Sending custom error data in the graphql response

Returning errors

All resolvers simply return an error to be sent to the user. The assumption is that any error message returned here is appropriate for end users. If certain messages aren’t safe, customise the error presenter.

Multiple errors

To return multiple errors you can call the graphql.Error functions like so:

package foo

import (
	"context"
	"errors"

	"github.com/vektah/gqlparser/v2/gqlerror"
	"github.com/99designs/gqlgen/graphql"
)

// DoThings add errors to the stack.
func (r Query) DoThings(ctx context.Context) (bool, error) {
	// Print a formatted string
	graphql.AddErrorf(ctx, "Error %d", 1)

	// Pass an existing error out
	graphql.AddError(ctx, gqlerror.Errorf("zzzzzt"))

	// Or fully customize the error
	graphql.AddError(ctx, &gqlerror.Error{
		Path:       graphql.GetPath(ctx),
		Message:    "A descriptive error message",
		Extensions: map[string]interface{}{
			"code": "10-4",
		},
	})

	// And you can still return an error if you need
	return false, gqlerror.Errorf("BOOM! Headshot")
}

They will be returned in the same order in the response, eg:

{
  "data": {
    "todo": null
  },
  "errors": [
    { "message": "Error 1", "path": [ "todo" ] },
    { "message": "zzzzzt", "path": [ "todo" ] },
    { "message": "A descriptive error message", "path": [ "todo" ], "extensions": { "code": "10-4" } },
    { "message": "BOOM! Headshot", "path": [ "todo" ] }
  ]
}

or you can simply return multiple errors

package foo

import (
	"context"
	"errors"

	"github.com/vektah/gqlparser/v2/gqlerror"
	"github.com/99designs/gqlgen/graphql"
)

var errSomethingWrong = errors.New("some validation failed")

// DoThingsReturnMultipleErrors collect errors and returns it if any.
func (r Query) DoThingsReturnMultipleErrors(ctx context.Context) (bool, error) {
	errList := gqlerror.List{}

	// Add existing error
	errList = append(errList, gqlerror.Wrap(errSomethingWrong))

	// Create new formatted and append
	errList = append(errList, gqlerror.Errorf("invalid value: %s", "invalid"))

	// Or fully customize the error and append
	errList = append(errList, &gqlerror.Error{
		Path:       graphql.GetPath(ctx),
		Message:    "A descriptive error message",
		Extensions: map[string]interface{}{
			"code": "10-4",
		},
	})

	return false, errList
}

They will be returned in the same order in the response, eg:

{
  "data": {
    "todo": null
  },
  "errors": [
    { "message": "some validation failed", "path": [ "todo" ] },
    { "message": "invalid value: invalid", "path": [ "todo" ] },
    { "message": "A descriptive error message", "path": [ "todo" ], "extensions": { "code": "10-4" } },
  ]
}

Hooks

The error presenter

All errors returned by resolvers, or from validation, pass through a hook before being displayed to the user. This hook gives you the ability to customise errors however makes sense in your app.

The default error presenter will capture the resolver path and use the Error() message in the response.

You change this when creating the server:

package bar

import (
	"context"
	"errors"

	"github.com/vektah/gqlparser/v2/gqlerror"
	"github.com/99designs/gqlgen/graphql"
	"github.com/99designs/gqlgen/graphql/handler"
)

func main() {
	server := handler.New(MakeExecutableSchema(resolvers))

	// Server setup...

	server.SetErrorPresenter(func(ctx context.Context, e error) *gqlerror.Error {
		err := graphql.DefaultErrorPresenter(ctx, e)

		var myErr *MyError
		if errors.As(e, &myErr) {
			err.Message = "Eeek!"
		}

		return err
	})
}

This function will be called with the same resolver context that generated it, so you can extract the current resolver path and whatever other state you might want to notify the client about.

The panic handler

There is also a panic handler, called whenever a panic happens to gracefully return a message to the user before stopping parsing. This is a good spot to notify your bug tracker and send a custom message to the user. Any errors returned from here will also go through the error presenter.

You change this when creating the server:

server := handler.New(MakeExecutableSchema(resolvers))

// Server setup...

server.SetRecoverFunc(func(ctx context.Context, err interface{}) error {
    // Notify bug tracker...

		return gqlerror.Errorf("Internal server error!")
})

While these handlers are useful in production to make sure the program does not crash, even if a user finds an issue that causes a crash-condition. During development, it can sometimes be more useful to properly crash, potentially generating a coredump to enable further debugging.

To allow your program to crash on a panic, add this to your config file:

omit_panic_handler: true