Data source integration in GraphQL

Introduction

Data source integration helps you connect GraphQL to different places where data lives. This way, you can ask for data from many sources in one simple way.

You want to get user info from a database and product info from another service.

You need to combine data from an API and a local database in one query.

You want to fetch data from multiple places but show it together in your app.

You want to keep your data sources separate but still use GraphQL to get data.

You want to add a new data source without changing your whole app.

Syntax

GraphQL

type Query {
  fieldName: ReturnType
}

const resolvers = {
  Query: {
    fieldName: (parent, args, context, info) => {
      // fetch data from data source here
      return data;
    }
  }
};

Resolvers are functions that tell GraphQL how to get data from each source.

You can connect to databases, REST APIs, or other services inside resolvers.

Examples

This example shows how to get a user by ID from a database inside a resolver.

GraphQL

type Query {
  getUser(id: ID!): User
}

const resolvers = {
  Query: {
    getUser: (parent, args, context) => {
      return database.getUserById(args.id);
    }
  }
};

This example fetches weather data from an external API using a resolver.

GraphQL

type Query {
  getWeather(city: String!): Weather
}

const resolvers = {
  Query: {
    getWeather: async (parent, args) => {
      const response = await fetch(`https://api.weather.com/${args.city}`);
      return response.json();
    }
  }
};

Sample Program

This GraphQL schema and resolver connect to a simple data source (an array) to get a book by its ID.

GraphQL

type Query {
  book(id: ID!): Book
}

type Book {
  id: ID!
  title: String
  author: String
}

const books = [
  { id: "1", title: "1984", author: "George Orwell" },
  { id: "2", title: "The Hobbit", author: "J.R.R. Tolkien" }
];

const resolvers = {
  Query: {
    book: (parent, args) => {
      return books.find(book => book.id === args.id);
    }
  }
};

Important Notes

Resolvers can be async to handle data from APIs or databases.

Keep data sources separate to make your code easier to manage.

Test your resolvers to make sure they return the right data.

Summary

Data source integration connects GraphQL to where your data lives.

Resolvers fetch data from databases, APIs, or other services.

This lets you get data from many places with one simple query.

Practice

1. What is the main purpose of data source integration in GraphQL?

easy

A. To style the GraphQL interface for better user experience

B. To encrypt data sent over the network

C. To create new GraphQL schema types automatically

D. To connect GraphQL queries to the actual data storage or services

Solution

1. Step 1: Understand what data source integration means

   It means linking GraphQL queries to where data is stored or fetched from, like databases or APIs.

2. Step 2: Identify the main goal of this integration

   The goal is to let GraphQL get data from these sources when a query runs.

3. Final Answer:

   To connect GraphQL queries to the actual data storage or services -> Option D

4. Quick Check:

   Data source integration = connect queries to data [OK]

Hint: Think: GraphQL needs to reach data somewhere [OK]

Common Mistakes:

• Confusing integration with UI styling
• Thinking it creates schema types automatically
• Assuming it handles data encryption

2. Which of the following is the correct way to define a resolver that fetches data from a REST API in GraphQL?

easy

A. const resolver = () => fetch('https://api.example.com/data').then(res => res.json());

B. const resolver = fetch('https://api.example.com/data').json();

C. const resolver = () => fetch('https://api.example.com/data').json();

D. const resolver = () => fetch('https://api.example.com/data').then(res => res.text());

Solution

1. Step 1: Check how fetch works in JavaScript

   fetch returns a promise; to get JSON, you call .then(res => res.json()).

2. Step 2: Verify the resolver syntax

   const resolver = () => fetch('https://api.example.com/data').then(res => res.json()); correctly returns a function that fetches and parses JSON asynchronously.

3. Final Answer:

   const resolver = () => fetch('https://api.example.com/data').then(res => res.json()); -> Option A

4. Quick Check:

   fetch + then + json() = correct resolver [OK]

Hint: Remember fetch returns a promise; use then() to parse JSON [OK]

Common Mistakes:

• Calling .json() directly on fetch without then()
• Not wrapping fetch in a function
• Using res.text() instead of res.json() for JSON data

3. Given this resolver code snippet, what will be the output of the GraphQL query if the API returns [{"id":1,"name":"Alice"}]?

const resolvers = {
  Query: {
    users: async () => {
      const response = await fetch('https://api.example.com/users');
      const data = await response.json();
      return data;
    }
  }
};

medium

A. undefined

B. {"id":1,"name":"Alice"}

C. [{"id":1,"name":"Alice"}]

D. Error: fetch is not defined

Solution

1. Step 1: Analyze the resolver's return value

   The resolver awaits the fetch, parses JSON, and returns the data array as is.

2. Step 2: Match the returned data with the query output

   The API returns an array with one user object, so the query returns the same array.

3. Final Answer:

   [{"id":1,"name":"Alice"}] -> Option C

4. Quick Check:

   Returned data array = output array [OK]

Hint: Async resolver returns exactly what API sends after parsing [OK]

Common Mistakes:

• Assuming it returns a single object instead of an array
• Forgetting async/await means data is resolved
• Thinking fetch is undefined in server environment

4. Identify the error in this resolver code that fetches data from a database:

const resolvers = {
  Query: {
    products: () => {
      const data = db.query('SELECT * FROM products');
      return data;
    }
  }
};

medium

A. The resolver should be async and await the db query

B. The query string is incorrect SQL syntax

C. The resolver should return a promise, not data

D. The resolver is missing a try-catch block

Solution

1. Step 1: Understand db.query behavior

   db.query usually returns a promise, so it must be awaited in async functions.

2. Step 2: Check resolver function signature

   The resolver is not async and does not await the promise, so it returns a pending promise instead of data.

3. Final Answer:

   The resolver should be async and await the db query -> Option A

4. Quick Check:

   Async + await needed for promises [OK]

Hint: If db query returns promise, resolver must be async and await it [OK]

Common Mistakes:

• Ignoring async/await for promises
• Assuming SQL syntax is wrong without checking
• Thinking try-catch is mandatory for correctness

5. You want to integrate two data sources in a GraphQL resolver: a REST API and a SQL database. Which approach correctly combines data from both sources before returning it?

const resolvers = {
  Query: {
    combinedData: async () => {
      const apiData = await fetch('https://api.example.com/items').then(res => res.json());
      const dbData = await db.query('SELECT * FROM items');
      // What next?
    }
  }
};

hard

A. Return {apiData, dbData} as an object without merging

B. Return [...apiData, ...dbData] to merge both arrays

C. Return dbData only, ignoring apiData

D. Return apiData only, ignoring dbData

Solution

1. Step 1: Understand data types returned

   Both apiData and dbData are arrays of items fetched asynchronously.

2. Step 2: Combine arrays before returning

   Using spread syntax [...apiData, ...dbData] merges both arrays into one list.

3. Final Answer:

   Return [...apiData, ...dbData] to merge both arrays -> Option B

4. Quick Check:

   Spread arrays to combine data sources [OK]

Hint: Use spread operator to merge arrays from multiple sources [OK]

Common Mistakes:

• Returning only one data source ignoring the other
• Returning an object without merging arrays
• Forgetting to await both data sources