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executor.go
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executor.go
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package graphql
import (
"errors"
"fmt"
"reflect"
"strings"
"github.com/graphql-go/graphql/gqlerrors"
"github.com/graphql-go/graphql/language/ast"
"golang.org/x/net/context"
)
type ExecuteParams struct {
Schema Schema
Root interface{}
AST *ast.Document
OperationName string
Args map[string]interface{}
// Context may be provided to pass application-specific per-request
// information to resolve functions.
Context context.Context
}
func Execute(p ExecuteParams) (result *Result) {
result = &Result{}
exeContext, err := buildExecutionContext(BuildExecutionCtxParams{
Schema: p.Schema,
Root: p.Root,
AST: p.AST,
OperationName: p.OperationName,
Args: p.Args,
Errors: nil,
Result: result,
Context: p.Context,
})
if err != nil {
result.Errors = append(result.Errors, gqlerrors.FormatError(err))
return
}
defer func() {
if r := recover(); r != nil {
var err error
if r, ok := r.(error); ok {
err = gqlerrors.FormatError(r)
}
exeContext.Errors = append(exeContext.Errors, gqlerrors.FormatError(err))
result.Errors = exeContext.Errors
}
}()
return executeOperation(ExecuteOperationParams{
ExecutionContext: exeContext,
Root: p.Root,
Operation: exeContext.Operation,
})
}
type BuildExecutionCtxParams struct {
Schema Schema
Root interface{}
AST *ast.Document
OperationName string
Args map[string]interface{}
Errors []gqlerrors.FormattedError
Result *Result
Context context.Context
}
type ExecutionContext struct {
Schema Schema
Fragments map[string]ast.Definition
Root interface{}
Operation ast.Definition
VariableValues map[string]interface{}
Errors []gqlerrors.FormattedError
Context context.Context
}
func buildExecutionContext(p BuildExecutionCtxParams) (*ExecutionContext, error) {
eCtx := &ExecutionContext{}
var operation *ast.OperationDefinition
fragments := map[string]ast.Definition{}
for _, definition := range p.AST.Definitions {
switch definition := definition.(type) {
case *ast.OperationDefinition:
if (p.OperationName == "") && operation != nil {
return nil, errors.New("Must provide operation name if query contains multiple operations.")
}
if p.OperationName == "" || definition.GetName() != nil && definition.GetName().Value == p.OperationName {
operation = definition
}
case *ast.FragmentDefinition:
key := ""
if definition.GetName() != nil && definition.GetName().Value != "" {
key = definition.GetName().Value
}
fragments[key] = definition
default:
return nil, fmt.Errorf("GraphQL cannot execute a request containing a %v", definition.GetKind())
}
}
if operation == nil {
if p.OperationName != "" {
return nil, fmt.Errorf(`Unknown operation named "%v".`, p.OperationName)
}
return nil, fmt.Errorf(`Must provide an operation.`)
}
variableValues, err := getVariableValues(p.Schema, operation.GetVariableDefinitions(), p.Args)
if err != nil {
return nil, err
}
eCtx.Schema = p.Schema
eCtx.Fragments = fragments
eCtx.Root = p.Root
eCtx.Operation = operation
eCtx.VariableValues = variableValues
eCtx.Errors = p.Errors
eCtx.Context = p.Context
return eCtx, nil
}
type ExecuteOperationParams struct {
ExecutionContext *ExecutionContext
Root interface{}
Operation ast.Definition
}
func executeOperation(p ExecuteOperationParams) *Result {
operationType, err := getOperationRootType(p.ExecutionContext.Schema, p.Operation)
if err != nil {
return &Result{Errors: gqlerrors.FormatErrors(err)}
}
fields := collectFields(CollectFieldsParams{
ExeContext: p.ExecutionContext,
RuntimeType: operationType,
SelectionSet: p.Operation.GetSelectionSet(),
})
executeFieldsParams := ExecuteFieldsParams{
ExecutionContext: p.ExecutionContext,
ParentType: operationType,
Source: p.Root,
Fields: fields,
}
if p.Operation.GetOperation() == ast.OperationTypeMutation {
return executeFieldsSerially(executeFieldsParams)
}
return executeFields(executeFieldsParams)
}
// Extracts the root type of the operation from the schema.
func getOperationRootType(schema Schema, operation ast.Definition) (*Object, error) {
if operation == nil {
return nil, errors.New("Can only execute queries and mutations")
}
switch operation.GetOperation() {
case ast.OperationTypeQuery:
return schema.QueryType(), nil
case ast.OperationTypeMutation:
mutationType := schema.MutationType()
if mutationType.PrivateName == "" {
return nil, gqlerrors.NewError(
"Schema is not configured for mutations",
[]ast.Node{operation},
"",
nil,
[]int{},
nil,
)
}
return mutationType, nil
case ast.OperationTypeSubscription:
subscriptionType := schema.SubscriptionType()
if subscriptionType.PrivateName == "" {
return nil, gqlerrors.NewError(
"Schema is not configured for subscriptions",
[]ast.Node{operation},
"",
nil,
[]int{},
nil,
)
}
return subscriptionType, nil
default:
return nil, gqlerrors.NewError(
"Can only execute queries, mutations and subscription",
[]ast.Node{operation},
"",
nil,
[]int{},
nil,
)
}
}
type ExecuteFieldsParams struct {
ExecutionContext *ExecutionContext
ParentType *Object
Source interface{}
Fields map[string][]*ast.Field
}
// Implements the "Evaluating selection sets" section of the spec for "write" mode.
func executeFieldsSerially(p ExecuteFieldsParams) *Result {
if p.Source == nil {
p.Source = map[string]interface{}{}
}
if p.Fields == nil {
p.Fields = map[string][]*ast.Field{}
}
finalResults := map[string]interface{}{}
for responseName, fieldASTs := range p.Fields {
resolved, state := resolveField(p.ExecutionContext, p.ParentType, p.Source, fieldASTs)
if state.hasNoFieldDefs {
continue
}
finalResults[responseName] = resolved
}
return &Result{
Data: finalResults,
Errors: p.ExecutionContext.Errors,
}
}
// Implements the "Evaluating selection sets" section of the spec for "read" mode.
func executeFields(p ExecuteFieldsParams) *Result {
if p.Source == nil {
p.Source = map[string]interface{}{}
}
if p.Fields == nil {
p.Fields = map[string][]*ast.Field{}
}
finalResults := map[string]interface{}{}
for responseName, fieldASTs := range p.Fields {
resolved, state := resolveField(p.ExecutionContext, p.ParentType, p.Source, fieldASTs)
if state.hasNoFieldDefs {
continue
}
finalResults[responseName] = resolved
}
return &Result{
Data: finalResults,
Errors: p.ExecutionContext.Errors,
}
}
type CollectFieldsParams struct {
ExeContext *ExecutionContext
RuntimeType *Object // previously known as OperationType
SelectionSet *ast.SelectionSet
Fields map[string][]*ast.Field
VisitedFragmentNames map[string]bool
}
// Given a selectionSet, adds all of the fields in that selection to
// the passed in map of fields, and returns it at the end.
// CollectFields requires the "runtime type" of an object. For a field which
// returns and Interface or Union type, the "runtime type" will be the actual
// Object type returned by that field.
func collectFields(p CollectFieldsParams) map[string][]*ast.Field {
fields := p.Fields
if fields == nil {
fields = map[string][]*ast.Field{}
}
if p.VisitedFragmentNames == nil {
p.VisitedFragmentNames = map[string]bool{}
}
if p.SelectionSet == nil {
return fields
}
for _, iSelection := range p.SelectionSet.Selections {
switch selection := iSelection.(type) {
case *ast.Field:
if !shouldIncludeNode(p.ExeContext, selection.Directives) {
continue
}
name := getFieldEntryKey(selection)
if _, ok := fields[name]; !ok {
fields[name] = []*ast.Field{}
}
fields[name] = append(fields[name], selection)
case *ast.InlineFragment:
if !shouldIncludeNode(p.ExeContext, selection.Directives) ||
!doesFragmentConditionMatch(p.ExeContext, selection, p.RuntimeType) {
continue
}
innerParams := CollectFieldsParams{
ExeContext: p.ExeContext,
RuntimeType: p.RuntimeType,
SelectionSet: selection.SelectionSet,
Fields: fields,
VisitedFragmentNames: p.VisitedFragmentNames,
}
collectFields(innerParams)
case *ast.FragmentSpread:
fragName := ""
if selection.Name != nil {
fragName = selection.Name.Value
}
if visited, ok := p.VisitedFragmentNames[fragName]; (ok && visited) ||
!shouldIncludeNode(p.ExeContext, selection.Directives) {
continue
}
p.VisitedFragmentNames[fragName] = true
fragment, hasFragment := p.ExeContext.Fragments[fragName]
if !hasFragment {
continue
}
if fragment, ok := fragment.(*ast.FragmentDefinition); ok {
if !doesFragmentConditionMatch(p.ExeContext, fragment, p.RuntimeType) {
continue
}
innerParams := CollectFieldsParams{
ExeContext: p.ExeContext,
RuntimeType: p.RuntimeType,
SelectionSet: fragment.GetSelectionSet(),
Fields: fields,
VisitedFragmentNames: p.VisitedFragmentNames,
}
collectFields(innerParams)
}
}
}
return fields
}
// Determines if a field should be included based on the @include and @skip
// directives, where @skip has higher precedence than @include.
func shouldIncludeNode(eCtx *ExecutionContext, directives []*ast.Directive) bool {
defaultReturnValue := true
var skipAST *ast.Directive
var includeAST *ast.Directive
for _, directive := range directives {
if directive == nil || directive.Name == nil {
continue
}
if directive.Name.Value == SkipDirective.Name {
skipAST = directive
break
}
}
if skipAST != nil {
argValues, err := getArgumentValues(
SkipDirective.Args,
skipAST.Arguments,
eCtx.VariableValues,
)
if err != nil {
return defaultReturnValue
}
if skipIf, ok := argValues["if"].(bool); ok {
if skipIf == true {
return false
}
}
}
for _, directive := range directives {
if directive == nil || directive.Name == nil {
continue
}
if directive.Name.Value == IncludeDirective.Name {
includeAST = directive
break
}
}
if includeAST != nil {
argValues, err := getArgumentValues(
IncludeDirective.Args,
includeAST.Arguments,
eCtx.VariableValues,
)
if err != nil {
return defaultReturnValue
}
if includeIf, ok := argValues["if"].(bool); ok {
if includeIf == false {
return false
}
}
}
return defaultReturnValue
}
// Determines if a fragment is applicable to the given type.
func doesFragmentConditionMatch(eCtx *ExecutionContext, fragment ast.Node, ttype *Object) bool {
switch fragment := fragment.(type) {
case *ast.FragmentDefinition:
typeConditionAST := fragment.TypeCondition
if typeConditionAST == nil {
return true
}
conditionalType, err := typeFromAST(eCtx.Schema, typeConditionAST)
if err != nil {
return false
}
if conditionalType == ttype {
return true
}
if conditionalType.Name() == ttype.Name() {
return true
}
if conditionalType, ok := conditionalType.(*Interface); ok {
return eCtx.Schema.IsPossibleType(conditionalType, ttype)
}
if conditionalType, ok := conditionalType.(*Union); ok {
return eCtx.Schema.IsPossibleType(conditionalType, ttype)
}
case *ast.InlineFragment:
typeConditionAST := fragment.TypeCondition
if typeConditionAST == nil {
return true
}
conditionalType, err := typeFromAST(eCtx.Schema, typeConditionAST)
if err != nil {
return false
}
if conditionalType == ttype {
return true
}
if conditionalType.Name() == ttype.Name() {
return true
}
if conditionalType, ok := conditionalType.(*Interface); ok {
return eCtx.Schema.IsPossibleType(conditionalType, ttype)
}
if conditionalType, ok := conditionalType.(*Union); ok {
return eCtx.Schema.IsPossibleType(conditionalType, ttype)
}
}
return false
}
// Implements the logic to compute the key of a given field’s entry
func getFieldEntryKey(node *ast.Field) string {
if node.Alias != nil && node.Alias.Value != "" {
return node.Alias.Value
}
if node.Name != nil && node.Name.Value != "" {
return node.Name.Value
}
return ""
}
// Internal resolveField state
type resolveFieldResultState struct {
hasNoFieldDefs bool
}
// Resolves the field on the given source object. In particular, this
// figures out the value that the field returns by calling its resolve function,
// then calls completeValue to complete promises, serialize scalars, or execute
// the sub-selection-set for objects.
func resolveField(eCtx *ExecutionContext, parentType *Object, source interface{}, fieldASTs []*ast.Field) (result interface{}, resultState resolveFieldResultState) {
// catch panic from resolveFn
var returnType Output
defer func() (interface{}, resolveFieldResultState) {
if r := recover(); r != nil {
var err error
if r, ok := r.(string); ok {
err = NewLocatedError(
fmt.Sprintf("%v", r),
FieldASTsToNodeASTs(fieldASTs),
)
}
if r, ok := r.(error); ok {
err = gqlerrors.FormatError(r)
}
// send panic upstream
if _, ok := returnType.(*NonNull); ok {
panic(gqlerrors.FormatError(err))
}
eCtx.Errors = append(eCtx.Errors, gqlerrors.FormatError(err))
return result, resultState
}
return result, resultState
}()
fieldAST := fieldASTs[0]
fieldName := ""
if fieldAST.Name != nil {
fieldName = fieldAST.Name.Value
}
fieldDef := getFieldDef(eCtx.Schema, parentType, fieldName)
if fieldDef == nil {
resultState.hasNoFieldDefs = true
return nil, resultState
}
returnType = fieldDef.Type
resolveFn := fieldDef.Resolve
if resolveFn == nil {
resolveFn = defaultResolveFn
}
// Build a map of arguments from the field.arguments AST, using the
// variables scope to fulfill any variable references.
// TODO: find a way to memoize, in case this field is within a List type.
args, _ := getArgumentValues(fieldDef.Args, fieldAST.Arguments, eCtx.VariableValues)
info := ResolveInfo{
FieldName: fieldName,
FieldASTs: fieldASTs,
ReturnType: returnType,
ParentType: parentType,
Schema: eCtx.Schema,
Fragments: eCtx.Fragments,
RootValue: eCtx.Root,
Operation: eCtx.Operation,
VariableValues: eCtx.VariableValues,
}
var resolveFnError error
result, resolveFnError = resolveFn(ResolveParams{
Source: source,
Args: args,
Info: info,
Context: eCtx.Context,
})
if resolveFnError != nil {
panic(gqlerrors.FormatError(resolveFnError))
}
completed := completeValueCatchingError(eCtx, returnType, fieldASTs, info, result)
return completed, resultState
}
func completeValueCatchingError(eCtx *ExecutionContext, returnType Type, fieldASTs []*ast.Field, info ResolveInfo, result interface{}) (completed interface{}) {
// catch panic
defer func() interface{} {
if r := recover(); r != nil {
//send panic upstream
if _, ok := returnType.(*NonNull); ok {
panic(r)
}
if err, ok := r.(gqlerrors.FormattedError); ok {
eCtx.Errors = append(eCtx.Errors, err)
}
return completed
}
return completed
}()
if returnType, ok := returnType.(*NonNull); ok {
completed := completeValue(eCtx, returnType, fieldASTs, info, result)
return completed
}
completed = completeValue(eCtx, returnType, fieldASTs, info, result)
return completed
}
func completeValue(eCtx *ExecutionContext, returnType Type, fieldASTs []*ast.Field, info ResolveInfo, result interface{}) interface{} {
resultVal := reflect.ValueOf(result)
if resultVal.IsValid() && resultVal.Type().Kind() == reflect.Func {
if propertyFn, ok := result.(func() interface{}); ok {
return propertyFn()
}
err := gqlerrors.NewFormattedError("Error resolving func. Expected `func() interface{}` signature")
panic(gqlerrors.FormatError(err))
}
// If field type is NonNull, complete for inner type, and throw field error
// if result is null.
if returnType, ok := returnType.(*NonNull); ok {
completed := completeValue(eCtx, returnType.OfType, fieldASTs, info, result)
if completed == nil {
err := NewLocatedError(
fmt.Sprintf("Cannot return null for non-nullable field %v.%v.", info.ParentType, info.FieldName),
FieldASTsToNodeASTs(fieldASTs),
)
panic(gqlerrors.FormatError(err))
}
return completed
}
// If result value is null-ish (null, undefined, or NaN) then return null.
if isNullish(result) {
return nil
}
// If field type is List, complete each item in the list with the inner type
if returnType, ok := returnType.(*List); ok {
return completeListValue(eCtx, returnType, fieldASTs, info, result)
}
// If field type is a leaf type, Scalar or Enum, serialize to a valid value,
// returning null if serialization is not possible.
if returnType, ok := returnType.(*Scalar); ok {
return completeLeafValue(returnType, result)
}
if returnType, ok := returnType.(*Enum); ok {
return completeLeafValue(returnType, result)
}
// If field type is an abstract type, Interface or Union, determine the
// runtime Object type and complete for that type.
if returnType, ok := returnType.(*Union); ok {
return completeAbstractValue(eCtx, returnType, fieldASTs, info, result)
}
if returnType, ok := returnType.(*Interface); ok {
return completeAbstractValue(eCtx, returnType, fieldASTs, info, result)
}
// If field type is Object, execute and complete all sub-selections.
if returnType, ok := returnType.(*Object); ok {
return completeObjectValue(eCtx, returnType, fieldASTs, info, result)
}
// Not reachable. All possible output types have been considered.
err := invariant(false,
fmt.Sprintf(`Cannot complete value of unexpected type "%v."`, returnType),
)
if err != nil {
panic(gqlerrors.FormatError(err))
}
return nil
}
// completeAbstractValue completes value of an Abstract type (Union / Interface) by determining the runtime type
// of that value, then completing based on that type.
func completeAbstractValue(eCtx *ExecutionContext, returnType Abstract, fieldASTs []*ast.Field, info ResolveInfo, result interface{}) interface{} {
var runtimeType *Object
resolveTypeParams := ResolveTypeParams{
Value: result,
Info: info,
Context: eCtx.Context,
}
if unionReturnType, ok := returnType.(*Union); ok && unionReturnType.ResolveType != nil {
runtimeType = unionReturnType.ResolveType(resolveTypeParams)
} else if interfaceReturnType, ok := returnType.(*Interface); ok && interfaceReturnType.ResolveType != nil {
runtimeType = interfaceReturnType.ResolveType(resolveTypeParams)
} else {
runtimeType = defaultResolveTypeFn(resolveTypeParams, returnType)
}
err := invariant(runtimeType != nil,
fmt.Sprintf(`Could not determine runtime type of value "%v" for field %v.%v.`, result, info.ParentType, info.FieldName),
)
if err != nil {
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, result)
}
// completeObjectValue complete an Object value by executing all sub-selections.
func completeObjectValue(eCtx *ExecutionContext, returnType *Object, fieldASTs []*ast.Field, info ResolveInfo, result interface{}) interface{} {
// If there is an isTypeOf predicate function, call it with the
// current result. If isTypeOf returns false, then raise an error rather
// than continuing execution.
if returnType.IsTypeOf != nil {
p := IsTypeOfParams{
Value: result,
Info: info,
Context: eCtx.Context,
}
if !returnType.IsTypeOf(p) {
panic(gqlerrors.NewFormattedError(
fmt.Sprintf(`Expected value of type "%v" but got: %T.`, returnType, result),
))
}
}
// Collect sub-fields to execute to complete this value.
subFieldASTs := map[string][]*ast.Field{}
visitedFragmentNames := map[string]bool{}
for _, fieldAST := range fieldASTs {
if fieldAST == nil {
continue
}
selectionSet := fieldAST.SelectionSet
if selectionSet != nil {
innerParams := CollectFieldsParams{
ExeContext: eCtx,
RuntimeType: returnType,
SelectionSet: selectionSet,
Fields: subFieldASTs,
VisitedFragmentNames: visitedFragmentNames,
}
subFieldASTs = collectFields(innerParams)
}
}
executeFieldsParams := ExecuteFieldsParams{
ExecutionContext: eCtx,
ParentType: returnType,
Source: result,
Fields: subFieldASTs,
}
results := executeFields(executeFieldsParams)
return results.Data
}
// completeLeafValue complete a leaf value (Scalar / Enum) by serializing to a valid value, returning nil if serialization is not possible.
func completeLeafValue(returnType Leaf, result interface{}) interface{} {
serializedResult := returnType.Serialize(result)
if isNullish(serializedResult) {
return nil
}
return serializedResult
}
// completeListValue complete a list value by completing each item in the list with the inner type
func completeListValue(eCtx *ExecutionContext, returnType *List, fieldASTs []*ast.Field, info ResolveInfo, result interface{}) interface{} {
resultVal := reflect.ValueOf(result)
parentTypeName := ""
if info.ParentType != nil {
parentTypeName = info.ParentType.Name()
}
err := invariant(
resultVal.IsValid() && resultVal.Type().Kind() == reflect.Slice,
fmt.Sprintf("User Error: expected iterable, but did not find one "+
"for field %v.%v.", parentTypeName, info.FieldName),
)
if err != nil {
panic(gqlerrors.FormatError(err))
}
itemType := returnType.OfType
completedResults := []interface{}{}
for i := 0; i < resultVal.Len(); i++ {
val := resultVal.Index(i).Interface()
completedItem := completeValueCatchingError(eCtx, itemType, fieldASTs, info, val)
completedResults = append(completedResults, completedItem)
}
return completedResults
}
// defaultResolveTypeFn If a resolveType function is not given, then a default resolve behavior is
// used which tests each possible type for the abstract type by calling
// isTypeOf for the object being coerced, returning the first type that matches.
func defaultResolveTypeFn(p ResolveTypeParams, abstractType Abstract) *Object {
possibleTypes := p.Info.Schema.PossibleTypes(abstractType)
for _, possibleType := range possibleTypes {
if possibleType.IsTypeOf == nil {
continue
}
isTypeOfParams := IsTypeOfParams{
Value: p.Value,
Info: p.Info,
Context: p.Context,
}
if res := possibleType.IsTypeOf(isTypeOfParams); res {
return possibleType
}
}
return nil
}
// defaultResolveFn If a resolve function is not given, then a default resolve behavior is used
// which takes the property of the source object of the same name as the field
// and returns it as the result, or if it's a function, returns the result
// of calling that function.
func defaultResolveFn(p ResolveParams) (interface{}, error) {
// try to resolve p.Source as a struct first
sourceVal := reflect.ValueOf(p.Source)
if sourceVal.IsValid() && sourceVal.Type().Kind() == reflect.Ptr {
sourceVal = sourceVal.Elem()
}
if !sourceVal.IsValid() {
return nil, nil
}
if sourceVal.Type().Kind() == reflect.Struct {
// find field based on struct's json tag
// we could potentially create a custom `graphql` tag, but its unnecessary at this point
// since graphql speaks to client in a json-like way anyway
// so json tags are a good way to start with
for i := 0; i < sourceVal.NumField(); i++ {
valueField := sourceVal.Field(i)
typeField := sourceVal.Type().Field(i)
// try matching the field name first
if typeField.Name == p.Info.FieldName {
return valueField.Interface(), nil
}
tag := typeField.Tag
jsonTag := tag.Get("json")
jsonOptions := strings.Split(jsonTag, ",")
if len(jsonOptions) == 0 {
continue
}
if jsonOptions[0] != p.Info.FieldName {
continue
}
return valueField.Interface(), nil
}
return nil, nil
}
// try p.Source as a map[string]interface
if sourceMap, ok := p.Source.(map[string]interface{}); ok {
property := sourceMap[p.Info.FieldName]
val := reflect.ValueOf(property)
if val.IsValid() && val.Type().Kind() == reflect.Func {
// try type casting the func to the most basic func signature
// for more complex signatures, user have to define ResolveFn
if propertyFn, ok := property.(func() interface{}); ok {
return propertyFn(), nil
}
}
return property, nil
}
// last resort, return nil
return nil, nil
}
// This method looks up the field on the given type definition.
// It has special casing for the two introspection fields, __schema
// and __typename. __typename is special because it can always be
// queried as a field, even in situations where no other fields
// are allowed, like on a Union. __schema could get automatically
// added to the query type, but that would require mutating type
// definitions, which would cause issues.
func getFieldDef(schema Schema, parentType *Object, fieldName string) *FieldDefinition {
if parentType == nil {
return nil
}
if fieldName == SchemaMetaFieldDef.Name &&
schema.QueryType() == parentType {
return SchemaMetaFieldDef
}
if fieldName == TypeMetaFieldDef.Name &&
schema.QueryType() == parentType {
return TypeMetaFieldDef
}
if fieldName == TypeNameMetaFieldDef.Name {
return TypeNameMetaFieldDef
}
return parentType.Fields()[fieldName]
}