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CHAPTER 7 TAGGING SUPPORT
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Setting Up for Tagging
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As in previous chapters, we will use ActiveRecord migrations to modify the database schema to implement tagging. We will add tables, create a model, and develop unit tests. We ll also introduce testing with the console.
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Updating the Database Schema
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When George assigns a set of tags to a book, we must be able to store them somewhere, and also be able to associate them with the book. For this purpose, we will add two new tables to the database schema (see Figure 7-1): The tags table is where the unique id and name of all tags are stored. The books_tags table is used to associate a set of tags with one or more books, through a many-to-many relationship (ActiveRecord database relationships are covered in 3). The books_tags table includes foreign key references to the tags and books tables.
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books id title publisher_id published_at isbn blurb page_count price created_at updated_at cover_image
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books_tags * tag_id book_id *
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Figure 7-1. Tables used by the tagging system Next, create the migration by executing the generate command: $ script/generate migration CreateTagsAndBooksTags exists db/migrate create db/migrate/006_create_tags_and_books_tags.rb The generate script creates an empty migration script for you. Next, change the script as shown in Listing 7-1.
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CHAPTER 7 TAGGING SUPPORT
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Listing 7-1. Migration Script for the Tagging Functionality class CreateTagsAndBooksTags < ActiveRecord::Migration def self.up create_table :tags do |table| table.column :name, :string, :limit => 255, :null => false, :unique => true end create_table :books_tags, :id => false do |table| table.column :tag_id, :integer, :null => false table.column :book_id, :integer, :null => false end say_with_time 'Adding foreign keys' do # Add foreign key reference to books_tags table execute 'ALTER TABLE books_tags ADD CONSTRAINT fk_tb_tags FOREIGN KEY ( tag_id ) REFERENCES tags( id ) ON DELETE CASCADE' execute 'ALTER TABLE books_tags ADD CONSTRAINT fk_tb_books FOREIGN KEY ( book_id ) REFERENCES books( id ) ON DELETE CASCADE' end say_with_time 'Adding default tags' do execute(insert_tags_sql) end end def self.down drop_table :books_tags drop_table :tags end def self.insert_tags_sql <<-END_OF_DATA insert into tags values (1,"Romance"), (2,"Cooking"), (3,"Mystery"), (4,"History"), (5,"Politics"), (6,"Elvis"), (7,"Science Fiction") END_OF_DATA end end
CHAPTER 7 TAGGING SUPPORT
The script creates the two tables, tags and books_tags. The migration also adds foreign key references to the books and tags tables by executing raw SQL with the execute command. At the end of the migration, the script adds a default set of tags to the tags table by again calling the execute command.
Note The create_table method creates an id column by default. When creating the books_tags join table, we are telling the create_table command not to add an id column by setting the id parameter to false.
Now, run the migrations by executing the following command: rake db:migrate You should see the command run without errors.
Note Remember to clone the database structure from the development to the test database by executing
rake db:test:clone_structure. You can also perform the migration by executing the rake command
without parameters. This will run the tests and the migrations.
Preparing the Models
In 3, we created the Book model. Before creating the Tag model, we need to modify the Book model so that it can be tagged. This is a simple operation. Just add the acts_as_taggable method call to the model, as shown here: class Book < ActiveRecord::Base acts_as_taggable This gives us access to the acts_as_taggable API methods listed in Tables 7-1 and 7-2. We can now do things like Book.find_by_title('The Satanic Verses').tag('Novel, Blasphemous') and Book.find_tagged_with(:any => 'Blasphemous') with the model. As you can see, the code reads almost like a sentence written in English. Next, create the ActiveRecord model for the tags table by executing the generate script: $ script/generate model Tag --skip-migration
CHAPTER 7 TAGGING SUPPORT
exists exists exists create create create
app/models/ test/unit/ test/fixtures/ app/models/tag.rb test/unit/tag_test.rb test/fixtures/tags.yml
We tell the generate script not to generate a migration file, since we already created it manually. You can open the app/models/tag.rb and examine it. You should see the following code: class Tag < ActiveRecord::Base end The Tag model doesn t include a mapping to the Book model. This means you can t access the books that are associated with a tag by calling, for example, Tag.find(1).books. This won t be a problem, as we will use the acts_as_taggable API instead, which does the same work with the Book.find_tagged_with method. However, if you need it, you can add the mapping by adding has_and_belongs_to_many :books to the Tag model.
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