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Trying to emulate `belongs_to trough` using `has_one trough` but having problems

Rails • Asked by TL


Gravatar TL commented on :

Imagine a travel website where you have HotelOwners and Tourists. When they start a conversation, the app creates a join model named Conversation using has_many through. It's a classic many to many association:

class HotelOwner
  has_many :tourists, through: :conversations
  has_many :conversations
end

class Tourist
  has_many :hotel_owners, through: :conversations
  has_many :conversations
end

class Conversation
  belongs_to :hotel_owner
  belongs_to :tourist
end

Now we can use hotel_owner.tourists and tourist.hotel_owners. Also, the join model Conversation is also being used to keep some state on that association between them both (like, HotelOwner comments on Tourist and vice-versa).

But now we need a Reservation model. My initial ideia was this:

class Reservation
  belongs_to :hotel_owner
  belongs_to :tourist
end

But we also need to create the Conversation join model, since app logic requires that there cannot be a Reservation without a previous Conversation, even if a blank one. Also, the hotel_owner notes on tourist and vice-versa should be kept there and need to exist if a reservation exists.

After thinking about using manual callbacks to manually create the join model Conversation, I read that it would not be a good idea to add a belongs_to :conversation on Reservation because it could lead to database inconsistencies (like the problem if reservation.conversation.tourist pointed to a different tourist then reservation.tourist .. there should be a single source of truth to this association right?)

I then had the idea of using Conversation as a proxy to Reservations, like this:

class HotelOwner
  has_many :tourists, through: :conversations
  has_many :conversations
  has_many :reservations, through: :conversations
end

class Tourist
  has_many :hotel_owners, through: :conversations
  has_many :conversations
  has_many :reservations, through: :conversations
end

class Conversation
  belongs_to :hotel_owner
  belongs_to :tourist
  has_many   :reservations
end

class Reservation
  has_one :hotel_owner, through: :conversation
  has_one :tourist,     through: :conversation
  belongs_to :conversation
end

Since there is no belongs_to through in Rails to use in Reservation, other posts in SO suggest using has_one trough instead, just like I did above.

The problem is that conversation has_many reservations, and does not belong_to a reservation (like it does belong to a Tourist and HotelOwner).

It's not only semantics that bother me. If I do hotel_owner.reservations.create(tourist: Tourist.last), it does create the Reservation, but the join model Conversation is not created, leaving reservation.conversation nil.

After a simple hotel_owner.reload, hotel_owner.reservations return nil.

What is the correct database design and Rails association model for something like this?

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