Rails associations - ideas for best practice - ruby-on-rails

Question

A question regarding Rails associations.
Consider the following models:
People
Events
Images
People and events can have many images.
Images uploaded to an event, need to have the ability to be associated with multiple people.
This means that there are two relationships between people and images.
One where images are uploaded directly on the person. And one where a person is tagged in an event.
Can there also be a relationship between a person and an event based on the fact they were tagged in one (or multiple) images in an event? In this regard, it's a sort of image tagging system where associations are created based on what event people are tagged in.
Wondering what is the best practice in Rails to create this association? Any help or advice is greatly appreciated!

Answer 1

I am not absolutely sure about best practice,
but there is a chance to solve your case in simple way, as far as i understand you:
We have:
Event ... has_many :photos; Photo ... belongs_to :event
and solution is to create some intermediary ActiveRecord class
People ... has_many :photos, :through photo_mappings
PhotoMapping ... belongs_to :user; belongs_to :photo
Have a nice AR associations best practice1

Answer 2

In rails it is definitely possible to define a join-table that has extra fields. So in this case I would define the following table:
class LinkedImage
belongs_to :person
belongs_to :image
OWNER=1
TAGGED=2
validates :relation_type, :inclusion => {:in => [OWNER, TAGGED]}
end
This table would link an image to a person, and has an extra field relation_type (you could think of a more appropriate name maybe), which now can have two values: 1 (for the OWNER, meaning the image was directly uploaded to the person), and 2 (the person is tagged in the image). Aside from the relation, maybe you want to store something extra, like the position in the image, an extra comment, then you could easily add that here as well.
And the person would look like:
class Person
has_many :linked_images
has_many :uploaded_images, :class_name => Image,
:through => :linked_images,
:conditions => "relation_type=#{LinkedImage::OWNER}"
has_many :tagged_in_images, :class_name => Image,
:through => :linked_images,
:conditions => 'relation_type=#{LinkedImage::TAGGED}"
end
and the code for Image could be similar.
I hope this helps.

Answer 3

I don't know if this is the best approach, but I would do something like this:
class User < ActiveRecord::Base
has_many :images, :as => :owner
has_many :tags
has_many :tagged_images, :through => :tags, :source => :image
has_many :tagged_events, :finder_sql => %q( SELECT `events`.* FROM `events`, `images` WHERE `events`.id = `images`.owner_id AND `images.owner_type ="Event" `
WHERE (`images`.id IN (#{tagged_image_ids.join(',')}) ))
end
class Event < ActiveRecord::Base
has_many :images, :as => :owner
has_many :tagged_users, :finder_sql => %q( SELECT `users`.* FROM `users`, `images` WHERE `users`.id = `images`.owner_id AND `images.owner_type ="User" `
WHERE (`images`.id IN (#{image_ids.join(',')}) ))
end
class Tag < ActiveRecord::Base
belongs_to :user
belongs_to :image
end
class Image < ActiveRecord::Base
belongs_to :owner, :polymorphic => true
has_many :tags
has_many :tagged_users, :through => :tags, :source => :user
end
Note: Using finder_sql is not the best in Rails, because for example it doesn't allow you to add new tagged images just using the relationship, but it works fine for reading.