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:

# $vehicleId = 3
subscription getLocation($vehicleId: Int!) {
  vehicle(where: {id: {_eq: $vehicleId}}) {
    id
    vehicle_number
    locations(order_by: {timestamp: desc}, limit: 1) {
      location
      timestamp
    }
  }
}
# $vehicleId = 3 subscription getLocation($vehicleId: Int!) { vehicle(where: {id: {_eq: $vehicleId}}) { id vehicle_number locations(order_by: {timestamp: desc}, limit: 1) { location timestamp } } }
{ "data": { "vehicle": [ { "id": 3, "vehicle_number": "KA04AD4583", "locations": [ { "location": "(12.93623,77.61701)", "timestamp": "2018-09-05T06:52:44.383588+00:00" } ] } ] } }

Check this sample app for a working demo (source code).

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:

subscription getMessages {
  message(order_by: {timestamp: desc}) {
    text
    timestamp
    author {
      username
    }
  }
}
subscription getMessages { message(order_by: {timestamp: desc}) { text timestamp author { username } } }
{ "data": { "message": [ { "text": "I am fine.", "timestamp": "2018-09-05T10:52:23.522223+00:00", "author": { "username": "Jane" } }, { "text": "Hi! How are you?", "timestamp": "2018-09-05T10:52:04.75283+00:00", "author": { "username": "Jose" }, }, { "text": "Hi!", "timestamp": "2018-09-05T10:51:43.622839+00:00", "author": { "username": "Jane" } } ] } }

Check this sample app for a working demo (source code).

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:

# $pollId = 1
subscription getResult($pollId: Int!) {
  poll_results (
    where: { poll_id: {_eq: $pollId} }
  ) {
    poll_id
    option {
      text
    }
    votes
  }
}
# $pollId = 1 subscription getResult($pollId: Int!) { poll_results ( where: { poll_id: {_eq: $pollId} } ) { poll_id option { text } votes } }
{ "data": { "poll_results": [ { "poll_id": 1, "votes": 1, "option": { "text": "Pizza" } }, { "poll_id": 1, "votes": 1, "option": { "text": "Salad" } }, { "poll_id": 1, "votes": 2, "option": { "text": "Sandwich" } }, { "poll_id": 1, "votes": 3, "option": { "text": "Burger" } }, { "poll_id": 1, "votes": 1, "option": { "text": "Lasagna" } } ] } }

Check this sample app for a working demo (source code).

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