Gatsby + Netlify CMS: One to Many relation

config.yml

  - name: "expertise"
    label: "Expertise"
    folder: "src/pages/expertise"
    create: true
    fields:
      - {label: "Kind", name: kind, widget: hidden, default: "expertise"}
      - {label: "Title", name: title, widget: string}
      - {label: "Image", name: image, widget: image}

  - name: "experts"
    label: "Expert"
    folder: "src/pages/experts"
    create: true
    fields:
      - {label: "Kind", name: kind, widget: hidden, default: "expert"}
      - {label: "Title", name: title, widget: string}
      - {label: "Position", name: position, widget: string}
      - {label: "LinkedIn Link", name: linkedinlink, widget: string}
      - {label: "Twitter Link", name: twitterlink, widget: string}
      - {label: "External Link", name: externallink, widget: string}
      - {label: "Image", name: image, widget: image}      
      - label: "Expertise"
        name: "expertise"
        widget: "relation"
        multiple: true
        collection: "expertise_en"
        searchFields: ["title"]
        valueField: "title"
        displayFields: ["title"]
      - {label: "Description", name: description, widget: markdown}

gatsby-config.js

plugins: [
  ...
],
mapping: {
  'MarkdownRemark.fields.expertises': `MarkdownRemark`
}
...

gatsby-node.js

exports.sourceNodes = ({ boundActionCreators, getNodes, getNode }) => {
  const { createNodeField } = boundActionCreators
  const expertiseOfExperts = {} 

  const getExpertiseNodeByName = name => getNodes().find(
  node_ =>
    node_.internal.type === `MarkdownRemark` &&
    node_.frontmatter.title === name 
  )

  getNodes()
    .filter(node => node.internal.type === `MarkdownRemark`)
    .forEach(node => {
      if (node.frontmatter.kind == 'expert') {
        const expertiseNodes = node.frontmatter.expertise instanceof Array 
          ? node.frontmatter.expertise.map(getExpertiseNodeByName) 
          : [getExpertiseNodeByName(node.frontmatter.expertise)]

        expertiseNodes.filter(authorNode=> authorNode).map(authorNode => {
          if (!(node.id in expertiseOfExperts )) {
            expertiseOfExperts[node.id] = []
          }
          expertiseOfExperts[node.id].push(authorNode.id)
        })
      }
    })
    
  Object.entries(expertiseOfExperts).forEach(([expertNodeId, expertiseIds]) => {
    createNodeField({
      node: getNode(expertNodeId),
      name: `expertises`,
      value: expertiseIds,
    })
  })
}

graphql-query.js

query MyQuery {
  allMarkdownRemark(sort: {order: ASC, fields: [frontmatter___order]}, filter: {frontmatter: {kind: {eq: "expert"}}}) {
    edges {
      node {
        frontmatter {
          title
          position
          linkedinlink
          twitterlink
          externallink
          expertise
        }
        fields {
          expertises {
            id
            frontmatter {
              title
            }
          }
        }
      }
    }
  }
}