title: Gatsby GraphQL error: Variable "$slug" is never used in operation "BlogPostQuery"
body: <p>I am unable to pull in the data of my Ghost blog using Gatsby. I am using Ghost as my back end and I am using a package to get the Ghost blog as a source. The problem is just getting the individual posts on the page. Here is <code>blog-post.js</code>:</p>
  
  <pre><code>import React from "react";
  
  export default ({ data }) =&gt; {
  //   const post = data.allGhostPost.edges;
    return (
      &lt;div&gt;
        {/* &lt;h1&gt;{post.title}&lt;/h1&gt; */}
        {/* &lt;div dangerouslySetInnerHTML={{ __html: post.html }} /&gt; */}
      &lt;/div&gt;
    );
  };
  
  export const query = graphql`
    query BlogPostQuery($slug: String!) {
      allGhostPost {
          edges {
            node {
              id
              slug
              title
              html
              published_at
            }
          }
        }
    }
  `;
  </code></pre>
  
  <p>Here is my gatsby node file:</p>
  
  <pre><code>exports.createPages = ({ graphql, boundActionCreators}) =&gt; {
      const {createPage} = boundActionCreators
  
      return new Promise((resolve, reject) =&gt; {
          const blogPostTemplate = path.resolve(`src/templates/blog-post.js`)
  
          resolve(
              graphql(
                  `
                  {
                      allGhostPost(sort: { order: DESC, fields: [published_at] }) {
                        edges {
                          node {
                            id
                            slug
                            title
                            html
                            published_at      
                          }
                        }
                      }
                    }
                  `
              )
              .then(result =&gt; {
                  result.data.allGhostPost.edges.forEach(edge =&gt; {
                      createPage({
                          path: edge.node.slug,
                          component: blogPostTemplate,
                          context: {
                              slug: edge.node.slug
                          }
                      })
                  })
                  return;
              })
          )
      })
  }
  </code></pre>