MongoDB with GraphQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a GraphQL query that fetches all users from MongoDB.

GraphQL

type Query { users: [User] [1] }

Drag options to blanks, or click blank then click option'

A!

C?

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Adding '!' which makes the list non-nullable and may cause errors if no users exist.

The users field returns a list of User objects. No exclamation mark means it can be null or empty.

2fill in blank

medium

Complete the resolver function to fetch all users from MongoDB collection.

GraphQL

const resolvers = { Query: { users: async () => await db.collection('[1]').find().toArray() } }

Drag options to blanks, or click blank then click option'

Acustomers

Busers

Corders

Dproducts

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong collection name like 'orders' or 'products' which returns wrong data.

The MongoDB collection name for users is usually 'users'.

3fill in blank

hard

Fix the error in the GraphQL schema for a User type with an ID field.

GraphQL

type User { id: [1] name: String }

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AInt

BID!

CString!

DBoolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Int' or 'Boolean' for ID which is not correct for unique identifiers.

The ID field should be of type 'ID!' to uniquely identify the user and be non-nullable.

4fill in blank

hard

Fill both blanks to define a mutation that adds a new user with a name argument.

GraphQL

type Mutation { addUser(name: [1]): User [2] }

Drag options to blanks, or click blank then click option'

AString!

BBoolean

C!

DID

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving out '!' which allows null values and may cause errors.

The name argument should be a non-nullable String, and the mutation returns a User type with a non-nullable result.

5fill in blank

hard

Fill all three blanks to write a resolver for addUser mutation that inserts a user and returns it.

GraphQL

const resolvers = { Mutation: { addUser: async (_, { [1] }) => { const result = await db.collection('users').insertOne({ [2] }); return result.[3]; } } }

Drag options to blanks, or click blank then click option'

Aname

Bname: name

Cops[0]

DinsertedId

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 'insertedId' instead of the full inserted document.

Not matching argument name and inserted document field.

The resolver takes 'name' argument, inserts a document with 'name: name', and returns the inserted document from 'ops[0]'.

Practice

1. What is the main purpose of using GraphQL with MongoDB?

easy

A. To store data in tables instead of documents

B. To replace MongoDB with a SQL database

C. To automatically backup MongoDB data

D. To query only the specific data fields needed from MongoDB

Solution

1. Step 1: Understand GraphQL's role

   GraphQL allows clients to request exactly the data they want, no more, no less.

2. Step 2: Connect to MongoDB data

   MongoDB stores flexible documents, and GraphQL queries these documents precisely.

3. Final Answer:

   To query only the specific data fields needed from MongoDB -> Option D

4. Quick Check:

   GraphQL queries = precise data retrieval [OK]

Hint: GraphQL fetches only requested fields from MongoDB [OK]

Common Mistakes:

• Thinking GraphQL replaces MongoDB
• Confusing document storage with tables
• Assuming GraphQL handles backups

2. Which of the following is the correct way to define a GraphQL query to fetch a list of users from MongoDB?

easy

A. query { users { id name email } }

B. mutation { users { id name email } }

C. query users { id name email }

D. fetch users from MongoDB

Solution

1. Step 1: Identify GraphQL query syntax

   GraphQL queries start with the keyword query followed by the selection set.

2. Step 2: Check the options

   query { users { id name email } } correctly uses query { users { id name email } } to fetch user fields.

3. Final Answer:

   query { users { id name email } } -> Option A

4. Quick Check:

   GraphQL query syntax = query + selection set [OK]

Hint: Queries start with 'query' keyword, mutations with 'mutation' [OK]

Common Mistakes:

• Using mutation instead of query for fetching data
• Omitting braces or keywords
• Writing invalid syntax like 'fetch users from MongoDB'

3. Given this GraphQL query to fetch a user by ID:
query { user(id: "123") { name age } }
And MongoDB has a user document { _id: "123", name: "Alice", age: 30, city: "NY" }, what will be the query result?

medium

A. { "data": { "user": { "name": "Alice", "age": 30 } } }

B. { "data": { "user": { "name": "Alice", "city": "NY" } } }

C. { "data": { "user": { "id": "123", "name": "Alice" } } }

D. { "error": "User not found" }

Solution

1. Step 1: Understand the query fields

   The query requests only name and age fields for user with id "123".

2. Step 2: Match MongoDB document fields

   The document has those fields: name "Alice" and age 30, so those are returned.

3. Final Answer:

   { "data": { "user": { "name": "Alice", "age": 30 } } } -> Option A

4. Quick Check:

   Requested fields returned exactly [OK]

Hint: GraphQL returns only requested fields, ignoring others [OK]

Common Mistakes:

• Expecting all document fields returned
• Confusing id with _id field
• Assuming error when user exists

4. You wrote this GraphQL mutation to add a new user:
mutation { addUser(name: "Bob", age: "25") { id name } }
But you get an error: "Field 'addUser' argument 'age' of type 'Int!' was provided invalid value." What is the likely cause?

medium

A. The mutation syntax is incorrect; should use query instead

B. The age argument is missing in the schema definition

C. The age argument was provided as a string, not an integer

D. The age value 25 is invalid because it is too low

Solution

1. Step 1: Analyze the error message

   The error says the 'age' argument type is 'Int!' but the value is invalid.

2. Step 2: Analyze the query literal

   The query provides age: "25" as a string literal, but the schema expects an integer literal for Int!, causing the invalid value error.

3. Final Answer:

   The age argument was provided as a string, not an integer -> Option C

4. Quick Check:

   Type mismatch in argument causes invalid value error [OK]

Hint: Match argument types exactly as schema defines [OK]

Common Mistakes:

• Using query instead of mutation for data change
• Ignoring schema argument types
• Assuming numeric values are always valid

5. You want to update a user's city in MongoDB using GraphQL. The schema defines a mutation:
updateUser(id: ID!, city: String!): User
Which resolver code snippet correctly updates the city field in MongoDB?

hard

A. return UserModel.updateOne({ id }, { city });

B. return UserModel.findByIdAndUpdate(id, { city }, { new: true });

C. return UserModel.findOneAndDelete({ _id: id });

D. return UserModel.insertOne({ id, city });

Solution

1. Step 1: Identify correct MongoDB update method

   findByIdAndUpdate updates a document by its _id and returns the updated document.

2. Step 2: Check options for update and return

   return UserModel.findByIdAndUpdate(id, { city }, { new: true }); uses findByIdAndUpdate with { new: true } to return updated user, matching schema.

3. Final Answer:

   return UserModel.findByIdAndUpdate(id, { city }, { new: true }); -> Option B

4. Quick Check:

   Use findByIdAndUpdate to update and return new doc [OK]

Hint: Use findByIdAndUpdate with { new: true } to get updated doc [OK]

Common Mistakes:

• Using updateOne without returning updated document
• Deleting instead of updating
• Inserting new document instead of updating existing