Update schema-customization.md (#25109)

* Update schema-customization.md Adds paragraph at the end of section **Foreign-key fields** explaining that the `by` argument doesn't work with arrays. Closes #25089 * Update docs/docs/schema-customization.md Co-authored-by: Aisha Blake <[email protected]> Co-authored-by: gatsbybot <[email protected]>
gatsbyjs · Jun 20, 2020 · d708fa2 · d708fa2
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diff --git a/docs/docs/schema-customization.md b/docs/docs/schema-customization.md
@@ -396,6 +396,8 @@ otherwise the foreign-key has to be provided with the `by` argument. The
 optional `from` argument allows getting the field on the current type which acts as the foreign-key to the field specified in `by`.
 In other words, you `link` **on** `from` **to** `by`. This makes `from` especially helpful when adding a field for back-linking.
 
+Keep in mind that in the example above, the link of `posts` in `AuthorJson` works because `frontmatter` and `author` are both objects. If, for example, the `Frontmatter` type had a list of `authors` instead (`frontmatter.authors.email`), it wouldn't work since the `by` argument doesn't support arrays. In that case, you'd have to provide a custom resolver with [Gatsby Type Builders](/docs/schema-customization/#gatsby-type-builders) or [createResolvers API](/docs/schema-customization/#createresolvers-api).
+
 > Note that when using `createTypes` to fix type inference for a foreign-key field
 > created by a plugin, the underlying data will probably live on a field with
 > a `___NODE` suffix. Use the `from` argument to point the `link` extension to