some code snippets for gqlgen dataloader

dataloader.go

I learned a lot from watching this youtube video - https://www.youtube.com/watch?v=D3SrjuTaUQU



// Gin Route File

import (
  ...
	"github.com/mycompany/menu/graph/dataloader/meal_tag"
)

func Routes(r *gin.Engine) {
  ...
	mealTagDataloader := meal_tag.NewMealTagDataloader()
  ...
  
  
  rg := r.Group("/mysite")
	rg.Use(
		...
		meal_tag.MealTagLoaderMiddleware(productRepository),
    ...
	{
		rg.POST("query", serverutils.GraphqlHandler(productRepository, mealTagDataloader))
	}
}




// graphql handler file
func GraphqlHandler(productRepo productRepo.ProductRepository, mealTagDataloader meal_tag.MealTagDataloader) gin.HandlerFunc {
	h := handler.New(gqlGen.NewExecutableSchema(gqlGen.Config{
		Resolvers: &resolvers.Resolver{
			ProductRepository:             productRepo,
		  ...
			MealTagDataloader:             mealTagDataloader,
	    ...
		},
		Complexity: gqlGen.ComplexityRoot{},
	}))

	h.AroundOperations(func(ctx context.Context, next graphql.OperationHandler) graphql.ResponseHandler {
		rc := graphql.GetOperationContext(ctx)
		rc.DisableIntrospection = cfg.GraphQLDisableIntrospection
		return next(ctx)
	})

	h.Use(extension.FixedComplexityLimit(cfg.GraphQLComplexityCount))

	return func(c *gin.Context) {
		h.AddTransport(transport.GET{})
		h.AddTransport(transport.POST{})
		h.ServeHTTP(c.Writer, c.Request)
	}
}







// Dataloader file

const mealTagsLoaderKey = "mealTagsLoader"

func MealTagLoaderMiddleware(productRepo productRepo.ProductRepository) gin.HandlerFunc {
	return func(c *gin.Context) {
		mealTagLoader := MealTagLoader{
			maxBatch: 100,
			wait:     5 * time.Millisecond,
			fetch: func(ids []int) ([]*string, []error) {
				mealTags, err := productRepo.MealTagAllergens(c, ids)
				if err != nil {
					log.Errorf("error in mealTagLoaderMiddleware retrieving mealTags %v", err)
					return nil, []error{err}
				}

				return buildMealTagsAndSort(ids, mealTags)
			},
		}

		ctx := context.WithValue(c.Request.Context(), mealTagsLoaderKey, &mealTagLoader)
		c.Request = c.Request.WithContext(ctx)
		c.Next()
	}
}

func buildMealTagsAndSort(ids []int, mealTags []*model.MealTag) ([]*string, []error) {
  ... do some stuff
  result := make([]*string, len(ids))
  ... do some more stuff
	return result, nil
}

type MealTagDataloader interface {
	GetMealTagLoader(ctx context.Context) *MealTagLoader
}

type MealTagDataloaderImpl struct{}

func NewMealTagDataloader() *MealTagDataloaderImpl {
	return &MealTagDataloaderImpl{}
}

func (m *MealTagDataloaderImpl) GetMealTagLoader(ctx context.Context) *MealTagLoader {
	return ctx.Value(mealTagsLoaderKey).(*MealTagLoader)
}