How can I get values from a TypeORM property decorator

In TypeORM, property decorators are commonly used to define model properties for database tables, such as @Column, @PrimaryGeneratedColumn, etc. They not only define fields and data types but also include additional metadata. If you want to retrieve the values defined in these decorators, you need to use reflection technology, which typically involves using the reflect-metadata library—a standard library for TypeScript decorator metadata.

Step-by-Step Explanation

1. Install the required libraries: First, ensure that reflect-metadata and typeorm are installed in your project.

bash
npm install reflect-metadata typeorm

And import reflect-metadata in your entry file:

typescript
import "reflect-metadata";

1. Use decorators to define the model: Define the model by decorating class properties with TypeORM decorators.

typescript
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

@Entity()
class User {
    @PrimaryGeneratedColumn()
    id: number;

    @Column()
    name: string;
}

1. Retrieve decorator metadata using Reflect: To access metadata from decorators, utilize APIs provided by the Reflect module. For example, to obtain information about the @Column decorator:

typescript
import { getMetadataArgsStorage } from "typeorm";

function getColumnDecorators(entity: Function) {
    const columns = getMetadataArgsStorage().columns.filter(column => column.target === entity);
    columns.forEach(column => {
        console.log(`Property: ${column.propertyName}, Type: ${column.options.type || 'default'}`);
    });
}

getColumnDecorators(User);

Example Explanation

In the above example, we define a simple User entity using @Entity and @Column decorators to annotate its properties. By calling getMetadataArgsStorage() from TypeORM, we retrieve all decorator metadata, filter for column information specific to the entity, and print the property name and type for each column.

Important Notes

• Retrieving decorator metadata occurs at runtime, so ensure the application is properly configured and all necessary modules and libraries are loaded before using this technique.
• TypeORM's getMetadataArgsStorage() method provides extensive access to internal decorator metadata, enabling you to retrieve additional information about entities and decorators.