问题描述
我正在学习Sinatra，并且阅读了datamapper文档，并找到了该n对n关系示例：
class Photo
包括DataMapper :: Resource
属性：id，序列
具有n，：taggings
具有n，：tags，：through => ：taggings
end
类Tag
包括DataMapper :: Resource
属性：id，序列
具有n，：taggings
具有n，：photos， ：through => ：taggings
end
类标记
包括DataMapper :: Resource
属居到：tag，：key =>真
属于：photo，：key =>真正的
结束
我从上面的代码了解到，一张照片可能有很多零个标签，并且一个标签可能包含很多或零张照片。如何检索带有已关联标签的照片列表。我知道datamapper使用惰性方法，因此它不会自动加载关联的类（在本例中为photo.tag）。因此：
照片= Photo.all
会导致数组中包含带标签的Photo对象。有没有一种方法可以自动检索它，或者我必须遍历数组并手动进行设置？
预先感谢！
我也有一个具有相似关系的数据库。 Author ， Post ， Tag 是主要模型，而 Subscribedtag 和 Tagging 是通过 has n，：through 构建的。
class作者
include DataMapper :: Resource
property：id，Serial
属性：email，String，：unique => true
属性：password，字符串
属性：first_name，字符串
属性：last_name，字符串
属性：bio，文本
属性：phone，String，：unique = > true
属性：twitter，String，：unique => true
属性：facebook，String，：unique => true
属性：show_phone，布尔值，：default => false
属性：show_facebook，布尔值，：default => false
属性：show_twitter，布尔值，：default => false
属性：is_admin，布尔值，：default => false
属性：this_login，DateTime
属性：last_login，DateTime
属性：session_lasting，Integer，：default => 0
具有n，：posts
具有n，：subscribedtags
具有n，：tags，：through => ：subscribedtags
结束
类Post
包括DataMapper :: Resource
属性：id，序列
属性：title，String，：必填=> true
属性：body，Text，：required => true
属性：is_blog_post，布尔值，：default => true
属性：已查看，整数，：默认=> 0
属性：featured，Boolean，：default => false
属性：created_at，DateTime
属性：updated_at，DateTime
归属_至：作者
属性_至：类别
具有n，：注释
具有n，：taggings
具有n，：tags，：through => ：taggings
validates_length_of：title，：min => 3
validates_length_of：body，：min => 20
validates_format_of：title，：with => / \w /
＃一些其他方法
end
class Tag
include DataMapper :: Resource
属性：id，序列
属性：name，String，：unique => true
具有n，：taggings
具有n，：posts，：through => ：taggings
具有n，：subscribedtags
具有n，：authors，：through => ：subscribedtags
validates_length_of：name，：min => 1
validates_format_of：name，：with => / \w /
＃其他一些方法
end
类标记
包括DataMapper :: Resource
属于：tag，：key =>真
属于：post，：key =>真
结束
类Subscribedtag
include DataMapper :: Resource
属于：tag，：key =>真
属于：作者，：key => true
end
定义模型的方式使您可以编写查询，例如那。
2.2.0：016> kant = Tag.get（25）＃获取ID为25的标签实例，并将其分配给名为kant
=>的变量。 ＃< Tag @ id = 25 @ name =İmmanuilKant>
2.2.0：017> kant.posts
=> #return具有此标签的帖子实例。
2.2.0：018> kant.posts.count＃具有此标签的帖子数。
=> 2
2.2.0：021> kant.posts.first.author.first_name
=> Ziya＃检查我的作者类和first_name属性。
假设我要检索没有帖子的标签实例。
一个简单的ruby命令。
2.2.0：024> Tag.each {| tnp |如果tnp.posts.count == 0，则输入tnp.name。
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或基于帖子检索标签。
2.2 .0：034> p = Post.get（9）
=> ＃< Post @ id = 9 @ title =我们为什么使用Lorem Ipsum @ body =<未加载> @ is_blog_post = false @ viewed = 0 @ featured = false @created_at =＃< DateTime：2015-08-02T23：14：04 + 05：00（（2457237j，65644s，0n），+ 18000s，2299161j）> @updated_at =＃< DateTime：2015-08-02T23：14：04 + 05：00（（2457237j，65644s，0n），+ 18000s，2299161j）> @ author_id = 1 @ category_id = 1>
2.2.0：035> p.tags
=> [＃< Tag @ id = 19 @ name = Bundesliqa>]
检索帖子没有标签。
2.2.0：043> Post.each {| pnt |如果pnt.tags.count.zero？放置pnt.id？}
8＃id为#post的标签没有标签
2.2.0：044> Post.get（8）.tags.count
=> 0
您还可以通过其他属性进行查询。
2.2.0：046> Tag.first（：name => Lorem）。id
=> 30
对结果重复
2.2.0：050> Tag.first（：name => Lorem）。posts.each {| lorempost | puts lorempost.title}＃打印带有lorem标签的帖子标题。
全部获取
qwerty
我还通过 Subscribedtags 模型，我可以轻松地检查哪个作者订阅了哪个标签，反之亦然。
2.2.0：055> z = Author.get（1）
=> ＃返回作者实例
2.2.0：056>的详细信息z.tags
=> [＃，＃，＃，＃]
或通过Subscribedtag
2.2.0：057> z.subscribedtags
=> [＃< Subscribedtag @ tag_id = 2 @ author_id = 1>，＃< Subscribedtag @ tag_id = 4 @ author_id = 1>，＃< Subscribedtag @ tag_id = 25 @ author_id = 1>，＃< Subscribedtag @ tag_id = 30 @ author_id = 1>]
您还可以定义自己的函数来利用查询。我定义了一个subscriptiond_tags方法，该方法返回一个已订阅标签名称的数组。
2.2.0：058> z.subscribed_tags
=> [Həyat，Məstan，İmmanuilKant， Lorem]
如果我要检索订阅了名为 Lorem标签的随机作者的first_name属性，
2.2.0：062> ; Tag.first（：name => Lorem）。authors.sample.first_name
=> Ziya
作为第二个问题的答案，是的，大多数时候您必须进行迭代。 / p>
因为 Photos.all 返回Photo对象实例的集合。而且此实例分别具有标签属性，而不是由Photo实例组成的数组。
如果您调用 p = Photo.all; print p.tags; 它将返回与所有照片关联的所有标签，可能不是您想要的东西。
如果这些问题还不够，请随时提出更多问题。
I'm learning Sinatra, and I have read datamapper documentation and found this n to n relationship example:
class Photo
include DataMapper::Resource
property :id, Serial
has n, :taggings
has n, :tags, :through => :taggings
end
class Tag
include DataMapper::Resource
property :id, Serial
has n, :taggings
has n, :photos, :through => :taggings
end
class Tagging
include DataMapper::Resource
belongs_to :tag, :key => true
belongs_to :photo, :key => true
end
What I understood from the code above is that one photo may have many or zero tags, and a tag may have many or zero photos. How do I retrieve a list of photos with the tags associated to it already loaded. I know datamapper uses the lazy approach, so it does not automatically loads the associated classes (in this case photo.tag). So this:
photos = Photo.all
would result in an array with Photo objects without the tags. Is there a way to automatically retrieve it or do I have to iterate over the array and set that manually?
Thanks in advance!
I also have a database which has similar relations. Author, Post, Tag are main models, and Subscribedtag and Tagging are built through has n, :through.
class Author
include DataMapper::Resource
property :id, Serial
property :email, String, :unique => true
property :password, String
property :first_name, String
property :last_name, String
property :bio, Text
property :phone, String, :unique => true
property :twitter, String, :unique => true
property :facebook, String, :unique => true
property :show_phone, Boolean, :default => false
property :show_facebook, Boolean, :default => false
property :show_twitter, Boolean, :default => false
property :is_admin, Boolean, :default => false
property :this_login, DateTime
property :last_login, DateTime
property :session_lasting, Integer, :default => 0
has n, :posts
has n, :subscribedtags
has n, :tags, :through => :subscribedtags
end
class Post
include DataMapper::Resource
property :id, Serial
property :title, String, :required => true
property :body, Text, :required => true
property :is_blog_post, Boolean, :default => true
property :viewed, Integer, :default => 0
property :featured, Boolean, :default => false
property :created_at, DateTime
property :updated_at, DateTime
belongs_to :author
belongs_to :category
has n, :comments
has n, :taggings
has n, :tags, :through => :taggings
validates_length_of :title, :min => 3
validates_length_of :body, :min => 20
validates_format_of :title, :with => /\w/
#some other methods
end
class Tag
include DataMapper::Resource
property :id, Serial
property :name, String, :unique => true
has n, :taggings
has n, :posts, :through => :taggings
has n, :subscribedtags
has n, :authors, :through => :subscribedtags
validates_length_of :name, :min => 1
validates_format_of :name, :with => /\w/
# some other methods
end
class Tagging
include DataMapper::Resource
belongs_to :tag, :key => true
belongs_to :post, :key => true
end
class Subscribedtag
include DataMapper::Resource
belongs_to :tag, :key => true
belongs_to :author, :key => true
end
The way you've defined models, allows you to write queries, like that.
2.2.0 :016 > kant = Tag.get(25) # getting tag instance with id 25 and assign it to variable named kant
=> #<Tag @id=25 @name="İmmanuil Kant">
2.2.0 :017 > kant.posts
=> #returns post instances which has this tag.
2.2.0 :018 > kant.posts.count # count of posts with this tag.
=> 2
2.2.0 :021 > kant.posts.first.author.first_name
=> "Ziya" # check my author class and first_name attribute.
Let's say I want to retrieve the tag instances which has no posts.a simple ruby command.
2.2.0 :024 > Tag.each {|tnp| puts tnp.name if tnp.posts.count == 0}
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Or retrieving tags based on posts.
2.2.0 :034 > p = Post.get(9)
=> #<Post @id=9 @title="Why we use Lorem Ipsum" @body=<not loaded> @is_blog_post=false @viewed=0 @featured=false @created_at=#<DateTime: 2015-08-02T23:14:04+05:00 ((2457237j,65644s,0n),+18000s,2299161j)> @updated_at=#<DateTime: 2015-08-02T23:14:04+05:00 ((2457237j,65644s,0n),+18000s,2299161j)> @author_id=1 @category_id=1>
2.2.0 :035 > p.tags
=> [#<Tag @id=19 @name="Bundesliqa">]
retrieve posts which has no tag.
2.2.0 :043 > Post.each {|pnt| puts pnt.id if pnt.tags.count.zero?}
8 #post with id has no tags
2.2.0 :044 > Post.get(8).tags.count
=> 0
you can also query via other attributes.
2.2.0 :046 > Tag.first(:name => "Lorem").id
=> 30
iterate over results
2.2.0 :050 > Tag.first(:name => "Lorem").posts.each {|lorempost| puts lorempost.title} # printing post titles which are tagged with lorem.
Get'em all
qwerty
I also associated authors with tags through Subscribedtags model, which I can easily check which author is subscribed to which tag, and vice versa.
2.2.0 :055 > z = Author.get(1)
=> # returns details of author instance
2.2.0 :056 > z.tags
=> [#, #, #, #]
or querying via Subscribedtag
2.2.0 :057 > z.subscribedtags
=> [#<Subscribedtag @tag_id=2 @author_id=1>, #<Subscribedtag @tag_id=4 @author_id=1>, #<Subscribedtag @tag_id=25 @author_id=1>, #<Subscribedtag @tag_id=30 @author_id=1>]
you can also define your own functions to utilize querying. I've defined a subscribed_tags method which returns an array of subscribed tags' names.
2.2.0 :058 > z.subscribed_tags
=> ["Həyat", "Məstan", "İmmanuil Kant", "Lorem"]
If I want to retrieve the first_name attribute of a random author, who is subscribed to tag named "Lorem",
2.2.0 :062 > Tag.first(:name => "Lorem").authors.sample.first_name
=> "Ziya"
As an answer to your 2nd question, yes, most times you have to iterate.
Because Photos.all return a collection of Photo object instances. And this instances individually has tag attributes, not the array consists of Photo instances.
if you call p = Photo.all; print p.tags; it will return all tags associated with all photos, which may or may not be the thing you want.
Feel free to ask more questions, if these are not enough.
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