@OneToOne relationship in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the @OneToOne annotation represent in Spring Boot?

It represents a one-to-one relationship between two entities, meaning each instance of one entity is linked to exactly one instance of another entity.

intermediate

How do you specify the owning side in a @OneToOne relationship?

The owning side is the entity that contains the foreign key and uses the @JoinColumn annotation to specify the column that joins the two tables.

intermediate

What is the purpose of the mappedBy attribute in a @OneToOne relationship?

It is used on the inverse side to indicate that the relationship is controlled by the other entity's field, avoiding duplicate foreign keys.

advanced

Can a @OneToOne relationship be lazy loaded in Spring Boot?

Yes, by default @OneToOne is eager, but you can set fetch=FetchType.LAZY to load the related entity only when accessed.

beginner

What happens if you don't specify @JoinColumn in a @OneToOne relationship?

Spring Boot will create a default foreign key column with a generated name, which might not match your database schema or naming conventions.

In a @OneToOne relationship, which side owns the foreign key?

A The side with the @JoinColumn annotation

B The side with the mappedBy attribute

C Both sides share the foreign key equally

D Neither side owns the foreign key

The owning side is the entity with the @JoinColumn annotation that holds the foreign key.

What does the mappedBy attribute do in a @OneToOne relationship?

A Removes the relationship

B Defines the owning side of the relationship

C Creates a new foreign key column

D Specifies the inverse side and points to the owning side's field

mappedBy tells Spring Boot which field owns the relationship to avoid duplicate foreign keys.

How do you make a @OneToOne relationship load lazily?

A Lazy loading is not possible with @OneToOne

B Use @Lazy annotation on the entity class

C Set fetch=FetchType.LAZY on the @OneToOne annotation

D Set fetch=FetchType.EAGER

Setting fetch=FetchType.LAZY delays loading the related entity until it is accessed.

What is the default fetch type for @OneToOne in Spring Boot?

A EAGER

B NONE

C LAZY

D MANUAL

By default, @OneToOne relationships are eagerly fetched.

If you omit @JoinColumn in a @OneToOne relationship, what happens?

A An error occurs at runtime

B Spring Boot creates a default foreign key column

C The relationship is ignored

D The database schema is not updated

Spring Boot generates a default foreign key column name if @JoinColumn is not specified.

Explain how to set up a @OneToOne relationship between two entities in Spring Boot, including owning and inverse sides.

Think about which entity holds the foreign key and how to avoid duplicate keys.

Describe the difference between the owning side and the inverse side in a @OneToOne relationship.

Consider who 'owns' the link in the database.

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