From 302aa264714b240fd3f5fbfb29e1993f15d64630 Mon Sep 17 00:00:00 2001
From: stefanprobst <stefanprobst@kolabnow.com>
Date: Mon, 14 Oct 2019 23:16:33 +0200
Subject: [PATCH] fix(gatsby): Extend fields when merging types (#18500)

---
 .../src/schema/__tests__/build-schema.js      | 27 +++++++++++++++
 packages/gatsby/src/schema/schema.js          | 33 ++++++++++++++++++-
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/packages/gatsby/src/schema/__tests__/build-schema.js b/packages/gatsby/src/schema/__tests__/build-schema.js
index e5e36fb251280..a8477e2d5cd59 100644
--- a/packages/gatsby/src/schema/__tests__/build-schema.js
+++ b/packages/gatsby/src/schema/__tests__/build-schema.js
@@ -814,6 +814,33 @@ describe(`Build schema`, () => {
       )
     })
 
+    it(`extends fieldconfigs when merging types`, async () => {
+      createTypes(
+        buildObjectType({
+          name: `Mdx`,
+          interfaces: [`Node`],
+          fields: {
+            body: {
+              type: `String`,
+              resolve: () => `Mdx!`,
+            },
+          },
+        })
+      )
+      createTypes(`
+        type Mdx implements Node {
+          body: String!
+        }
+      `)
+
+      const schema = await buildSchema()
+      const fields = schema.getType(`Mdx`).getFields()
+
+      expect(fields.body.type.toString()).toBe(`String!`)
+      expect(typeof fields.body.resolve).toBe(`function`)
+      expect(fields.body.resolve()).toBe(`Mdx!`)
+    })
+
     it(`displays error message for reserved Node interface`, () => {
       const typeDefs = [
         `interface Node { foo: Boolean }`,
diff --git a/packages/gatsby/src/schema/schema.js b/packages/gatsby/src/schema/schema.js
index 8e1b17643711c..e8d7a1e602447 100644
--- a/packages/gatsby/src/schema/schema.js
+++ b/packages/gatsby/src/schema/schema.js
@@ -7,6 +7,8 @@ const {
   parse,
   GraphQLNonNull,
   GraphQLList,
+  GraphQLObjectType,
+  GraphQLInterfaceType,
 } = require(`graphql`)
 const {
   ObjectTypeComposer,
@@ -15,6 +17,7 @@ const {
   InputTypeComposer,
   ScalarTypeComposer,
   EnumTypeComposer,
+  defineFieldMapToConfig,
 } = require(`graphql-compose`)
 
 const apiRunner = require(`../utils/api-runner-node`)
@@ -315,11 +318,30 @@ const mergeTypes = ({
     plugin.name === `default-site-plugin` ||
     plugin.name === typeOwner
   ) {
-    typeComposer.merge(type)
+    if (type instanceof ObjectTypeComposer) {
+      mergeFields({ typeComposer, fields: type.getFields() })
+      type.getInterfaces().forEach(iface => typeComposer.addInterface(iface))
+    } else if (type instanceof InterfaceTypeComposer) {
+      mergeFields({ typeComposer, fields: type.getFields() })
+    } else if (type instanceof GraphQLObjectType) {
+      mergeFields({
+        typeComposer,
+        fields: defineFieldMapToConfig(type.getFields()),
+      })
+      type.getInterfaces().forEach(iface => typeComposer.addInterface(iface))
+    } else if (type instanceof GraphQLInterfaceType) {
+      mergeFields({
+        typeComposer,
+        fields: defineFieldMapToConfig(type.getFields()),
+      })
+    }
+
     if (isNamedTypeComposer(type)) {
       typeComposer.extendExtensions(type.getExtensions())
     }
+
     addExtensions({ schemaComposer, typeComposer, plugin, createdFrom })
+
     return true
   } else {
     report.warn(
@@ -1160,3 +1182,12 @@ const checkQueryableInterfaces = ({ schemaComposer }) => {
     )
   }
 }
+
+const mergeFields = ({ typeComposer, fields }) =>
+  Object.entries(fields).forEach(([fieldName, fieldConfig]) => {
+    if (typeComposer.hasField(fieldName)) {
+      typeComposer.extendField(fieldName, fieldConfig)
+    } else {
+      typeComposer.setField(fieldName, fieldConfig)
+    }
+  })