Expose selected fields in context

README.md

@@ -76,7 +76,7 @@ When using `UseFieldResolvers` schema option, a struct field will be used *only*
 
 The method has up to two arguments:
 
-- Optional `context.Context` argument.
+- Optional `context.Context` argument. If the graphql query had nested subfields, then use the `SelctedFieldsFromContext(ctx context.Context)` getter method
 - 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.
 
 The method has up to two results:

graphql.go

@@ -16,6 +16,7 @@ import (
 	"github.com/graph-gophers/graphql-go/internal/validation"
 	"github.com/graph-gophers/graphql-go/introspection"
 	"github.com/graph-gophers/graphql-go/log"
+	"github.com/graph-gophers/graphql-go/selection"
 	"github.com/graph-gophers/graphql-go/trace"
 )
 
@@ -144,6 +145,12 @@ type Response struct {
 	Extensions map[string]interface{} `json:"extensions,omitempty"`
 }
 
+// SelectedFieldsFromContext retrieves the selected fields passed via the context during the request
+// execution
+func SelectedFieldsFromContext(ctx context.Context) []*selection.SelectedField {
+	return exec.SelectedFieldsFromContext(ctx)
+}
+
 // Validate validates the given query with the schema.
 func (s *Schema) Validate(queryString string) []*errors.QueryError {
 	doc, qErr := query.Parse(queryString)
@@ -184,11 +191,11 @@ func (s *Schema) exec(ctx context.Context, queryString string, operationName str
 
 	// Subscriptions are not valid in Exec. Use schema.Subscribe() instead.
 	if op.Type == query.Subscription {
-		return &Response{Errors: []*errors.QueryError{&errors.QueryError{ Message: "graphql-ws protocol header is missing" }}}
+		return &Response{Errors: []*errors.QueryError{&errors.QueryError{Message: "graphql-ws protocol header is missing"}}}
 	}
 	if op.Type == query.Mutation {
 		if _, ok := s.schema.EntryPoints["mutation"]; !ok {
-			return &Response{Errors: []*errors.QueryError{{ Message: "no mutations are offered by the schema" }}}
+			return &Response{Errors: []*errors.QueryError{{Message: "no mutations are offered by the schema"}}}
 		}
 	}
 

internal/exec/exec.go

@@ -15,9 +15,16 @@ import (
 	"github.com/graph-gophers/graphql-go/internal/query"
 	"github.com/graph-gophers/graphql-go/internal/schema"
 	"github.com/graph-gophers/graphql-go/log"
+	"github.com/graph-gophers/graphql-go/selection"
 	"github.com/graph-gophers/graphql-go/trace"
 )
 
+type ctxKey string
+
+const (
+	selectedFieldsKey ctxKey = "selectedFields"
+)
+
 type Request struct {
 	selected.Request
 	Limiter chan struct{}
@@ -160,6 +167,32 @@ func typeOf(tf *selected.TypenameField, resolver reflect.Value) string {
 	return ""
 }
 
+func selectionToSelectedFields(internalSelection []selected.Selection) []*selection.SelectedField {
+	fieldSelection := []*selection.SelectedField{}
+	for _, element := range internalSelection {
+		if field, ok := element.(*selected.SchemaField); ok {
+			nestedSelections := selectionToSelectedFields(field.Sels)
+			fieldSelection = append(fieldSelection, &selection.SelectedField{
+				Name:           field.Name,
+				SelectedFields: nestedSelections,
+			})
+		}
+	}
+	return fieldSelection
+}
+
+// SelectedFieldsFromContext exposes the fields selected in the GraphQL request
+// using the public-facing selection.SelectedField struct
+func SelectedFieldsFromContext(ctx context.Context) []*selection.SelectedField {
+	selection := ctx.Value(selectedFieldsKey).([]selected.Selection)
+	selectedFields := selectionToSelectedFields(selection)
+	return selectedFields
+}
+
+func contextWithSelectedFields(parentContext context.Context, selection []selected.Selection) context.Context {
+	return context.WithValue(parentContext, selectedFieldsKey, selection)
+}
+
 func execFieldSelection(ctx context.Context, r *Request, s *resolvable.Schema, f *fieldToExec, path *pathSegment, applyLimiter bool) {
 	if applyLimiter {
 		r.Limiter <- struct{}{}
@@ -195,6 +228,9 @@ func execFieldSelection(ctx context.Context, r *Request, s *resolvable.Schema, f
 		if f.field.UseMethodResolver() {
 			var in []reflect.Value
 			if f.field.HasContext {
+				if len(f.sels) != 0 {
+					traceCtx = contextWithSelectedFields(traceCtx, f.sels)
+				}
 				in = append(in, reflect.ValueOf(traceCtx))
 			}
 			if f.field.ArgsPacker != nil {

selection/selected_field.go

@@ -0,0 +1,8 @@
+package selection
+
+// SelectedField is the public representation of a field selection
+// during a graphql query
+type SelectedField struct {
+	Name           string
+	SelectedFields []*SelectedField
+}