Data Transformations

One of the realtime features of the todo app is to display the list of online users. We need a way to fetch this information based on the value of last_seen which tells when the user was last online.

So far we were building tables and relationships. Postgres allows you to perform data transformations using:

• Views
• SQL Functions

In this example, we are going to make use of Views. This view is required by the app to find the users who have logged in and are online in the last 30 seconds.

Create View

The SQL statement for creating this view looks like this:

Copy
CREATE OR REPLACE VIEW "public"."online_users" AS
 SELECT users.id,
    users.last_seen
   FROM users
  WHERE (users.last_seen >= (now() - '00:00:30'::interval));

Let's add this view and track the view with Hasura to be able to query it.

Head to Console -> DATA -> SQL page.

Click on Run to create the view.

Subscription to Online Users

Now let's test by making a subscription query to the online_users view.

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subscription {
  online_users {
    id
    last_seen
  }
}

In another tab, update an existing user's last_seen value to see the subscription response getting updated.

Enter the value as now() for the last_seen column and click on Save.

Now switch back to the tab where your subscription query is running to see the updated response.