A Model Belongs to Many Model

rubyonrails-talk
1

Hi,

I have a design question. I have 4 models, Photo, Trip, Destination, and Hotel. I want Trip, Destination, and Hotel to have Photos. A Photo always belongs to a Destination but at the same time it can belongs to Trip and Hotel. Most of the time a Photo belongs to Trip & Destination. The way I have it right now is the following:

[code] class Trip < ActiveRecord::Base   :has_many :photos

class Destination < ActiveRecord::Base   :has_many :photos

class Hotel < ActiveRecord::Base   :has_many :photos

class Photos < ActiveRecord::Base   :belongs_to :trip   :belongs_to :destination   :belongs_to :hotel [/code]

photos table will have trip_id, destination_id, and hotel_id. And since a record in photo table doesn't always belongs to trip or hotel, trip_id and hotel_id can be null. That's should be ok right to have null in the foreign key? Is this the best way to do it or there's a better design to this problem? Moreover, I want to be able to easily add another model that I can attach photos to it.

Thanks!

2

Hi,

I have a design question. I have 4 models, Photo, Trip, Destination, and Hotel. I want Trip, Destination, and Hotel to have Photos. A Photo always belongs to a Destination but at the same time it can belongs to Trip and Hotel. Most of the time a Photo belongs to Trip & Destination. The way I have it right now is the following:

photos table will have trip_id, destination_id, and hotel_id. And since a record in photo table doesn't always belongs to trip or hotel, trip_id and hotel_id can be null. That's should be ok right to have null in the foreign key?

Unless you've explicitly asked for it not to be null

Is this the best way to do it or there's a better design to this problem? Moreover, I want to be able to easily add another model that I can attach photos to it.

You may be interested in a polymorphic association: make photos have columns subject_id, and subject_type. then class Photo < ActiveRecord::Base   belongs_to :subject, :polymorphic => true end

class Trip < AR::Base    has_many :photos, :as => :subject end

class Destination < AR::Base    has_many :photos, :as => :subject end

etc...

photo.subject will return an instance of the appropriate class.

Fred

3

Frederick Cheung wrote:

4

Fajar A b wrote:

Thanks for the reply Fred. I'd like to be able to do destination.photos which will returns all photos that belongs to destination. Can I do this with polymorphic? What I understand is when I use your setup and do destination.photos it will only return photos that has subject_type = "Destination". SOme photos belong to a trip and also a destination.

Thanks!

Polymorphics are certainly the way to go here. Once you understand them you will use them everywhere :slight_smile:

If you declare the associations as Fred described you will be able to call destination.photos, which will return an array of photo objects.

You are also correct about one photo belonging to one model. However, are you sure that photos really need to belong to destinations, trips, and hotels or would it be better attaching photos to one, then using the associations to pull them back again?

(as an aside, I am a little confused about the difference between a destination and a hotel. Surely a hotel is one kind of destination?)

5

Thanks for the reply Fred. I'd like to be able to do destination.photos which will returns all photos that belongs to destination. Can I do this with polymorphic? What I understand is when I use your setup and do destination.photos it will only return photos that has subject_type = "Destination". SOme photos belong to a trip and also a destination.

Oops, I'd glossed over the fact that some photos belong to more than one of destination/trip/etc... Perhaps a has_many :through, where the association to destination/trip/etc on the join table is polymorphic ?

Fred

6

Frederick Cheung wrote:

Thanks for the reply Fred. I'd like to be able to do
destination.photos which will returns all photos that belongs to destination. Can I do
this with polymorphic? What I understand is when I use your setup and do destination.photos it will only return photos that has subject_type = "Destination". SOme photos belong to a trip and also a destination.

Oops, I'd glossed over the fact that some photos belong to more than one of destination/trip/etc... Perhaps a has_many :through, where the association to destination/trip/ etc on the join table is polymorphic ?

Fred

This is what I ended up doing:

class Photo < ActiveRecord::Base   belongs_to :user   belongs_to :destination   belongs_to :content, :polymorphic => true

class Destination < ActiveRecord::Base   has_many :photos

class Trip < ActiveRecord::Base   belongs_to :user   belongs_to :destination

This way I can do destination.photos and get all photos for that destination even though the content is Trip or anything else. What do you guys think?

Andrew, Destination usually a city and hotel must belongs to a destination.