Subscription filtering in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Filter subscription events by user ID

Given a GraphQL subscription that listens for new messages, which option correctly filters messages to only those sent by user with ID 42?

GraphQL

subscription OnNewMessage($userId: ID!) {
  newMessage(filter: { senderId: $userId }) {
    id
    content
    senderId
  }
}

A

subscription OnNewMessage {
  newMessage(filter: { senderId: 42 }) {
    id
    content
    senderId
  }
}

B

subscription OnNewMessage($userId: ID!) {
  newMessage(filter: { id: $userId }) {
    id
    content
    senderId
  }
}

C

subscription OnNewMessage($userId: ID!) {
  newMessage(filter: { senderId: $userId }) {
    id
    content
    senderId
  }
}

D

subscription OnNewMessage($userId: ID!) {
  newMessage(filter: { sender: $userId }) {
    id
    content
    senderId
  }
}

💡 Hint

Check the filter field name matches the message sender's ID field.

Option C correctly uses a variable $userId and filters on senderId, which matches the message sender's ID field. Option C hardcodes the ID and does not use the variable. Option C filters on the wrong field 'id' instead of 'senderId'. Option C uses 'sender' which is not the correct field name.

🧠 Conceptual

intermediate

Understanding subscription filter behavior

What happens if a subscription filter does not match any events?

A The subscription immediately closes with an error.

B The subscription stays open but no events are sent until a matching event occurs.

C The subscription sends all events regardless of the filter.

D The subscription sends a single empty event and then closes.

💡 Hint

Think about how subscriptions work over time.

Subscriptions stay open waiting for events. If no event matches the filter, no data is sent but the connection remains active.

📝 Syntax

advanced

Identify the syntax error in subscription filter

Which option contains a syntax error in the subscription filter?

GraphQL

subscription OnOrderUpdate($status: String!) {
  orderUpdated(filter: { status: $status }) {
    id
    status
  }
}

A

ubscription OnOrderUpdate($status: String!) {
  orderUpdated(filter: { status: $status }) {
    id
    status
  }
}

B

subscription OnOrderUpdate($status: String!) {
  orderUpdated(filter: { status: $status }) {
    id
    status
  }
}

C

}
}  
sutats    
di    
{ )} sutats$ :sutats { :retlif(detadpUredro  
{ )!gnirtS :sutats$(etadpUredrOnO noitpircsbus

D

subscription OnOrderUpdate($status: String!) {
  orderUpdated(filter: { status $status }) {
    id
    status
  }
}

💡 Hint

Look carefully at the colon usage in the filter object.

Option D is missing the colon between 'status' and '$status', causing a syntax error. The others are correct.

❓ optimization

advanced

Optimize subscription filter for multiple conditions

You want to subscribe to new comments that are either from user ID 10 or have the tag 'urgent'. Which filter is the most efficient and correct?

A filter: { OR: [{ authorId: 10 }, { tags_contains: "urgent" }] }

B filter: { authorId: 10, tags_contains: "urgent" }

C filter: { AND: [{ authorId: 10 }, { tags_contains: "urgent" }] }

D filter: { authorId: 10 OR tags_contains: "urgent" }

💡 Hint

Think about how to combine conditions to match either one or the other.

Option A uses the OR operator to match comments from user 10 or with tag 'urgent'. Option A and C require both conditions to be true, which is not the requirement. Option A is invalid syntax.

🔧 Debug

expert

Debug why subscription filter does not receive expected events

A subscription is set up to receive updates for products with price greater than 100. The filter is written as filter: { price_gt: 100 }. However, no events are received even when products with price 150 are updated. What is the most likely cause?

A The subscription server does not support the 'price_gt' filter operator.

B The filter should use 'price > 100' syntax instead of 'price_gt'.

C The subscription query is missing the variable declaration for price.

D The filter field name 'price_gt' is incorrect; it should be 'priceGreaterThan'.

💡 Hint

Check the filter operators supported by the GraphQL server.

Many GraphQL servers support specific filter operators like 'price_gt'. If the server does not support 'price_gt', the filter silently fails and no events are sent. The other options are incorrect because 'price > 100' is not valid GraphQL filter syntax, variable declaration is unrelated, and 'priceGreaterThan' is not a standard operator.