I'm trying to filter the slug generated by Gatsby so I can remove white space and replace it with "-". I have Gatsby's reporter set to fire when onCreateNode is ran but it never seems to fire regardless of clearing the cache first. When I check in GraphiQL it shows no change to the slug. Here is my gastby-node.js onCreateNode:

exports.onCreateNode = ({ node, getNode, actions, reporter }) => {
reporter.info(`onCreateNode called from gatsby-node`)
const { createNodeField } = actions
if (node.internal.type === `MarkdownRemark`) {
    const slug = createFilePath({ node, getNode, basePath: `posts` }).replace(
        ' ',
        '-'
    )
    createNodeField({
        node,
        name: `slug`,
        value: slug
    })
}}

Any input would be greatly appreciated!