How to actually use has_many :through and set up relationships - ruby-on-rails

Question

So many tutorials on how to set up a has_many :through but not enough on how to actually do it!
I have a Inventories and Requests table joined by Bookings. Example: there could be 3 lenders who have tents in inventory, each of which is requested by 3 other borrowers. What I want to do is for each of the 3 tents in inventory, show that lender the list of 3 borrowers who requested the tent. Then the lender can pick who s/he wants to be the ultimate borrower.
I have thoughts on how this should work, but no idea if it's right, so please give advice on the below! The action is driven all by the Requests controller. Let's run through an example where the Inventories table already has 3 tents, ids [1, 2, 3]. Let's say Borrower Pat submits a Request_ID 1 for a tent.
Am I then supposed to create 3 new Bookings all with Request_ID 1 and then Inventory_ID [1, 2, 3] to get all the conceivable combinations? Something like
Inventory.where(name: "tent").each { |inventory| #request.bookings.create(inventory_id: inventory.id) }
And then is it right to use the Bookings primary key as the foreign key in both the Request and Inventory? Which means that after Borrower Pat submits his request, the bookings_id will be blank until say Lender 2 accepts, at which point bookings_id equals the id that matches the combination of Request_ID 1 and Inventory_ID 2
Now let's say when a Request is posted and a Bookings is made, I email the lender. However, I realized I don't want to bother Lender Taylor if 3 borrowers want her tent over the same time period. I'll just email her the first time, and then the subsequent ones she'll find out about when she logs in to say yes or no. In this situation is it OK to just query the Bookings table in the create action, something like (expanding off above)
-
Inventory.where(name: "tent").each do |inventory|
if !Bookings.find_by_inventory_id(inventory.id).exists?
# If there are no other bookings for this inventory, then create the booking and send an email
#request.bookings.create(inventory_id: inventory.id)
AlertMail.mail_to_lender(inventory).deliver
else
# If there are other bookings for this inventory, do any of those bookings have a request ID where the requested time overlaps with this new request's requested time? If so then just create a booking, don't bother with another email
if Bookings.where(inventory_id: inventory.id).select { |bookings_id| Bookings.find_by_id(bookings_id).request.time overlaps_with current_request.time }.count > 0
#request.bookings.create(inventory_id: inventory.id)
# If there are other bookings for this inventory but without overlapping times, go ahead and send an new email
else
#request.bookings.create(inventory_id: inventory.id)
AlertMail.mail_to_lender(inventory).deliver
end
end
end
Code above is probably flawed, I just want to know the theory of how this is supposed to be working.

Answer 1

Join Table
Firstly, has_many :through works by using a join table - a central table used to identify two different foreign_keys for your other tables. This is what provides the through functionality:
Some trivia for you:
has_and_belongs_to_many tables are called [plural_model_1]_[plural_model_2] and the models need to be in alphabetical order (entries_users)
has_many :through join tables can be called anything, but are typically called [alphabetical_model_1_singular]_[alphabetical_model_2_plural]
--
Models
The has_many :through models are generally constructed as such:
#app/models/inventory.rb
Class Inventory < ActiveRecord::Base
has_many :bookings
has_many :requests, through: :bookings
end
#app/models/booking.rb
Class Booking < ActiveRecord::Base
belongs_to :inventory
belongs_to :request
end
#app/models/request.rb
Class Request < ActiveRecord::Base
has_many :bookings
has_many :requests, through: :bookings
end
--
Code
Your code is really quite bloated - you'll be much better doing something like this:
#app/controllers/inventories_controller.rb
Class InventoriesController < ApplicationController
def action
#tents = Inventory.where name: "tent"
#tents.each do |tent|
booking = Booking.find_or_create_by inventory_id: tend.id
AlertMail.mail_to_lender(tent).deliver if booking.is_past_due?
end
end
end
#app/models/booking.rb
Class Booking < ActiveRecord::Base
def is_past_due?
...logic here for instance method
end
end
Used find_or_create_by
You should only be referencing things once - it's called DRY (don't repeat yourself)

Answer 2

I did a poor job of asking this question. What I wanted to know was how to create the actual associations once everything is set up in the DB and Model files.
If you want to create a record of B that is in a many-to-many relationship with an existing record of A, it's the same syntax of A.Bs.create. What was more important for me, was how to link an A and B that already existed, in which case the answer was A.B_ids += B_id.
Two other things:
More obvious: if you created/ linked something one way, was the other way automatic? And yes, of course. In a many-to-many relationship, if you've done say A.B_ids += B_id, you no longer have to do 'B.A_ids += A_id`.
Less obvious: if A and B are joined by table AB, the primary key of table AB doesn't need to be added to A or B. Rails wants you to worry about the AB table as less as possible, so searches, builds, etc. can all be done by A.B or B.A instead of A.AB.B or B.AB.A