下面是实现父子分类的代码：
migrate:
class CreateCategories < ActiveRecord::Migration
def change
create_table :categories do |t|
t.integer :parent_id
t.string :name
t.timestamps null: false
end
end
end
model:
class Category < ActiveRecord::Base
has_many :sub_categories, class_name: "Category", foreign_key: "parent_id", dependent: :destroy
belongs_to :parent_category, class_name: "Category", foreign_key: "parent_id"
end
查询语句：
categories = Category.includes(:sub_categories).where(parent_id: nil)
控制台返回的 SQL：
Category Load (1.5ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" IS NULL
Category Load (1.8ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" IN (1, 5)
Category Load (1.9ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 2]]
Category Load (1.2ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 3]]
Category Load (1.3ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 4]]
Category Load (1.3ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 6]]
Category Load (1.0ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 7]]
Category Load (1.2ms) SELECT "categories".* FROM "categories" WHERE "categories"."parent_id" = ? [["parent_id", 8]]
发现会为子分类再查询一遍子分类，控制台返回的 SQL 语句从第 2 行之后，都是无用的。
另外数据是要最终渲染成 API 提供的 json 数据。
哪位同学有好的解决办法？
categories = Category.all
list = []
parent_hash = {}
categories.each do |category|
if category.parent_id.blank?
pcategory = category.attributes
pcategory["sub_categories"] = []
list.push(pcategory)
parent_hash[category.id] = list.index(pcategory)
end
end
categories.each do |category|
if !category.parent_id.blank?
list[parent_hash[category.parent_id]]["sub_categories"].push(category)
end
end
最后的 list 就是想要的结果。
def self.to_tree(options={})
options = { ignore_empty: false }
Rails.cache.fetch('community_tree') do
mappings = {}
communities = Community.order(parent_id: :desc, position: :asc)
communities.each do |community|
if community.parent_id
hash = community.to_hash
mappings[community.parent_id.to_s][:children].push(hash)
else
hash = community.to_hash.merge(children: [])
mappings[community.id.to_s] = hash
end
end
mappings.values
.sort_by { |c| c[:position] }
.reject { |c| options[:ignore_empty] && c[:children].empty? }
end
end
我是取一回，然后做 cache
#2 楼 @flowerwrong 如果是用 awesome_nested_set 的话，如果我想输出树型结构的 JSON 数据，是不是也要先数据库一次读出，然后自己手动组织结构再转出 JSON？
