Gatsby GraphQL Query Error Fix

In this tutorial, you'll learn about Gatsby GraphQL Query Error Fix. We cover key concepts, practical examples, and best practices.

The Problem

A Gatsby GraphQL query returns null, throws Cannot query field "title" on type "Query", or returns empty results. The GraphQL schema depends on source plugins and the data they provide.

Quick Fix

Step 1: Use GraphiQL to explore the schema

npx gatsby develop
# Open http://localhost:8000/___graphql

Expected output: GraphiQL shows the complete schema with autocomplete and documentation.

Step 2: Check the query for correct node types

# Wrong — incorrect type name
query {
    allMardownRemark {  # misspelled
        nodes { title }
    }
}

# Right
query {
    allMarkdownRemark {
        nodes {
            title
            frontmatter {
                date
                slug
            }
        }
    }
}

Expected output: The query matches the available schema types.

Step 3: Handle null values in queries

query {
    allMarkdownRemark {
        nodes {
            title
            frontmatter {
                # Wrong — fails if featuredImage is null
                featuredImage
                # Right — use nullable access
                featuredImage
            }
        }
    }
}

In your component, handle null:

const image = node.frontmatter?.featuredImage;

Expected output: Null values do not crash the component.

Step 4: Use query variables correctly

// Wrong — hardcoded filter
export const query = graphql`
    query {
        markdownRemark(slug: { eq: "hello-world" }) {
            title
        }
    }
`;

// Right — with variable
export const query = graphql`
    query($slug: String!) {
        markdownRemark(frontmatter: { slug: { eq: $slug } }) {
            frontmatter {
                title
            }
        }
    }
`;

Expected output: Page context variables are passed to queries.

Step 5: Verify source plugins are configured

// gatsby-config.js
module.exports = {
    plugins: [
        {
            resolve: 'gatsby-source-filesystem',
            options: {
                name: 'blog',
                path: `${__dirname}/content/blog`,
            },
        },
        'gatsby-transformer-remark',
    ],
};

Expected output: Source plugins provide the expected data to the schema.

Step 6: Use aliases for field disambiguation

query {
    published: allMarkdownRemark(filter: { frontmatter: { draft: { ne: true } } }) {
        nodes { title }
    }
    drafts: allMarkdownRemark(filter: { frontmatter: { draft: { eq: true } } }) {
        nodes { title }
    }
}

Expected output: Query aliases let you request the same type with different filters.

Step 7: Check for missing gatsby-transformer plugins

If your data files (MDX, YAML, JSON) are not showing in the schema, you may need a transformer plugin:

npm install gatsby-transformer-remark gatsby-transformer-yaml

Expected output: File contents are transformed into queryable GraphQL nodes.

Prevention

• Use GraphiQL (/___<a href="/apis/graphql/">Graphql</a>) to explore available queries
• Handle null values with optional chaining
• Verify source plugins match your content types
• Use query variables for dynamic page queries

Common Mistakes with graphql error

1. Misunderstanding that String is [Char] with poor performance for large text operations
2. Using foldl instead of foldl' causing stack overflow on large lists
3. Forgetting deriving (Show, Eq) on custom data types needed for debugging

These mistakes appear frequently in real-world GATSBY code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ