Boot

// Review your code and add any missing imports or annotations
// Your code here

Hint

Make sure all imports and annotations are present and both classes are complete.

Practice

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

easy

A. A relationship where one entity is linked to exactly one other entity

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

C. A relationship where many entities are linked to many entities

D. A relationship where entities are not linked at all

Solution

1. Step 1: Understand the meaning of @OneToOne

   The @OneToOne annotation defines a direct one-to-one link between two entities in JPA.

2. Step 2: Compare with other relationship types

   Unlike @OneToMany or @ManyToMany, @OneToOne means exactly one entity matches exactly one other entity.

3. Final Answer:

   A relationship where one entity is linked to exactly one other entity -> Option A

4. Quick Check:

   @OneToOne = one-to-one link [OK]

Hint: One entity matches exactly one other entity [OK]

Common Mistakes:

• Confusing @OneToOne with @OneToMany
• Thinking it allows multiple linked entities
• Ignoring the uniqueness of the relationship

2. Which annotation is used on the owning side of a @OneToOne relationship to specify the foreign key column?

easy

A. @MappedBy

B. @Column

C. @JoinColumn

D. @Entity

Solution

1. Step 1: Identify the owning side annotation

   The owning side uses @JoinColumn to specify the foreign key column in the database.

2. Step 2: Differentiate from mappedBy

   @MappedBy is used on the inverse side, not the owning side.

3. Final Answer:

   @JoinColumn -> Option C

4. Quick Check:

   Owning side uses @JoinColumn [OK]

Hint: Owning side uses @JoinColumn for foreign key [OK]

Common Mistakes:

• Using @MappedBy on owning side
• Confusing @Column with @JoinColumn
• Forgetting to specify @JoinColumn

3. Given the following code snippet, what will be the output when fetching Person and accessing person.getPassport().getNumber()?

@Entity
class Person {
  @Id
  private Long id;

  @OneToOne
  @JoinColumn(name = "passport_id")
  private Passport passport;

  // getters and setters
}

@Entity
class Passport {
  @Id
  private Long id;
  private String number;

  // getters and setters
}

medium

A. Throws NullPointerException because passport is not initialized

B. Returns the passport number linked to the person

C. Returns null because @OneToOne is missing mappedBy

D. Compilation error due to missing @MappedBy

Solution

1. Step 1: Analyze the @OneToOne mapping

   The Person entity owns the relationship with @JoinColumn, so passport is linked properly.

2. Step 2: Understand the data fetching

   When fetching Person, accessing person.getPassport().getNumber() returns the linked Passport's number if data exists.

3. Final Answer:

   Returns the passport number linked to the person -> Option B

4. Quick Check:

   Proper @OneToOne with @JoinColumn returns linked entity [OK]

Hint: Owning side with @JoinColumn returns linked entity [OK]

Common Mistakes:

• Assuming mappedBy is required on owning side
• Expecting NullPointerException without data check
• Confusing compilation errors with runtime behavior

4. Identify the error in this @OneToOne mapping:

@Entity
class User {
  @Id
  private Long id;

  @OneToOne(mappedBy = "user")
  private Profile profile;
}

@Entity
class Profile {
  @Id
  private Long id;

  @OneToOne
  private User user;
}

medium

A. Missing @JoinColumn on Profile entity owning side

B. mappedBy should be on Profile, not User

C. User entity should not have @OneToOne annotation

D. Profile entity must use mappedBy instead of @OneToOne

Solution

1. Step 1: Check owning side annotations

   Profile is the owning side but lacks @JoinColumn to specify the foreign key.

2. Step 2: Understand mappedBy usage

   mappedBy is correctly on User side, indicating inverse side.

3. Final Answer:

   Missing @JoinColumn on Profile entity owning side -> Option A

4. Quick Check:

   Owning side needs @JoinColumn [OK]

Hint: Owning side must have @JoinColumn [OK]

Common Mistakes:

• Placing mappedBy on owning side
• Omitting @JoinColumn on owning side
• Confusing owning and inverse sides

5. You want to create a bidirectional @OneToOne relationship between Employee and Desk. Which is the correct way to define the relationship so that Employee owns the relationship and the foreign key is in the Desk table?

hard

A. In Employee: @OneToOne(mappedBy = "desk") Desk desk; In Desk: @OneToOne @JoinColumn Employee employee;

B. In Employee: @OneToOne @JoinColumn Desk desk; In Desk: @OneToOne(mappedBy = "desk") Employee employee;

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D. In Employee: @OneToOne(mappedBy = "employee") Desk desk; In Desk: @OneToOne @JoinColumn Employee employee;

Solution

1. Step 1: Determine owning side and foreign key location

   The foreign key is in Desk table, so Desk owns the relationship.

2. Step 2: Correct annotation placement

   Desk must have @JoinColumn and no mappedBy; Employee uses mappedBy to point to Desk's field.

3. Step 3: Match option with correct annotations

   In Employee: @OneToOne(mappedBy = "employee") Desk desk; In Desk: @OneToOne @JoinColumn Employee employee; matches this.

4. Final Answer:

   In Employee: @OneToOne(mappedBy = "employee") Desk desk; In Desk: @OneToOne @JoinColumn Employee employee; -> Option D

5. Quick Check:

   Foreign key side owns with @JoinColumn, other side uses mappedBy [OK]

Hint: Foreign key side owns with @JoinColumn, other side uses mappedBy [OK]

Common Mistakes:

• Assigning owning side incorrectly
• Placing mappedBy on owning side
• Confusing which table holds foreign key