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LAB Java | Intermediate JPA

Introduction

We have just learned how to use JPQL and native SQL statements with Spring JPA to query a database as well as how to model object inheritance and component objects with JPA so let's practice a bit more.

LAB Java | Modeling Complex Relationships

Introduction

We have just learned how to use Relationship Mapping to model 1 to 1 relationship and 1 to many relationships so let's practice a bit more.

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Requirements

  1. Fork this repo.
  2. Clone this repo.
  3. Add your instructor and the class graders as collaborators to your repository. If you are unsure who your class graders are, ask your instructor or refer to the day 1 slide deck.
  4. In the repository, create a Java project and add the code for the following prompts.

Submission

Once you finish the assignment, submit a URL link to your repository or your pull request in the field below.


Instructions

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  1. Model the relationships of a Task Management software using Spring JPA. Assume that you have the following requirements for data storage:
  • Tasks have a title, dueDate and a boolean status.
  • There are 2 types of tasks: BillableTask and InternalTask.
  • BillableTask have an additional hourlyRate attribute.
  1. Model the relationships of a PR company using Spring JPA. Assume that you have the following requirements for data storage:
  • The database consists of Contacts. Each Contact has a name, title and company.
  • Names are their own object and have a firstName, lastName, middleName and salutation.
  • Names should be embedded within the Contact entity and not be stored in a separate table.

  1. List 3 tasks that can be accomplished with native SQL that you cannot do with JPQL. Explain why you would choose to use native SQL over JPQL for these specific tasks. =======

  2. Using the Spring JPA model, create the relationships for an application for an Association of Nurses. The Association has 7 Chapters and each Chapter has the following properties:

    • name: the name of the chapter
    • district: the geographical district the chapter belongs to
    • president: a Member object representing the chapter's president
    • list: a list of Member objects representing the chapter's members

  • Each Member has the following properties:

    • name: the member's name
    • status: the member's status, which can be ACTIVE or LAPSED
    • renewal date: the date on which the member's membership will be renewed
  1. Using the Spring JPA model, create the relationships for an Event Management application. The application needs to model upcoming events, which can be of two types: Conferences or Expositions.

  • Each event has the following properties:

    • date: the date the event will take place
    • duration: the duration of the event
    • location: the location of the event
    • title: the title of the event
    • list: a list of guests that are invited to the event.

  • Each guest has the following properties:

    • name: the guest's name
    • status: the guest's response, which can be ATTENDING, NOT_ATTENDING, or NO_RESPONSE

In addition to the typical event properties, Conferences also have a list of Speakers.

  • Each Speaker has the following properties:
    • name: the speaker's name
    • presentationDuration: the duration of the presentation.

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FAQs


I am stuck and don't know how to solve the problem or where to start. What should I do?


If you are stuck in your code and don't know how to solve the problem or where to start, you should take a step back and try to form a clear, straight forward question about the specific issue you are facing. The process you will go through while trying to define this question, will help you narrow down the problem and come up with potential solutions.

For example, are you facing a problem because you don't understand the concept or are you receiving an error message that you don't know how to fix? It is usually helpful to try to state the problem as clearly as possible, including any error messages you are receiving. This can help you communicate the issue to others and potentially get help from classmates or online resources.

Once you have a clear understanding of the problem, you should be able to start working toward the solution.

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How do I create a Spring boot project?


Spring boot is a framework for creating stand-alone, production-grade applications that are easy to launch and run. The best way to create a Spring boot project is to use the Spring Initializer website. The website provides a convenient way to generate a basic project structure with all the necessary dependencies and configurations.

  • Step 1: Go to start.spring.io
  • Step 2: Choose the type of project you want to create, such as Maven or Gradle.
  • Step 3: Select the version of Spring Boot you want to use.
  • Step 4: Choose the dependencies you need for your project. Some common dependencies include web, jpa and data-jpa.
  • Step 5: Click the "Generate" button to download the project files.

Alternatively, you can use an Integrated Development Environment (IDE) such as Eclipse or IntelliJ IDEA. These IDEs have plugins for creating Spring boot projects, making it easy to set up the environment and get started with coding.

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What is JPA and how can I use it in Java?


JPA stands for Java Persistence API, which is a Java specification for accessing, persisting and managing data between Java objects and a relational database. JPA provides a standard interface for accessing databases, reducing the need for custom data access code and enabling efficient management of database connections.

To use JPA in Java, you will need to include the necessary dependencies in your project, such as the Hibernate JPA implementation and create entity classes to represent your data. These entity classes will be annotated with JPA-specific annotations, such as @Entity and @Id, to indicate the mapping between the Java class and the database table.

Here is a code snippet to show you how to create a JPA entity class in Java:

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

  private String name;
  private int age;
  private String position;

  // Getters and Setters for the attributes
}

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What is the purpose of using the "@Entity", "@Table" and "@Column" annotations in JPA (Java Persistence API)?


The @Entity, @Table and @Column annotations in JPA (Java Persistence API) are used to map Java objects to relational database tables.

@Entity is used to mark a class as a persistent entity. This means that instances of the class can be stored in a database.

@Table is used to define the name of the database table that the entity will be mapped to.

@Column is used to define the columns in the table that correspond to the attributes of the entity.

Here is an example of how to use these annotations:

@Entity
@Table(name="employee")
public class Employee {

  @Id
  @GeneratedValue(strategy=GenerationType.AUTO)
  @Column(name="id")
  private int id;

  @Column(name="first_name")
  private String firstName;

  @Column(name="last_name")
  private String lastName;

  //getters and setters
}

In this example, the Employee class is marked as a persistent entity using the @Entity annotation. The name of the database table is defined using the @Table annotation as "employee". The id, firstName and lastName attributes are mapped to columns in the "employee" table using the @Column annotation.

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What is the difference between "JOINED", "TABLE_PER_CLASS" and "SINGLE_TABLE" inheritance strategies in JPA and when to use them?


The @Inheritance annotation in JPA can be used to specify inheritance strategy for entities in JPA. There are three strategies available in JPA: SINGLE_TABLE, JOINED and TABLE_PER_CLASS. The inheritance strategy is specified using the strategy attribute in the @Inheritance annotation.

The inheritance strategies in JPA determine how the data is stored in the database for the entities that inherit from a parent entity.

  • JOINED strategy: creates separate tables for each concrete entity and a join between them is used to retrieve the data.

    @Inheritance(strategy=InheritanceType.JOINED)
@Entity
public class Parent {
// ...
}

  • TABLE_PER_CLASS strategy: creates a separate table for each concrete entity and includes all the columns of the parent entity as well.

    @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
@Entity
public class Parent {
// ...
}

  • SINGLE_TABLE strategy: creates a single table for all the entities in the hierarchy, including a discriminator column to distinguish between the entities.

    @Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@Entity
public class Parent {
// ...
}

The choice of inheritance strategy depends on the requirements of the application, such as the need for normalization, the number of columns in the table and the number of joins required to retrieve data. For example, if the entities have many columns and normalization is important, then JOINED strategy is appropriate. If there are fewer columns, the TABLE_PER_CLASS strategy may be more suitable. The SINGLE_TABLE strategy is the simplest and most compact but may result in a less flexible data model.

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What is "JpaRepository" and how can I use it in Java?


JpaRepository is a Spring Data interface that extends the PagingAndSortingRepository interface. It provides all the basic CRUD (Create, Read, Update, Delete) operations and additional methods to work with JPA (Java Persistence API) to interact with the database.

To use JpaRepository in your project, follow the below steps:

Step 1: Import the necessary libraries

import org.springframework.data.jpa.repository.JpaRepository;

Step 2: Create an interface that extends JpaRepository

public interface MyRepository extends JpaRepository<MyEntity, Long> {

}

Note: In the above code, MyEntity is the entity class that you want to interact with the database and Long is the type of the primary key of MyEntity.

Step 3: Inject the interface in the class where you want to use it.

@Autowired
private MyRepository myRepository;

Step 4: You can now use the methods provided by JpaRepository to interact with the database, for example:

MyEntity myEntity = new MyEntity();
myRepository.save(myEntity);

With the above steps, you can now use JpaRepository to interact with the database in your Java project.

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The @Embedded and @Embeddable annotations in JPA are used to map an embedded field or class in a JPA entity.

These annotations are used to store a set of related fields within another JPA entity. This allows the developer to store complex data within a single entity, as opposed to having multiple entities for different data fields.

The @Embeddable annotation is used to annotate the embedded class, while the @Embedded annotation is used to annotate the field within the JPA entity that will store the embedded class.

Example:

@Embeddable
public class Address {
  private String street;
  private String city;
  private String state;
  private String zipCode;
  // getters and setters
}

@Entity
public class Employee {
  @Id
  private long id;
  private String name;
  @Embedded
  private Address address;
  // getters and setters
}

JPA will store the data in the embedded class as part of the owning entity in the database. When retrieving the entity from the database, the embedded class data will also be retrieved and stored as part of the entity.

In conclusion, the @Embedded and @Embeddable annotations in JPA allow developers to store complex data within a single JPA entity, providing a more organized and efficient way to store and retrieve data from the database.


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How can I use the JPA annotations "@OneToOne", "@OneToMany", "@ManyToOne", "@ManyToMany", "@JoinColumn" and "@JoinTable" in Java?


The Java Persistence API (JPA) provides several annotations for mapping relationships between entities in Java applications. These annotations include:

  1. @OneToOne: This annotation is used to define a one-to-one relationship between two entities. The following code shows how to use the @OneToOne annotation:
@Entity
public class Employee {

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

  private String name;

  @OneToOne(cascade = CascadeType.ALL)
  @JoinColumn(name = "address_id", referencedColumnName = "id")
  private Address address;

  // Getters and setters ...
}

@Entity
public class Address {

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

  private String street;

  private String city;

  // Getters and setters ...
}
  1. @OneToMany & @ManyToOne: These annotations are used to define one-to-many and many-to-one relationships between two entities.

The following code shows how to use the @OneToMany and the @ManyToOne annotation:

@Entity
public class Department {

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

  private String name;

  @OneToMany(mappedBy = "department")
  private List<Employee> employees;

  // Getters and setters ...
}

@Entity
public class Employee {

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

  private String name;

  @ManyToOne
  @JoinColumn(name = "department_id", referencedColumnName = "id")
  private Department department;

  // Getters and setters ...
}
  1. @ManyToMany & @JoinColumn & @JoinTable: The @ManyToMany annotation is used in Java to define a many-to-many relationship between two entities. This means that multiple instances of one entity can be related to multiple instances of another entity.

The @JoinColumn annotation is used in Java to specify the foreign key column that will be used to join the two entities. The foreign key column is used to establish a relationship between the entities.

The @JoinTable annotation is used in Java to define a join table for a many-to-many relationship. The join table is used to store the relationship information between the two entities.

The following code shows how to use the @ManyToMany, @JoinColumn and @JoinTable annotations:

@Entity
public class User {

    @ManyToMany
    @JoinTable(name = "user_role",
    joinColumns = @JoinColumn(name = "user_id"),
    inverseJoinColumns = @JoinColumn(name = "role_id"))
    private List<Role> roles;

}

@Entity
public class Role {

    @ManyToMany(mappedBy = "roles")
    private List<User> users;

}

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I am unable to push changes to my repository. What should I do?


If you are unable to push changes to your repository, here are a few steps that you can follow:

  1. Check your internet connection: Ensure that your internet connection is stable and working.
  2. Verify your repository URL: Make sure that you are using the correct repository URL to push your changes.
  3. Check Git credentials: Ensure that your Git credentials are up-to-date and correct. You can check your credentials using the following command:
git config --list
  1. Update your local repository: Before pushing changes, make sure that your local repository is up-to-date with the remote repository. You can update your local repository using the following command:
git fetch origin
  1. Check for conflicts: If there are any conflicts between your local repository and the remote repository, resolve them before pushing changes.
  2. Push changes: Once you have resolved any conflicts and updated your local repository, you can try pushing changes again using the following command:
git push origin <branch_name>