**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Working with Databases: TypeORM and Data Transfer Objects (DTOs) **Topic:** Creating database entities and migrations In this topic, we will explore how to create database entities and migrations using TypeORM, a popular ORM (Object-Relational Mapping) library for TypeScript and JavaScript. We will cover the basics of creating entities, defining relationships, and generating migrations. ### What are database entities? Database entities are classes that represent tables in your database. They are used to define the structure of your data, including the columns, data types, and relationships between tables. ### Creating database entities To create a database entity, you need to create a class that extends the `Entity` class from TypeORM. Here is an example of a simple entity: ```typescript import { Entity, Column, PrimaryGeneratedColumn } from 'typeorm'; @Entity() export class User { @PrimaryGeneratedColumn() id: number; @Column() name: string; @Column() email: string; } ``` In this example, we have created an entity called `User` with three columns: `id`, `name`, and `email`. The `@PrimaryGeneratedColumn()` decorator is used to define the primary key of the table, which is automatically generated by the database. The `@Column()` decorator is used to define the other columns. ### Defining relationships TypeORM supports several types of relationships between entities, including one-to-one, one-to-many, and many-to-many. Here is an example of a one-to-many relationship: ```typescript import { Entity, Column, PrimaryGeneratedColumn, ManyToOne } from 'typeorm'; @Entity() export class Order { @PrimaryGeneratedColumn() id: number; @ManyToOne(() => User, (user) => user.orders) user: User; } @Entity() export class User { @PrimaryGeneratedColumn() id: number; @OneToMany(() => Order, (order) => order.user) orders: Order[]; } ``` In this example, we have defined a one-to-many relationship between the `User` and `Order` entities. The `@ManyToOne()` decorator is used to define the relationship from the `Order` entity to the `User` entity, and the `@OneToMany()` decorator is used to define the relationship from the `User` entity to the `Order` entity. ### Generating migrations TypeORM provides a command-line tool called `typeorm migration:generate` to generate migrations for your entities. Here is an example of how to use it: ```bash npx typeorm migration:generate -n CreateUsers ``` This will generate a migration file called `CreateUsers.ts` in the `migrations` directory. ### Running migrations To run the migrations, you need to use the `typeorm migration:run` command. Here is an example: ```bash npx typeorm migration:run ``` This will apply the migrations to your database. ### Conclusion In this topic, we have covered how to create database entities and migrations using TypeORM. We have also covered how to define relationships between entities and generate migrations using the `typeorm migration:generate` command. We have also covered how to run migrations using the `typeorm migration:run` command. ### Exercise Create a new entity called `Product` with the following columns: `id`, `name`, `price`, and `description`. Define a one-to-many relationship between the `Product` entity and a new entity called `Review`. Generate a migration for the `Product` entity and run it. ### Leave a comment If you have any questions or need help with the exercise, please leave a comment below. ### External resources * TypeORM documentation: <https://typeorm.io/> * TypeORM migration guide: <https://typeorm.io/#/migrations> * TypeORM entity documentation: <https://typeorm.io/#/entities> ### Next topic In the next topic, we will cover how to handle data with DTOs for validation and transformation.