Getting custom fields in gatsby-plugin-feed - rss

Question

I can't figure out how to use the gatsby-plugin-feed to add some custom content from my frontmatter to the rss feed. Here is my config:
{
resolve: `gatsby-plugin-feed`,
options: {
query: `
{
site {
siteMetadata {
title
description
siteUrl
site_url: siteUrl
}
}
}
`,
feeds: [
{
serialize: ({ query: { site, allMarkdownRemark } }) => {
return allMarkdownRemark.edges.map(edge => ({
...edge.node.frontmatter,
description: edge.node.excerpt,
url: site.siteMetadata.siteUrl + edge.node.fields.slug,
guid: site.siteMetadata.siteUrl + edge.node.fields.slug,
}));
},
query: `
{
allMarkdownRemark(
limit: 1000,
sort: { order: DESC, fields: [fields___prefix] },
filter: { fields: { source: {eq: "posts"}, slug: { ne: null } } }
) {
edges {
node {
excerpt
html
fields {
slug
prefix
}
frontmatter {
title
subtitle
tags
categories
youtube
}
}
}
}
}
`,
output: '/rss.xml',
},
],
},
},
I thought that using the spread operator on the edge.node.frontmatter object would add all of the fields but that resulting item is missing a ton of the frontmatter fields.
I am sure I am missing something, or I don't understand something about rss feeds. Can anyone point me in the correct direction? Thank you.