dataloaders.go

func newAuthorsByBookID(ctx context.Context, repo pg.Repository) *AuthorSliceLoader {
	return NewAuthorSliceLoader(AuthorSliceLoaderConfig{
		MaxBatch: 100,
		Wait:     5 * time.Millisecond,
		Fetch: func(bookIDs []int64) ([][]pg.Author, []error) {
			// db query
			res, err := repo.ListAuthorsByBookIDs(ctx, bookIDs)
			if err != nil {
				return nil, []error{err}
			}
			// group
			groupByBookID := make(map[int64][]pg.Author, len(bookIDs))
			for _, r := range res {
				groupByBookID[r.BookID] = append(groupByBookID[r.BookID], pg.Author{
					ID:      r.ID,
					Name:    r.Name,
					Website: r.Website,
					AgentID: r.AgentID,
				})
			}
			// order
			result := make([][]pg.Author, len(bookIDs))
			for i, bookID := range bookIDs {
				result[i] = groupByBookID[bookID]
			}
			return result, nil
		},
	})
}