how to generate barcode in c# net with example has_many :events, :order => "occurs_on DESC, title ASC", :dependent => :nullify in Font

Generator QR-Code in Font has_many :events, :order => "occurs_on DESC, title ASC", :dependent => :nullify

has_many :events, :order => "occurs_on DESC, title ASC", :dependent => :nullify
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So, if we did that and deleted a user, Active Record would automatically update the events and break all the foreign key references. If we didn t do this, we might have events.user_id = 1037 when there is no User with an id of 1037. For our application, we want to keep it as :destroy.
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Creating Many-to-Many Associations
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Often, the relationship between two models is many-to-many. This describes a pattern where two tables are connected to multiple rows on both sides. We ll use this in our events application to add categories to events. If we wanted to allow only one category to be selected for a given event, we could use has_many. But we want to be able to apply multiple categories. Think about this for a minute: an event can have many categories, and a category can have many events where would the belongs_to go in this situation Neither model belongs to the other in the traditional sense. In Active Record-speak, we refer to this kind of association as has_and_belongs_to_many (often referred to as habtm for short). The has_and_belongs_to_many association works by relying on a join table that keeps a reference to the foreign keys involved in the relationship. The join table sits between the tables we want to join: categories and events. Not surprisingly, then, the join table in this
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CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
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case will be called categories_events. Pay particular attention to the table name. It s formed from the names of each table in alphabetical order, separated by an underscore. In our case, the c in categories comes before the e in events, hence, categories_events. Figure 5-4 illustrates this relationship.
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Figure 5-4. The many-to-many relationship between events and categories
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Let s start by adding the Category model. This is a simple matter of generating the model. Since the category schema definition is so simple (consisting of just a name column), we ll let the generator fill in the migration for us by passing field arguments directly to the generator. Run the following command inside your application root:
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./script/generate model Category name:string
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You ll notice that we generated this model a bit differently than the others. We added
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name:string onto the end of the generate method. This is a shortcut to have your migrations automatically generated with the field_name:type that you specify. This is a handy
trick when you re generating simple schemas. Let s look at that migration and see what was generated, as shown in Listing 5-8.
Listing 5-8. The db/migrate/004_create_categories.rb File
class CreateCategories < ActiveRecord::Migration def self.up create_table :categories do |t| t.column :name, :string end end def self.down drop_table :categories end end
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
We need another migration to create the join table. Let s do that now by running the following command:
$ ./script/generate migration create_categories_events
Listing 5-9 shows the result.
Listing 5-9. The db/migrate/005_create_categories_events.rb File
class CreateCategoriesEvents < ActiveRecord::Migration def self.up create_table :categories_events, :id => false do |t| t.column :event_id, :integer t.column :category_id, :integer end end def self.down drop_table :categories_events end end
Remember that when using create_table, you don t need to specify the primary key, as it will be created automatically. Well, in the case of a join table, we actually don t want a primary key. This is because the join table isn t a first-class entity in its own right. Since creating tables without primary keys is the exception and not the rule, we need to explicitly tell create_table that we don t want to create an id. Take a close look at the call to create_table in Listing 5-9. We pass in the option :id => false. This prevents create_table from creating the primary key. Go ahead and run this migration:
$ rake db:migrate
== CreateCategories: migrating ================================================ -- create_table(:categories) -> 0.0033s == CreateCategories: migrated (0.0034s) ======================================= == CreateCategoriesEvents: migrating ========================================== -- create_table(:categories_events, {:id=>false}) -> 0.0034s == CreateCategoriesEvents: migrated (0.0035s) =================================
CHAPTER 5 ADVANCED ACTIVE RECORD: ENHANCING YOUR MODELS
With the Category model and the join table in place, we re ready to let Active Record in on our association. Open the Event and Category models and add the has_and_belongs_ to_many declarations to them, as shown in Listings 5-10 and 5-11.
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