ruby-on-rails - 从头开始标记：标签云问题

Question

我正在按照http://railscasts.com/episodes/382-tagging的说明从头开始构建标签系统。一切都很好，但不是 tag_cloud 助手。它在搜索 tag_counts 时引发数据库错误。

以下范围：

#Picture.rb

class Picture < ActiveRecord::Base

  attr_accessible :description, :title, :tag_list

  has_many :taggings
  has_many :tags, through: :taggings

#Because of the following I'm getting an error from the Posgresql (showed in "Database Error")

  def self.tag_counts 

    Tag.select("tags.*, count(taggings.tag_id) as count").
      joins(:taggings).group("taggings.tag_id")
  end
end

Application_helper.rb

def tag_cloud(tags, classes)
  max = tags.sort_by(&:count).last
  tags.each do |tag|
    index = tag.count.to_f / max.count * (classes.size - 1)
    yield(tag, classes[index.round])
end

数据库错误

ActiveRecord::StatementInvalid in Pictures#index

PG::Error: column "tags.id" ​​should appear in the GROUP BY clause or be used in an aggregate function

LINE 1: SELECT tags.*, count(taggings.tag_id) as count FROM "tags" I...

               ^
: SELECT tags.*, count(taggings.tag_id) as count FROM "tags" INNER JOIN "taggings" ON "taggings"."tag_id" = "tags"."id" GROUP BY taggings.tag_id

Answer 1

您应该按 tags.id 和/或 count taggings.id 分组

Tag.select("tags.*, count(taggings.id) as count").
  joins(:taggings).group("tags.id")

您不能在查询中同时分组和聚合。