has_many :through association

This relationship is used when you want to set up a many-to-many relationship through a third model, often called a join model.

For example in the code below, subject would be the join model.

class Teacher < ActiveRecord::Base
  has_many :subjects
  has_many :students, through: :subjects
end

class Subject < ActiveRecord::Base
  belongs_to :teacher
  belongs_to :student
end

class Student < ActiveRecord::Base
  has_many :subjects
  has_many :teachers, through: :subjects
end

The join model typically contains foreign keys that connect the two other models (Teacher and Student).

Using the association has_many :through allows the primary model (Teacher) to interact with the associated models (Student) indirectly through the join model. This provides direct access to associated data across a multi step association.

teacher.students # Returns all students associated with this teacher through subjects teacher.subjects # Returns all subjects taught by this teacher

Each teacher object can use .students to get a collection of students associated through the subjects join table and similarly .subjects to access all of the subjects they teach.

This setup effectively maps a many-to-many relationship between Teacher and Student using Subject as a connector, illustrating a practical use of the has_many :through association in rails.