class Blog < ActiveRecord::Base
  has_many :comments, dependent: :destroy
end
 
class Comment < ActiveRecord::Base
  belongs_to :blog
end

By using this creation of a comment becomes easier as:

@comment = @Blog.comments.create(text: "A helpful Blog")

And deleting the Blog will delete all the associated comments.

@blog.destroy

Types Of Associations:
Rails support six types of associations. Those are:

(a) belongs_to : - It sets up one-to-one connection with another model. So that one instance of model can be related to only one instance of another model. Like a comment can be associated to only one Blog. So it will be declared as in previously given example:

class Order < ActiveRecord::Base
  belongs_to :customer
end

(b) belongs_to : - It also sets up one-to-one connection with another model. The difference is that in this case each instance contains or possesses one instance of another model. As an example if we store the analytics of a blog in one table, then a blog can have only one analytics. So there relationship would be blog has_one analytic. It would be written in rails as:

class Blog < ActiveRecord::Base
  has_one :analytic
end

Note: To check whether to use has_one and belongs.Check this out Choosing Between belongs_to and has_one

(c) has_many : - It indicates one-to-many relationship between two models. It means one instance of a model can have 0 to many instance of another model. In our case a blog can have many comments. So there will be blog has_many comments at one end and on other end it would be comment belongs_to blog.In rails we will write as:

class Blog < ActiveRecord::Base
  has_many :comments
end

(d) has_many :through : - It is used to set up many-to-many connection with another model. It represents that the declaring model can be matched to 0 or more model instance of another model via third model. As an example if a blog can have many comments and a user can have many blogs, then :

class Blogger < ActiveRecord::Base
  has_many :blogs
  has_many :comments, through: :blogs
end
 
class Comment < ActiveRecord::Base
  belongs_to :blogger
  belongs_to :blog
end

(e) has_one :through : - It is used to set up one-to-one connection with another model. It represents that the declaring model can be matched to one instance of another model via third model. As an example, if a blog has one analytic and analytic has one analytic_history then blog can be related to analytic history via analytic. In rails we will write it as:

class Blog < ActiveRecord::Base
  has_one :analytic
  has_one :analytic_history, through: :analytic
end
 
class Analytic < ActiveRecord::Base
  belongs_to :blog
  has_one :analytic_history
end
 
class AnalyticHistory < ActiveRecord::Base
  belongs_to :analytic
end

(f) has_and_belongs_to_many : - It indicates a direct many-to-many connection with no third model between them. For a example if a tutionclass has many teacher and students and each teacher can have many students and each student can study from many teachers. In rails we will write this relationship as:

class TutionClass < ActiveRecord::Base
  has_and_belongs_to_many :students
end
 
class Student < ActiveRecord::Base
  has_and_belongs_to_many :teachers
end

I hope you liked associations in rails and my blog too. Some more topics are given below. Please give a look into them.

• Choosing Between has_many :through and has_and_belongs_to_many
• Polymorphic association
• Self Join in Rails