How to create this ViewEntity with TypeORM?

In TypeORM, ViewEntity is a decorator used to create a database view (View). Database views are virtual tables defined by queries. Unlike regular entities, ViewEntity does not map to a physical table in the database but instead maps to a result set defined by an SQL query.

The steps to create a ViewEntity typically include the following:

1. Define a class: First, define a regular class.
2. Use the decorator: Then, apply the @ViewEntity decorator to mark the class as a view entity.
3. Specify SQL: Within the @ViewEntity decorator, provide an expression that can be an SQL query string or a function returning an SQL query string. This SQL query defines the view's content.
4. Map properties: Inside the class, use regular TypeORM column decorators (e.g., @Column) to map view columns to class properties.

Here is a simple example demonstrating how to create a ViewEntity:

typescript
import { ViewEntity, Connection, ViewColumn } from 'typeorm';

@ViewEntity({
    name: 'user_summary', // View name
    expression: (connection: Connection) => connection.createQueryBuilder()
        .select("user.id", "id")
        .addSelect("user.name", "name")
        .addSelect("COUNT(posts.id)", "postCount")
        .from(User, "user")
        .leftJoin(Post, "posts", "posts.userId = user.id")
        .groupBy("user.id")
})
export class UserSummary {
    @ViewColumn()
    id: number;

    @ViewColumn()
    name: string;

    @ViewColumn()
    postCount: number;
}

In this example, we create a UserSummary view entity that maps to a database view named user_summary. This view is defined by an SQL query that counts the number of posts per user. The properties id, name, and postCount in the class map to the corresponding columns of the view.

Note that the actual SQL for creating the view entity may vary depending on the database type (e.g., MySQL, PostgreSQL), so the expression may need to be adjusted accordingly in different database environments.