Subscriptions Sample Use Cases

Introduction

The following are a few use cases for using subscriptions:

Subscribe to the latest value of a particular field

In case you are interested only in the latest value of a particular field, you can use subscriptions to fetch the field and get updated with its latest value whenever it changes.

Example: Live location tracking

Use subscriptions to show the current location of a vehicle on a map.

Let's say we have the following database schema:

vehicle (
  id INT PRIMARY KEY,
  vehicle_number TEXT
)

vehicle_location (
  id INT PRIMARY KEY,
  location TEXT,
  timestamp TIMESTAMP,
  /* used to create relationship 'locations' for vehicle */
  vehicle_id INT FOREIGN KEY REFERENCES vehicle(id)
)

Now we can use the following subscription to fetch the latest location of a vehicle to display it on a map:

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Subscribe to changes to a table's entries

In case you are interested in all the additions/changes to a table's entries, you can use subscriptions to fetch the table rows and get updates whenever there are any additions/changes to the table.

Example: Chat app

Use subscriptions to show new messages in a chatroom.

Let's say we have the following database schema:

user (
  id INT PRIMARY KEY,
  username TEXT UNIQUE
)

message (
  id INT PRIMARY KEY,
  text TEXT,
  timestamp TIMESTAMP,
  /* used to create relationship 'author' for message */
  user_id INT FOREIGN KEY REFERENCES user(id)
)

Now we can use the following subscription to display the latest messages in a chatroom:

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Subscribe to the latest value of some derived data

In case you are interested in the latest value of some derived data, you can create a view to query the derived data and then use subscriptions to fetch the derived value and get its latest value whenever it updates.

Example: A poll dashboard

Use subscriptions to show the result of a poll.

Let's say we have the following database schema:

poll (
  id INT PRIMARY KEY,
  question TEXT
)

option (
  id INT PRIMARY KEY
  poll_id INT FOREIGN KEY REFERENCES poll(id)
  text TEXT
)

user (
  id INT PRIMARY KEY
  name TEXT
)

vote (
  id INT PRIMARY KEY,
  option_id INT FOREIGN KEY REFERENCES option(id),
  user_id INT FOREIGN KEY REFERENCES user(id),
  timestamp TIMESTAMP
)

First, create a view poll_results to give the result of the poll:

CREATE OR REPLACE VIEW public."poll_results" AS
  SELECT poll.id AS poll_id,
         o.option_id,
         count(*) AS votes
    FROM (
      (
        SELECT vote.option_id,
               option.poll_id,
               option.text
          FROM (
            vote
              LEFT JOIN option ON ((option.id = vote.option_id))
          )
        ) o
            LEFT JOIN poll ON ((poll.id = o.poll_id))
      )
  GROUP BY poll.question, o.option_id, poll.id;

This view will have the following fields: poll_id, option_id and votes, i.e. it gives the number of votes received by each option for a poll.

Next, set up relationships poll and option between the poll_results view and the poll and option tables using the poll_id and option_id fields respectively.

Now we can use the following subscription to display the latest poll result:

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