使用ruby和datamapper检索具有多对多关联的记录

Question

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!

Answer 1

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}
Latın
Python
Ruby
Sosializm
Hegel

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; 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.