Merge pull request #5 from sipsma/enforce-iface-at-runtime

check whether object implements interface at runtime

abstract_test.go

@@ -424,7 +424,7 @@ func TestResolveTypeOnInterfaceYieldsUsefulError(t *testing.T) {
 	humanType = graphql.NewObject(graphql.ObjectConfig{
 		Name: "Human",
 		Fields: graphql.Fields{
-			"name": &graphql.Field{
+			"job": &graphql.Field{
 				Type: graphql.String,
 			},
 		},
@@ -507,7 +507,7 @@ func TestResolveTypeOnInterfaceYieldsUsefulError(t *testing.T) {
 		},
 		Errors: []gqlerrors.FormattedError{
 			{
-				Message: `Runtime Object type "Human" is not a possible type for "Pet".`,
+				Message: `"Pet" expects field "name" but "Human" does not provide it.`,
 				Locations: []location.SourceLocation{
 					{
 						Line:   2,

executor.go

@@ -794,8 +794,18 @@ func completeAbstractValue(eCtx *executionContext, returnType Abstract, fieldAST
 	}
 	if unionReturnType, ok := returnType.(*Union); ok && unionReturnType.ResolveType != nil {
 		runtimeType = unionReturnType.ResolveType(resolveTypeParams)
+		if !eCtx.Schema.IsPossibleType(returnType, runtimeType) {
+			panic(gqlerrors.NewFormattedError(
+				fmt.Sprintf(`Runtime Object type "%v" is not a possible type `+
+					`for "%v".`, runtimeType, returnType),
+			))
+		}
 	} else if interfaceReturnType, ok := returnType.(*Interface); ok && interfaceReturnType.ResolveType != nil {
 		runtimeType = interfaceReturnType.ResolveType(resolveTypeParams)
+		// verify the object matches the interface
+		if err := assertObjectImplementsInterface(&eCtx.Schema, runtimeType, interfaceReturnType); err != nil {
+			panic(err)
+		}
 	} else {
 		runtimeType = defaultResolveTypeFn(resolveTypeParams, returnType)
 	}
@@ -807,13 +817,6 @@ func completeAbstractValue(eCtx *executionContext, returnType Abstract, fieldAST
 		panic(err)
 	}
 
-	if !eCtx.Schema.IsPossibleType(returnType, runtimeType) {
-		panic(gqlerrors.NewFormattedError(
-			fmt.Sprintf(`Runtime Object type "%v" is not a possible type `+
-				`for "%v".`, runtimeType, returnType),
-		))
-	}
-
 	return completeObjectValue(eCtx, runtimeType, fieldASTs, info, path, result)
 }