Support exposing the selected fields in a resolver

Copied from graph-gophers#169 and
used package 'query' instead of 'selected'.

Author: dvic

README.md

@@ -51,6 +51,7 @@ The method has up to two arguments:
 
 - Optional `context.Context` argument.
 - Mandatory `*struct { ... }` argument if the corresponding GraphQL field has arguments. The names of the struct fields have to be [exported](https://golang.org/ref/spec#Exported_identifiers) and have to match the names of the GraphQL arguments in a non-case-sensitive way.
+- Optional `[]query.SelectedField` argument to receive the tree of selected subfields in the GraphQL query (useful for preloading of database relations)
 
 The method has up to two results:
 

graphql_test.go

@@ -2,12 +2,14 @@ package graphql_test
 
 import (
 	"context"
+	"reflect"
 	"testing"
 	"time"
 
 	"github.com/qdentity/graphql-go"
 	"github.com/qdentity/graphql-go/example/starwars"
 	"github.com/qdentity/graphql-go/gqltesting"
+	"github.com/qdentity/graphql-go/query"
 )
 
 type helloWorldResolver1 struct{}
@@ -16,6 +18,26 @@ func (r *helloWorldResolver1) Hello() string {
 	return "Hello world!"
 }
 
+type selectedFieldsResolver struct {
+	assert func(fields []query.SelectedField)
+}
+
+func (r *selectedFieldsResolver) Do1(fields []query.SelectedField) *helloWorldResolver1 {
+	r.assert(fields)
+	return &helloWorldResolver1{}
+}
+
+func (r *selectedFieldsResolver) Do2(_ context.Context, fields []query.SelectedField) *helloWorldResolver1 {
+	r.assert(fields)
+	return &helloWorldResolver1{}
+}
+
+func (r *selectedFieldsResolver) Do3(_ context.Context, args struct{ Name string },
+	fields []query.SelectedField) *helloWorldResolver1 {
+	r.assert(fields)
+	return &helloWorldResolver1{}
+}
+
 type helloWorldResolver2 struct{}
 
 func (r *helloWorldResolver2) Hello(ctx context.Context) (string, error) {
@@ -111,6 +133,107 @@ func TestHelloWorld(t *testing.T) {
 	})
 }
 
+func TestSelectedFields(t *testing.T) {
+	gqltesting.RunTests(t, []*gqltesting.Test{
+		{
+			Schema: graphql.MustParseSchema(`
+				schema {
+					query: Query
+				}
+				type Hello {
+					hello: String!
+				}
+				type Query {
+					do1: Hello!
+				}
+			`, &selectedFieldsResolver{
+				assert: func(got []query.SelectedField) {
+					want := []query.SelectedField{
+						{Name: "hello"},
+					}
+					if !reflect.DeepEqual(want, got) {
+						t.Errorf("want %#v, got %#v", want, got)
+					}
+				},
+			}),
+			Query: `
+				{
+					do1 { hello }
+				}
+			`,
+			ExpectedResult: `
+				{
+					"do1": { "hello": "Hello world!" }
+				}
+			`,
+		},
+		{
+			Schema: graphql.MustParseSchema(`
+				schema {
+					query: Query
+				}
+				type Hello {
+					hello: String!
+				}
+				type Query {
+					do2: Hello!
+				}
+			`, &selectedFieldsResolver{
+				assert: func(got []query.SelectedField) {
+					want := []query.SelectedField{
+						{Name: "hello"},
+					}
+					if !reflect.DeepEqual(want, got) {
+						t.Errorf("want %#v, got %#v", want, got)
+					}
+				},
+			}),
+			Query: `
+				{
+					do2 { hello }
+				}
+			`,
+			ExpectedResult: `
+				{
+					"do2": { "hello": "Hello world!" }
+				}
+			`,
+		},
+		{
+			Schema: graphql.MustParseSchema(`
+				schema {
+					query: Query
+				}
+				type Hello {
+					hello: String!
+				}
+				type Query {
+					do3(name: String!): Hello!
+				}
+			`, &selectedFieldsResolver{
+				assert: func(got []query.SelectedField) {
+					want := []query.SelectedField{
+						{Name: "hello"},
+					}
+					if !reflect.DeepEqual(want, got) {
+						t.Errorf("want %#v, got %#v", want, got)
+					}
+				},
+			}),
+			Query: `
+				{
+					do3(name: "") { hello }
+				}
+			`,
+			ExpectedResult: `
+				{
+					"do3": { "hello": "Hello world!" }
+				}
+			`,
+		},
+	})
+}
+
 func TestHelloSnake(t *testing.T) {
 	gqltesting.RunTests(t, []*gqltesting.Test{
 		{

internal/exec/exec.go

@@ -14,6 +14,7 @@ import (
 	"github.com/qdentity/graphql-go/internal/query"
 	"github.com/qdentity/graphql-go/internal/schema"
 	"github.com/qdentity/graphql-go/log"
+	pubquery "github.com/qdentity/graphql-go/query"
 	"github.com/qdentity/graphql-go/trace"
 )
 
@@ -142,6 +143,24 @@ func typeOf(tf *selected.TypenameField, resolver reflect.Value) string {
 	return ""
 }
 
+func selectionToSelectedFields(sels []selected.Selection) []pubquery.SelectedField {
+	n := len(sels)
+	if n == 0 {
+		return nil
+	}
+	selectedFields := make([]pubquery.SelectedField, 0, n)
+	for _, sel := range sels {
+		selField, ok := sel.(*selected.SchemaField)
+		if ok {
+			selectedFields = append(selectedFields, pubquery.SelectedField{
+				Name:     selField.Field.Name,
+				Selected: selectionToSelectedFields(selField.Sels),
+			})
+		}
+	}
+	return selectedFields
+}
+
 func execFieldSelection(ctx context.Context, r *Request, f *fieldToExec, path *pathSegment, applyLimiter bool) {
 	if applyLimiter {
 		r.Limiter <- struct{}{}
@@ -180,6 +199,9 @@ func execFieldSelection(ctx context.Context, r *Request, f *fieldToExec, path *p
 		if f.field.ArgsPacker != nil {
 			in = append(in, f.field.PackedArgs)
 		}
+		if f.field.HasSelected {
+			in = append(in, reflect.ValueOf(selectionToSelectedFields(f.sels)))
+		}
 		callOut := f.resolver.Method(f.field.MethodIndex).Call(in)
 		result = callOut[0]
 		if f.field.HasError && !callOut[1].IsNil() {

internal/exec/resolvable/resolvable.go

@@ -9,6 +9,7 @@ import (
 	"github.com/qdentity/graphql-go/internal/common"
 	"github.com/qdentity/graphql-go/internal/exec/packer"
 	"github.com/qdentity/graphql-go/internal/schema"
+	pubquery "github.com/qdentity/graphql-go/query"
 )
 
 type Schema struct {
@@ -34,6 +35,7 @@ type Field struct {
 	MethodIndex int
 	HasContext  bool
 	HasError    bool
+	HasSelected bool
 	ArgsPacker  *packer.StructPacker
 	ValueExec   Resolvable
 	TraceLabel  string
@@ -251,6 +253,7 @@ func (b *execBuilder) makeObjectExec(typeName string, fields schema.FieldList, p
 }
 
 var contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
+var selectedType = reflect.TypeOf([]pubquery.SelectedField(nil))
 var errorType = reflect.TypeOf((*error)(nil)).Elem()
 
 func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.Method, methodIndex int, methodHasReceiver bool) (*Field, error) {
@@ -280,6 +283,11 @@ func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.
 		in = in[1:]
 	}
 
+	hasSelected := len(in) > 0 && in[0] == selectedType
+	if hasSelected {
+		in = in[1:]
+	}
+
 	if len(in) > 0 {
 		return nil, fmt.Errorf("too many parameters")
 	}
@@ -300,6 +308,7 @@ func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.
 		TypeName:    typeName,
 		MethodIndex: methodIndex,
 		HasContext:  hasContext,
+		HasSelected: hasSelected,
 		ArgsPacker:  argsPacker,
 		HasError:    hasError,
 		TraceLabel:  fmt.Sprintf("GraphQL field: %s.%s", typeName, f.Name),

query/selected_field.go

@@ -0,0 +1,6 @@
+package query
+
+type SelectedField struct {
+	Name     string
+	Selected []SelectedField
+}