forgeplus/app/services/admins/drag_portal_image_service.rb

class Admins::DragPortalImageService < ApplicationService
  Error = Class.new(StandardError)

  attr_reader :laboratory, :move, :after

  def initialize(laboratory, move, after)
    @laboratory = laboratory
    @move  = move
    @after = after # 移动后下一个位置的元素
  end

  def call
    return if move.position + 1 == after&.position # 未移动

    images = laboratory.portal_images

    ActiveRecord::Base.transaction do
      if after.blank? || move.id == after.id # 移动至末尾
        total = images.count

        images.where('position > ?', move.position).update_all('position = position - 1')
        move.update!(position: total)
        return
      end

      if move.position > after.position # 前移
        images.where('position >= ? AND position < ?', after.position, move.position).update_all('position = position + 1')
        move.update!(position: after.position)
      else # 后移
        images.where('position > ? AND position < ?', move.position, after.position).update_all('position = position - 1')
        move.update!(position: after.position - 1)
      end
    end
  end
end