如何在mongodb集合中保存2個文檔而不是1個？

Question

在軌道控制檯上，我試圖在我的課程類中保存2個類別。但是，我的輸出返回1個類別。

我的代碼：

class Book 
    include Mongoid::Document 
    field :title, type: String 
    field :publisher, type: String 
    field :published_on, type: Date 
    field :votes, type: Array 
    belongs_to :author 
    has_and_belongs_to_many :categories 
    embeds_many :reviews 
end 


class Category 
    include Mongoid::Document 
    field :name, type: String 
    has_and_belongs_to_many :books 
end 


class Author 
    include Mongoid::Document 
    field :name, type: String 
    has_many :books 
end 

在終端

irb(main):001:0> b = Book.new(title: "Oliver Twist", publisher: "Dover Publications", published_on: Date.parse("2002-12-30")) 

=> #<Book _id: 5833129213197da7660c0adb, title: "Oliver Twist", publisher:  "Dover Publications", published_on: 2002-12-30 00:00:00 UTC, votes: nil, author_id: nil, category_ids: nil> 
irb(main):002:0> Author.create(name: "Charles Dickens") 
=> #<Author _id: 5833129a13197da7660c0adc, name: "Charles Dickens"> 

irb(main):003:0> Category.create(name: "Fiction") 
=> #<Category _id: 583312a113197da7660c0add, name: "Fiction", book_ids: nil> 

irb(main):004:0> Category.create(name: "Drama") 
=> #<Category _id: 583312a813197da7660c0ade, name: "Drama", book_ids: nil> 

irb(main):006:0* 
irb(main):007:0* b.author = Author.where(name: "Charles Dickens").first 
=> #<Author _id: 5832d97713197da0893c05b9, name: "Charles Dickens"> 

irb(main):008:0> b.categories << Category.first 
=> [#<Category _id: 58330d7d13197da0893c05ba, name: "Fiction", book_ids:   [BSON::ObjectId('5832d96913197da0893c05b8'), BSON::ObjectId('5833129213197da7660c0adb')]>] 

irb(main):009:0> b.categories << Category.last 
=> [#<Category _id: 58330d7d13197da0893c05ba, name: "Fiction", book_ids: [BSON::ObjectId('5832d96913197da0893c05b8'), BSON::ObjectId('5833129213197da7660c0adb')]>] 

irb(main):010:0> 
irb(main):011:0* 

irb(main):012:0* b 
=> #<Book _id: 5833129213197da7660c0adb, title: "Oliver Twist", publisher:  "Dover Publications", published_on: 2002-12-30 00:00:00 UTC, votes: nil, author_id: BSON::ObjectId('5832d97713197da0893c05b9'), category_ids: [BSON::ObjectId('58330d7d13197da0893c05ba')]> 

irb(main):013:0> b.save 
=> true 

irb(main):014:0> Category.first 
=> #<Category _id: 58330d7d13197da0893c05ba, name: "Fiction", book_ids: [BSON::ObjectId('5832d96913197da0893c05b8')]> 

irb(main):015:0> Category.last 
=> #<Category _id: 58330d7d13197da0893c05ba, name: "Fiction", book_ids: [BSON::ObjectId('5832d96913197da0893c05b8')]> 
irb(main):016:0> 

兩類輸出顯示的名字爲 '小說' 而不是 '戲劇' futhermore，在測試 -

> db.books.findOne() 
{ 
    "_id" : ObjectId("5833129213197da7660c0adb"), 
    "title" : "Oliver Twist", 
    "publisher" : "Dover Publications", 
    "published_on" : ISODate("2002-12-30T00:00:00Z"), 
    "author_id" : ObjectId("5832d97713197da0893c05b9"), 
    "category_ids" : [ 
    ObjectId("58330d7d13197da0893c05ba") 
] 
} 
> 
> 
> db.categories.findOne() 
{ 
    "_id" : ObjectId("58330d7d13197da0893c05ba"), 
    "name" : "Fiction", 
    "book_ids" : [ 
    ObjectId("5832d96913197da0893c05b8") 
    ] 
} 

如何我確保2個categorie_ids出現在db.books.findOne（）上？

會感謝你的建議..

Answer 1

按本document，你必須明確地提供以使第一和最後一個方法正常工作的排序順序。

這樣的事情，

b.categories << Category order_by(:created_at => 'asc').first 
b.categories << Category order_by(:created_at => 'asc').last 

來源：The bug of mongoid returning first document when invoking last?