fix(gatsby): merge resolveType when merging abstract types

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

@@ -979,6 +979,70 @@ describe(`Build schema`, () => {
       expect(interfaces).toEqual([`Foo`, `Bar`])
     })
 
+    it(`merges resolveType for abstract types (Type Builder)`, async () => {
+      createTypes(
+        [
+          `interface Foo { foo: String }`,
+          `
+            type Fizz { id: ID! }
+            type Buzz { id: ID! }
+            union FizzBuzz = Fizz | Buzz
+          `,
+          buildInterfaceType({
+            name: `Foo`,
+            fields: { id: `ID!` },
+            resolveType: source => source.expectedType,
+          }),
+          buildUnionType({
+            name: `FizzBuzz`,
+            resolveType: source => (source.isFizz ? `Fizz` : `Buzz`),
+          }),
+        ],
+        {
+          name: `default-site-plugin`,
+        }
+      )
+      const schema = await buildSchema()
+      const Foo = schema.getType(`Foo`)
+      expect(Foo.resolveType({ expectedType: `Bar` })).toEqual(`Bar`)
+
+      const FizzBuzz = schema.getType(`FizzBuzz`)
+      expect(FizzBuzz.resolveType({ isFizz: true })).toEqual(`Fizz`)
+      expect(FizzBuzz.resolveType({ isFizz: false })).toEqual(`Buzz`)
+    })
+
+    it(`merges resolveType for abstract types (graphql-js)`, async () => {
+      createTypes(
+        [
+          `interface Foo { foo: String }`,
+          `
+            type Fizz { id: ID! }
+            type Buzz { id: ID! }
+            union FizzBuzz = Fizz | Buzz
+          `,
+          new GraphQLInterfaceType({
+            name: `Foo`,
+            fields: { foo: { type: GraphQLString } },
+            resolveType: source => source.expectedType,
+          }),
+          new GraphQLUnionType({
+            name: `FizzBuzz`,
+            resolveType: source => (source.isFizz ? `Fizz` : `Buzz`),
+          }),
+        ],
+        {
+          name: `default-site-plugin`,
+        }
+      )
+      const schema = await buildSchema()
+      const Foo = schema.getType(`Foo`)
+      expect(Foo.resolveType({ expectedType: `Bar` })).toEqual(`Bar`)
+
+      const FizzBuzz = schema.getType(`FizzBuzz`)
+      expect(FizzBuzz.resolveType({ isFizz: true })).toEqual(`Fizz`)
+      expect(FizzBuzz.resolveType({ isFizz: false })).toEqual(`Buzz`)
+    })
+
     it(`merges plugin-defined type (Type Builder) with overridable built-in type without warning`, async () => {
       createTypes(
         [

packages/gatsby/src/schema/schema.js

@@ -8,6 +8,7 @@ const {
   GraphQLList,
   GraphQLObjectType,
   GraphQLInterfaceType,
+  GraphQLUnionType,
 } = require(`graphql`)
 const {
   ObjectTypeComposer,
@@ -406,6 +407,15 @@ const mergeTypes = ({
     type.getInterfaces().forEach(iface => typeComposer.addInterface(iface))
   }
 
+  if (
+    type instanceof GraphQLInterfaceType ||
+    type instanceof InterfaceTypeComposer ||
+    type instanceof GraphQLUnionType ||
+    type instanceof UnionTypeComposer
+  ) {
+    mergeResolveType({ typeComposer, type })
+  }
+
   if (isNamedTypeComposer(type)) {
     typeComposer.extendExtensions(type.getExtensions())
   }
@@ -1387,3 +1397,20 @@ const mergeFields = ({ typeComposer, fields }) =>
       typeComposer.setField(fieldName, fieldConfig)
     }
   })
+
+const mergeResolveType = ({ typeComposer, type }) => {
+  if (
+    (type instanceof GraphQLInterfaceType ||
+      type instanceof GraphQLUnionType) &&
+    type.resolveType
+  ) {
+    typeComposer.setResolveType(type.resolveType)
+  }
+  if (
+    (type instanceof InterfaceTypeComposer ||
+      type instanceof UnionTypeComposer) &&
+    type.getResolveType()
+  ) {
+    typeComposer.setResolveType(type.getResolveType())
+  }
+}