Rails has_many :through - ruby-on-rails

Question

I have a problem understanding the has_many and through relationship. I tried looking at a few other post here was not really clear.(Not sure if its not really because of what I am trying to achieve..)
I am trying to build and app where users have events in a calendar and each user can exchange events among each other.
I want to be able to retrieve data for each user with an API such as "user.trades" <-- which retrieve all trades that the user has made and the other methods, one "user.requested_trades" and the other "user.pending_trades". The requested_trades and pending_trades works but If I should make a 3 way table relations or this 2 way is good enough. Truth fully I really had to play around with the console to figure out what type of query it executes to be able to achieve the two methods to work. But its still very unclear the :through and :has_many.
class Trade < ActiveRecord::Base
belongs_to :seller,
:class_name => "User",
:foreign_key => "seller_id"
belongs_to :buyer,
:class_name => "User",
:foreign_key => "buyer_id"
end
class User < ActiveRecord::Base
has_many :events, :dependent => :destroy
has_many :requested_trades, -> { where(["trades.status = ?",'requested']).order("trades.created_at DESC") },
:class_name => "Trade",
:foreign_key => "buyer_id"
has_many :pending_trades, -> { where(["trades.status = ?",'pending']).order("trades.created_at DESC") },
:class_name => "Trade",
:foreign_key => "buyer_id"
has_many :sent_messages, -> { where(["messages.sender_deleted = ?", false]).order("messages.created_at DESC")},
:class_name => "Message",
:primary_key => "email",
:foreign_key => "sender_id"
has_many :received_messages, -> { where(["messages.recepient_deleted = ?", false]).order("messages.created_at DESC")},
:class_name => "Message",
:primary_key => "email",
:foreign_key => "recepient_id"

Answer 1

has_many :through is many to many relationship between two models. But this many to many relationship maintained through third model.
Suppose, two models teachers and departments. both are bi-directional(may-to-many relation).
But we can't able to maintain the relationship using only these two models so we need third model. Suppose, teacher_departments.
So as any department relation with teacher we can do using the making entry in 3rd model also vice versa.
class Teacher < ActiveRecord::Base
#It describes that teacher many-to many relationship with teacher_department model and
#also if teacher gets deleted depending entries in teacher_departments also gets deleted.
has_many :teacher_departments, dependent: :destroy
#It describe that teacher having multiple departments though 3rd teacher_departments
#model.
has_many :departments, through: :teacher_departments
end
class Department < ActiveRecord::Base
#It describes that many-to many relationship with teacher_department
#model and also if department gets deleted depending entries in teacher_departments
#also gets deleted.
has_many :teacher_departments, dependent: :destroy
#It describe that department having multiple teachers though 3rd teacher_departments
#model.
has_many :teachers, through: :teacher_departments
end
class TeacherDepartment < ActiveRecord::Base
#It describe that this model are belonging to both teacher and department model.
belongs_to :teacher
belongs_to :department
end
Example.
Teacher.first.departments.create()
It will create new entry in teacher_department table with teacher first id and newly created department id.
So you and access departments of first teacher by
Teacher.first.departments
You also take look at following link of Rails Guide
http://guides.rubyonrails.org/association_basics.html#choosing-between-has-many-through-and-has-and-belongs-to-many