docs: Example @link directive with array (by elemMatch)

[cpboyd]

Preliminary Checks

• This issue is not a duplicate. Before opening a new issue, please search existing issues: https://github.com/gatsbyjs/gatsby/issues
• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/gatsbyjs/gatsby/discussions

Summary

Previous issues:

• docs: Document caveat of @link directive (by parameter) #25089
• Using createSchemaCustomization in gatsby-node to define reverse mapping defined in gatsby-config #25077

After looking into a custom createFieldExtension , I discovered that elemMatch works and subsequently found this:
#25336 (comment)

Steps to Resolve this Issue

The docs should clarify that this works:

type Author implements Node {
  id: ID!
  name: String!
  email: String!
- posts: [Post!]! @link(from: "email", by: "authors.email")
+ posts: [Post!]! @link(from: "email", by: "authors.elemMatch.email")
}

type Post implements Node {
  id: ID!
  title: String!
  body: String!
  authors: [Author!]! @link(by: "email")
  tags: [Tag!]! @link
}

type Tag implements Node {
  id: ID!
  label: String!
  description: String!
- posts: [Post!]! @link(from: "id", by: "tags.id")
+ posts: [Post!]! @link(from: "id", by: "tags.elemMatch.id")
}

This works because how the query is formed by splitting on .:

gatsby/packages/gatsby/src/schema/resolvers.ts

Lines 416 to 423 in 564a8f7

	runQueryArgs.filter = options.by.split(`.`).reduceRight(
	(acc: Record<string, any>, key: string) => {
	const obj = {}
	obj[key] = acc
	return obj
	},
	Array.isArray(fieldValue) ? { in: fieldValue } : { eq: fieldValue }
	)

[LekoArts]

Happy to review a PR changing this!

https://www.gatsbyjs.com/contributing/how-to-open-a-pull-request/

[aryans1319]

Can I work on this issue @LekoArts ?

[marvinjude]

Absolutely @aryans1319! Feel free to jump on this.

[aryans1319]

Can you assign this to me?

[marvinjude]

Sure thing 👍

[kachida]

Hi @cpboyd @LekoArts , I'm interested in above task, can you pls let me know whether the above task is still open.