transmet/categories/categories.go

package categories

import (
    "database/sql"	
	_ "github.com/lib/pq"
	"fmt"
	"strings"
)

type Category struct {
    Id int
    Name string
    Parent sql.NullInt64
    Children []*Category
}

var CategoriesTree []*Category
var CategoriesFlat map[int]*Category

// Cavet: slight cheat. All parents must have Category_id < their children. 
func LoadCategories(db *sql.DB) {
    CategoriesTree = []*Category{}
   	CategoriesFlat = make(map[int]*Category)

   	rows, err := db.Query("select categories.id, categories.name, categories.parent_id from categories order by coalesce(parent_id, id), parent_id is not null")
    if err != nil {
        fmt.Println("DB Error loading Categories:", err)
        return
    }
    defer rows.Close()
	
	for rows.Next() {
		category := &Category{Children: []*Category{}}

		err = rows.Scan(&category.Id, &category.Name, &category.Parent)
		if err != nil {
			fmt.Println("Categories DB Error: ", err)
		}

		CategoriesFlat[category.Id] = category
		
		if category.Parent.Valid {
		    pid := int(category.Parent.Int64)
		    CategoriesFlat[pid].Children = append(CategoriesFlat[pid].Children, category)
		} else {
		    CategoriesTree = append(CategoriesTree, category)
		}    
	}
}

// get the number of parents a category has
func (category *Category) Depth() int {
    depth := 0
    current := category
    for current.Parent.Valid {
        current = CategoriesFlat[int(current.Parent.Int64)]
        depth ++
    }
    return depth
}

// Helper function for templates (add form select list)
func (category *Category) ToString() string {
    return strings.Repeat("- ", category.Depth()) + category.Name
}