Subscriptions sample use cases¶
Table of contents
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,
time_stamp TIMESTAMP with time zone DEFAULT now(),
/* 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: {time_stamp: desc}, limit: 1) {
location
time_stamp
}
}
}
with variables:
{
"vehicleId": 3
}
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,
texts TEXT,
time_stamp TIMESTAMP default now(),
/* used to create relationship 'author' with the 'user' table */
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: {time_stamp: desc}) {
texts
time_stamp
author {
username
}
}
}
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)
texts 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),
time_stamp 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.texts
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
poll {
question
}
option {
texts
}
votes
}
}
with variables:
{
"pollId": 1
}
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