Graphql: skip a parameter if its type is string

Question

I have a graphql query for Markdown files which requests the title, author and coverImg from the frontmatter of the Markdown.

Some Markdown files do not have a cover image, when there is no image, then it looks like the type of coverImg is String

I am using gatsby-image which uses sharp and I would like to descend into the children of the coverImg parameter, if the coverImg is an object and not a string

---

This is my query

query($path: String!) {
    markdownRemark(fields: { slug: { eq: $path } }) {
        frontmatter {
            title
            author
            coverImg {
              childImageSharp {
                fluid(maxWidth: 600) {
                  src
                  srcSet
                  sizes
                  aspectRatio
                }
              }
           }
        }
     }
 }

If I try this I receive the error

There was an error in your GraphQL query:

Field "cover" must not have a selection since type "String" has no subfields.

This can happen if you e.g. accidentally added { } to the field "cover". If you didn't expect "cover" to be of type "String" make sure that your input source and/or plugin is correct.

---

It looks like there's graphql if statements but I don't know how to use these to conditionally branch to children. It would be great if I could just add something like

 coverImg @skip(if: coverImg is String) {

Answer 1

AFAIK Gatsby doesn't infer the schema of your data on a per-file basis, so once it infer a field as String, that field will always be of type String.

You can use the schema customization hook to tell Gatsby that the field coverImg is a file (as suggested by @ksav in the comment):

exports.createSchemaCustomization = ({ actions }) => {
  actions.createTypes(`
    type MarkdownRemarkFrontmatter @infer {
      coverImg: File
    }

    type MarkdownRemark implements Node @infer {
      frontmatter: MarkdownRemarkFrontmatter
    }
  `)
}

Then you can check if coverImg exists.

if (!coverImg) {
  // no image
} else if (coverImg.childImageSharp) {
  // has image
} else {
  // file is not an image
}