title: Gatsby - Graphql error "not defined by type MarkdownRemarkFieldsFilterInput" for gatsby-plugin-categories
body: <h2>Expected Result</h2>
  
  <p>I should be able to see a category page. I did exactly what the documentation said at <a href="https://www.npmjs.com/package/gatsby-plugin-categories" rel="nofollow noreferrer">https://www.npmjs.com/package/gatsby-plugin-categories</a></p>
  
  <h2>Actual Result</h2>
  
  <p>Geting a build error</p>
  
  <pre><code>error GraphQL Error Field "category" is not defined by type MarkdownRemarkFieldsFilterInput.
  </code></pre>
  
  <hr>
  
  <h2>Code</h2>
  
  <h3>src/templates/category.js</h3>
  
  <pre><code>import React from "react";
  import Helmet from "react-helmet";
  import { graphql } from "gatsby";
  import Layout from "../layout";
  import PostListing from "../components/PostListing";
  
  export default class CategoryTemplate extends React.Component {
    render() {
      const { pageContext, data } = this.props;
      const { category } = pageContext;
      return (
        &lt;Layout&gt;
          &lt;div className="category-container"&gt;
            &lt;Helmet title={`Posts in category "${category}"`} /&gt;
            &lt;PostListing postEdges={data.allMarkdownRemark.edges} /&gt;
          &lt;/div&gt;
        &lt;/Layout&gt;
      );
    }
  }
  
  export const pageQuery = graphql`
    query CategoryPage($category: String) {
      allMarkdownRemark(
        limit: 1000
        filter: { fields: { category: { eq: $category } } }
      ) {
        totalCount
        edges {
          node {
            fields {
              slug
              category
            }
            excerpt
            timeToRead
            frontmatter {
              title
              tags
              date
            }
          }
        }
      }
    }
  `;
  </code></pre>
  
  <h3>gatsby-config.js</h3>
  
  <pre><code>module.exports = {
    siteMetadata: {
      title: `VLLG`,
      description: `Village | California Campsites.`,
      author: `Juan Gallardo`,
    },
    plugins: [
      `gatsby-plugin-react-helmet`,
      {
        resolve: `gatsby-source-filesystem`,
        options: {
          name: `images`,
          path: `${__dirname}/src/images`,
        },
      },
      {
        resolve: `gatsby-source-filesystem`,
        options: {
          name: `pages`,
          path: `${__dirname}/src/pages/`,
        },
      },
      {
      resolve: `gatsby-transformer-remark`,
      options: {
        // CommonMark mode (default: true)
        commonmark: true,
        // Footnotes mode (default: true)
        footnotes: true,
        // Pedantic mode (default: true)
        pedantic: true,
        // GitHub Flavored Markdown mode (default: true)
        gfm: true,
        // Plugins configs
        plugins: [],
      },
    },
      `gatsby-transformer-sharp`,
      `gatsby-plugin-sharp`,
      {
        resolve: `gatsby-plugin-manifest`,
        options: {
          name: `VLLG`,
          short_name: `vllg`,
          start_url: `/`,
          background_color: `#663399`,
          theme_color: `#663399`,
          display: `minimal-ui`,
          icon: `src/images/gatsby-icon.png`, // This path is relative to the root of the site.
        },
      },
      // this (optional) plugin enables Progressive Web App + Offline functionality
      // To learn more, visit: https://gatsby.dev/offline
      // 'gatsby-plugin-offline',
    ],
  }
  </code></pre>
  
  <h3>How I added a category to a post (in a .md file)</h3>
  
  <pre><code>category: 'lol'
  </code></pre>
  
  <h3>Structure for blog posts</h3>
  
  <p><a href="https://i.stack.imgur.com/GOawi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GOawi.png" alt="enter image description here"></a></p>
  
  <h3>Results from Graphql query</h3>
  
  <p><a href="https://i.stack.imgur.com/ozuNB.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ozuNB.png" alt="enter image description here"></a></p>