feat(gatsby): Allow proxying field values from nested fields (#16149)

* Allow proxying field values from nested fields

* Lint'n'Loki

* Test using args.from in custom field extension

* Add some extra safety

* Pass options via info

* Lint

* Add test example for proxying to parent node

* Add test todos for proxying to fields on other nodes

* Remove unnecessary restriction

docs/docs/schema-customization.md

@@ -580,7 +580,12 @@ exports.createSchemaCustomization = ({ actions }) => {
           sanitize: "Boolean",
         },
         resolve(source, args, context, info) {
-          const fieldValue = source[info.fieldName]
+          const fieldValue = context.defaultFieldResolver(
+            source,
+            args,
+            context,
+            info
+          )
           const shouldSanitize =
             args.sanitize != null ? args.sanitize : options.sanitize
           const processor = remark().use(html, { sanitize: shouldSanitize })
@@ -640,6 +645,8 @@ If multiple field extensions are added to a field, resolvers are processed in th
 first a custom resolver added with `createTypes` (or `createResolvers`) runs, then field
 extension resolvers execute from left to right.
 
+Finally, note that in order to get the current fieldValue, we use `context.defaultFieldResolver`.
+
 ## createResolvers API
 
 While it is possible to directly pass `args` and `resolvers` along the type

packages/gatsby/src/schema/__tests__/build-schema.js

@@ -15,6 +15,7 @@ const {
   buildUnionType,
   buildInterfaceType,
 } = require(`../types/type-builders`)
+const withResolverContext = require(`../context`)
 require(`../../db/__tests__/fixtures/ensure-loki`)()
 
 const nodes = require(`./fixtures/node-model`)
@@ -913,7 +914,7 @@ describe(`Build schema`, () => {
         fields[`name`].resolve(
           { name: `Mikhail` },
           { withHello: true },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `name`,
           }
@@ -923,7 +924,7 @@ describe(`Build schema`, () => {
         fields[`name`].resolve(
           { name: `Mikhail` },
           { withHello: false },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `name`,
           }
@@ -959,7 +960,7 @@ describe(`Build schema`, () => {
         fields[`name`].resolve(
           { name: `Mikhail` },
           { withHello: true },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `name`,
           }
@@ -969,7 +970,7 @@ describe(`Build schema`, () => {
         fields[`name`].resolve(
           { name: `Mikhail` },
           { withHello: false },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `name`,
           }
@@ -1105,7 +1106,7 @@ describe(`Build schema`, () => {
         await fields[`date`].resolve(
           { date: new Date(2019, 10, 10) },
           { formatString: `YYYY` },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `date`,
           }
@@ -1115,7 +1116,7 @@ describe(`Build schema`, () => {
         await fields[`date`].resolve(
           { date: new Date(2010, 10, 10) },
           { formatString: `YYYY` },
-          {},
+          withResolverContext({}, schema),
           {
             fieldName: `date`,
           }

packages/gatsby/src/schema/context.js

@@ -1,4 +1,5 @@
 const { LocalNodeModel } = require(`./node-model`)
+const { defaultFieldResolver } = require(`./resolvers`)
 
 const withResolverContext = (context, schema, customContext) => {
   const nodeStore = require(`../db/nodes`)
@@ -7,6 +8,7 @@ const withResolverContext = (context, schema, customContext) => {
   return {
     ...context,
     ...customContext,
+    defaultFieldResolver,
     nodeModel: new LocalNodeModel({
       nodeStore,
       schema,