Expose the tree of field subselections to the resolver on demand

README.md

@@ -77,10 +77,11 @@ When using `UseFieldResolvers` schema option, a struct field will be used *only*
 - a struct field does not implement an interface method
 - a struct field does not have arguments
 
-The method has up to two arguments:
+The method has up to three 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 `[]selected.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:
 

internal/exec/exec.go

@@ -15,6 +15,7 @@ import (
 	"github.com/graph-gophers/graphql-go/internal/query"
 	"github.com/graph-gophers/graphql-go/internal/schema"
 	"github.com/graph-gophers/graphql-go/log"
+	pubselected "github.com/graph-gophers/graphql-go/selected"
 	"github.com/graph-gophers/graphql-go/trace"
 )
 
@@ -160,6 +161,24 @@ func typeOf(tf *selected.TypenameField, resolver reflect.Value) string {
 	return ""
 }
 
+func selectionToSelectedFields(sels []selected.Selection) []pubselected.SelectedField {
+	var selectedFields []pubselected.SelectedField
+	selsLen := len(sels)
+	if selsLen != 0 {
+		selectedFields = make([]pubselected.SelectedField, 0, selsLen)
+		for _, sel := range sels {
+			selField, ok := sel.(*selected.SchemaField)
+			if ok {
+				selectedFields = append(selectedFields, pubselected.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{}{}
@@ -200,6 +219,9 @@ 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 := res.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/graph-gophers/graphql-go/internal/common"
 	"github.com/graph-gophers/graphql-go/internal/exec/packer"
 	"github.com/graph-gophers/graphql-go/internal/schema"
+	pubselected "github.com/graph-gophers/graphql-go/selected"
 )
 
 type Schema struct {
@@ -36,6 +37,7 @@ type Field struct {
 	FieldIndex  int
 	HasContext  bool
 	HasError    bool
+	HasSelected bool
 	ArgsPacker  *packer.StructPacker
 	ValueExec   Resolvable
 	TraceLabel  string
@@ -281,6 +283,7 @@ func (b *execBuilder) makeObjectExec(typeName string, fields schema.FieldList, p
 }
 
 var contextType = reflect.TypeOf((*context.Context)(nil)).Elem()
+var selectedType = reflect.TypeOf([]pubselected.SelectedField(nil))
 var errorType = reflect.TypeOf((*error)(nil)).Elem()
 
 func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.Method, sf reflect.StructField,
@@ -289,6 +292,7 @@ func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.
 	var argsPacker *packer.StructPacker
 	var hasError bool
 	var hasContext bool
+	var hasSelected bool
 
 	// Validate resolver method only when there is one
 	if methodIndex != -1 {
@@ -317,6 +321,11 @@ 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")
 		}
@@ -344,6 +353,7 @@ func (b *execBuilder) makeFieldExec(typeName string, f *schema.Field, m reflect.
 		MethodIndex: methodIndex,
 		FieldIndex:  fieldIndex,
 		HasContext:  hasContext,
+		HasSelected: hasSelected,
 		ArgsPacker:  argsPacker,
 		HasError:    hasError,
 		TraceLabel:  fmt.Sprintf("GraphQL field: %s.%s", typeName, f.Name),

selected/selected.go

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