MongoDB with GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of using GraphQL with MongoDB?

GraphQL lets you ask for exactly the data you want from MongoDB, making data fetching efficient and flexible.

beginner

In GraphQL, what is a resolver's role when working with MongoDB?

A resolver is a function that tells GraphQL how to get data from MongoDB for each query or mutation.

intermediate

How do you define a GraphQL schema to fetch a list of users from a MongoDB collection?

You define a Query type with a field like users: [User], where User is a type matching MongoDB user documents.

intermediate

What is the benefit of using GraphQL queries over REST endpoints with MongoDB?

GraphQL queries reduce over-fetching and under-fetching by letting clients specify exactly what fields they want from MongoDB.

advanced

How can you handle MongoDB ObjectId fields in GraphQL schemas?

You can use custom scalar types or convert ObjectId to strings in resolvers to work smoothly with GraphQL.

What does a GraphQL resolver do when connected to MongoDB?

A Defines the GraphQL schema types

B Runs MongoDB server

C Fetches or modifies data in MongoDB based on the query or mutation

D Creates indexes in MongoDB

Resolvers are functions that fetch or update data in MongoDB according to the GraphQL query or mutation.

Which GraphQL operation is used to retrieve data from MongoDB?

A Query

B Subscription

C Mutation

D Directive

Queries are used to read or fetch data, such as from MongoDB collections.

How can you represent MongoDB's ObjectId in a GraphQL schema?

A As a Boolean

B As a list

C As an integer

D As a custom scalar or string

ObjectId is usually handled as a string or custom scalar type in GraphQL schemas.

What is a key advantage of GraphQL over REST when querying MongoDB?

A GraphQL requires no schema

B GraphQL lets clients specify exactly which fields to get

C GraphQL only supports mutations

D GraphQL automatically indexes MongoDB

GraphQL allows clients to ask for only the data they need, reducing unnecessary data transfer.

Which GraphQL type would you use to define a list of MongoDB documents?

A List

B Scalar

C Object

D Enum

Lists in GraphQL represent arrays, perfect for multiple MongoDB documents.

Explain how GraphQL resolvers interact with MongoDB to fetch data.

Think about how GraphQL knows what data to get from MongoDB.

Describe the benefits of using GraphQL queries instead of REST endpoints when working with MongoDB.

Consider how clients ask for data differently in GraphQL.

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