MongoDB 对 HABTM 关系（Mongoid、RoR）进行条件聚合查询

MongoDB是一种流行的NoSQL数据库，它提供了强大的聚合查询功能，使我们能够轻松地对数据进行复杂的分析和处理。在本文中，我将介绍如何使用MongoDB对具有HABTM（Has and Belongs to Many）关系的数据进行条件聚合查询。同时，我还将使用Mongoid和RoR（Ruby on Rails）作为示例代码，以帮助读者更好地理解。

ruby
gem 'mongoid', '~> 7.0'

bash
rails g mongoid:config
rails g scaffold User name:string
rails g scaffold Role name:string

ruby
class User
  include Mongoid::Document
  field :name, type: String
  has_and_belongs_to_many :roles
end

ruby
class Role
  include Mongoid::Document
  field :name, type: String
  has_and_belongs_to_many :users
end

bash
rails db:create
rails db:migrate

ruby
user1 = User.create(name: 'John')
user2 = User.create(name: 'Jane')
role1 = Role.create(name: 'Admin')
role2 = Role.create(name: 'Editor')
role3 = Role.create(name: 'Viewer')
user1.roles << role1
user1.roles << role2
user2.roles << role2
user2.roles << role3

ruby
User.collection.aggregate([
  { "$match": { "roles._id": role1.id } },
  { "$lookup": {
      "from": "roles",
      "localField": "roles",
      "foreignField": "_id",
      "as": "roles"
    }
  }
])