How to dynamically get column names from TypeORM?

In TypeORM, you can use various methods to dynamically retrieve column names from entities. Here are several common approaches:

1. Utilizing the getMetadata() Method

TypeORM provides the getMetadata() method on the Connection object, which can be used to retrieve entity metadata, including information about column names. Here is an example:

typescript
import { getConnection } from 'typeorm';
import { YourEntity } from './entity/YourEntity';

const metadata = getConnection().getMetadata(YourEntity);
const columns = metadata.columns;
const columnNames = columns.map(column => column.propertyName);

In this example, YourEntity is the entity class from which you want to retrieve column names. The getMetadata() method returns entity metadata, where the columns array contains detailed information about all columns, from which you can extract the required column names.

2. Using EntityMetadata and ColumnMetadata

If you already have an EntityMetadata object for the corresponding entity, you can directly access the columns property, which is an array containing ColumnMetadata objects. Each ColumnMetadata object contains information about the column, such as the database column name and property name. Here is how to retrieve column names:

typescript
import { getConnection } from 'typeorm';

const entityMetadata = getConnection().getMetadata(YourEntity);
const columnNames = entityMetadata.columns.map(column => column.propertyName);

3. Using QueryBuilder

If you want to retrieve column names while executing a query, you can use TypeORM's QueryBuilder. This method allows you to obtain column names when dynamically building queries, for example:

typescript
import { getConnection } from 'typeorm';
import { YourEntity } from './entity/YourEntity';

const queryBuilder = getConnection()
  .getRepository(YourEntity)
  .createQueryBuilder("entity");

const columnNames = queryBuilder.expressionMap.mainAlias.metadata.columns.map(column => column.propertyName);

In this example, expressionMap is an internal object of QueryBuilder that stores metadata related to the current query. mainAlias is the alias for the main query subject, and its metadata property contains the entity metadata.

Ensure that when using any of the above methods, your code executes after the database connection is established. The code to retrieve column names is typically placed within an asynchronous function, ensuring it is called after the database connection is completed. For example, you might place this code in the handler function for API requests or in an initialization function that runs after the application starts and establishes the database connection.