Info argument in GraphQL - Mini Project: Build & Apply

Using the Info Argument in GraphQL Resolvers

📖 Scenario: You are building a simple GraphQL server for a bookstore. You want to understand how to use the info argument in your resolver functions to access details about the GraphQL query being executed.

🎯 Goal: Learn how to use the info argument in a GraphQL resolver to inspect the query's field name and selection set.

📋 What You'll Learn

Create a resolver function for a book query

Use the info argument to get the field name being queried

Extract the selection set (requested fields) from the info argument

Return a simple object representing a book with the requested fields

💡 Why This Matters

🌍 Real World

GraphQL servers often use the 'info' argument in resolvers to understand what data the client requests, enabling efficient data fetching and response shaping.

💼 Career

Understanding the 'info' argument is essential for backend developers working with GraphQL APIs to optimize performance and customize responses.

1

Setup a basic book query resolver

Create a resolver function called book that takes four arguments: parent, args, context, and info. For now, return a hardcoded object with title set to '1984' and author set to 'George Orwell'.

GraphQL

// Define the book resolver function
// Your code here

Hint

Remember the resolver function signature is (parent, args, context, info).

2

Add a variable to get the field name from info

Inside the book resolver, create a variable called fieldName and set it to info.fieldName. This will hold the name of the field being queried.

GraphQL

const book = (parent, args, context, info) => {
  // Get the field name from info
  // Your code here
  return { title: '1984', author: 'George Orwell' };
};

Hint

The info argument contains metadata about the query, including fieldName.

3

Extract the selection set from info

Inside the book resolver, create a variable called selectionSet and set it to info.fieldNodes[0].selectionSet.selections. This will hold the list of fields requested by the client.

GraphQL

const book = (parent, args, context, info) => {
  const fieldName = info.fieldName;
  // Get the selection set from info
  // Your code here
  return { title: '1984', author: 'George Orwell' };
};

Hint

The fieldNodes array contains AST nodes for the query fields. Access the first node's selectionSet to get requested fields.

4

Return only requested fields from the resolver

Modify the book resolver to return an object that includes only the fields requested in selectionSet. Create an object bookData with title as '1984' and author as 'George Orwell'. Then build a new object result by iterating over selectionSet and adding only those fields from bookData. Return result.

GraphQL

const book = (parent, args, context, info) => {
  const fieldName = info.fieldName;
  const selectionSet = info.fieldNodes[0].selectionSet.selections;
  const bookData = { title: '1984', author: 'George Orwell' };
  // Build result object with requested fields
  // Your code here
  return result;
};

Hint

Use a for loop to iterate over selectionSet and build the result object with only requested fields.

Practice

1. What is the purpose of the info argument in a GraphQL resolver?

easy

A. It stores the database connection details.

B. It contains the user's authentication token.

C. It tells the resolver which fields the client requested.

D. It defines the schema of the GraphQL server.

Solution

1. Step 1: Understand the role of resolver arguments

   Resolvers receive arguments like parent, args, context, and info. Each has a specific role.

2. Step 2: Identify what info provides

   The info argument contains details about the query, including which fields the client requested.

3. Final Answer:

   It tells the resolver which fields the client requested. -> Option C

4. Quick Check:

   Info argument = requested fields [OK]

Hint: Info shows requested fields to optimize data fetching [OK]

Common Mistakes:

• Confusing info with context or args
• Thinking info holds authentication data
• Assuming info defines schema

2. Which of the following is the correct way to include the info argument in a GraphQL resolver function?

easy

A. resolver(parent, args, context, info) { ... }

B. resolver(args, parent, info, context) { ... }

C. resolver(context, info, args, parent) { ... }

D. resolver(info, parent, args, context) { ... }

Solution

1. Step 1: Recall the standard resolver signature

   The standard order is (parent, args, context, info) in GraphQL resolvers.

2. Step 2: Match the correct order

   resolver(parent, args, context, info) { ... } matches this order exactly, making it the correct syntax.

3. Final Answer:

   resolver(parent, args, context, info) { ... } -> Option A

4. Quick Check:

   Resolver args order = parent, args, context, info [OK]

Hint: Remember resolver args order: parent, args, context, info [OK]

Common Mistakes:

• Mixing up the order of arguments
• Placing info before context
• Omitting info argument

3. Given this resolver snippet, what will info.fieldNodes[0].selectionSet.selections.map(f => f.name.value) return if the client queries { user { id name } }?

medium

A. ["id", "name"]

B. ["user"]

C. ["id"]

D. An empty array []

Solution

1. Step 1: Understand the query structure

   The client requests the fields id and name inside user.

2. Step 2: Analyze the info.fieldNodes access

   info.fieldNodes[0].selectionSet.selections lists the fields requested inside user, which are id and name.

3. Final Answer:

   ["id", "name"] -> Option A

4. Quick Check:

   info.fieldNodes selections = requested fields [OK]

Hint: info.fieldNodes lists requested fields inside current resolver [OK]

Common Mistakes:

• Returning parent field name instead of subfields
• Confusing selectionSet with fieldNodes
• Expecting only one field instead of all requested

4. You wrote this resolver: function resolver(parent, args, context) { return info.fieldNodes; }. Why does this cause an error?

medium

A. You cannot access fieldNodes inside resolvers.

B. The resolver is missing the info argument in its parameters.

C. The info argument must be the first parameter.

D. Resolvers cannot return objects like fieldNodes.

Solution

1. Step 1: Check resolver parameters

   The resolver only has three parameters: parent, args, context. It lacks the fourth parameter info.

2. Step 2: Understand usage of info

   Since info is not declared, trying to use it causes a reference error.

3. Final Answer:

   The resolver is missing the info argument in its parameters. -> Option B

4. Quick Check:

   Missing info param = error [OK]

Hint: Always include info as fourth parameter to use it [OK]

Common Mistakes:

• Forgetting to add info parameter
• Assuming info is global or in context
• Misordering resolver parameters

5. You want your resolver to return only the fields requested by the client to save resources. Which approach using the info argument is best?

hard

A. Fetch all fields but filter results after sending to client.

B. Ignore info and always fetch all fields from the database.

C. Use args to guess which fields the client wants.

D. Use info.fieldNodes to find requested fields and fetch only those from the database.

Solution

1. Step 1: Understand the goal

   The goal is to fetch only requested fields to save resources.

2. Step 2: Use info to get requested fields

   info.fieldNodes provides the exact fields the client asked for, so you can fetch only those.

3. Step 3: Compare options

   Use info.fieldNodes to find requested fields and fetch only those from the database. uses info correctly. Options B, C, and D either waste resources or guess incorrectly.

4. Final Answer:

   Use info.fieldNodes to find requested fields and fetch only those from the database. -> Option D

5. Quick Check:

   Use info to fetch requested fields only [OK]

Hint: Fetch only requested fields using info to save resources [OK]

Common Mistakes:

• Fetching all data regardless of request
• Using args instead of info for fields
• Filtering after fetching all data