Rails has_many :through Find by Extra Attributes in Join Model

问题

New to both Ruby and Rails but I'm book educated by now (which apparently means nothing, haha).

I've got two models, Event and User joined through a table EventUser

class User < ActiveRecord::Base
  has_many :event_users
  has_many :events, :through => :event_users
end

class EventUser < ActiveRecord::Base
  belongs_to :event
  belongs_to :user

  #For clarity's sake, EventUser also has a boolean column "active", among others
end

class Event < ActiveRecord::Base
  has_many :event_users
  has_many :users, :through => :event_users
end

This project is a calendar, in which I have to keep track of people signing up and scratching their name out for a given event. I figure the many to many is a good approach, but I can't do something like this:

u = User.find :first
active_events = u.events.find_by_active(true)

Because events don't actually HAVE that extra data, the EventUser model does. And while I could do:

u = User.find :first
active_events = []
u.event_users.find_by_active(true).do |eu|
  active_events << eu.event
end

This seems to be contrary to "the rails way". Can anyone enlighten me, this has been bugging me for a long time tonight (this morning)?

回答1:

How about adding something like this into your User model?

has_many  :active_events, :through => :event_users, 
          :class_name => "Event", 
          :source => :event, 
          :conditions => ['event_users.active = ?',true]

After that you should be able to get active events for a user just by calling:

User.first.active_events

回答2:

Milan Novota has a good solution – but :conditions is now deprecated and the :conditions => ['event_users.active = ?',true] bit just doesn't seem very rails anyways. I prefer something like this:

has_many :event_users
has_many :active_event_users, -> { where active: true }, class_name: 'EventUser'
has_many :active_events, :through => :active_event_users, class_name: 'Event', :source => :event

After that you should still be able to get active events for a user just by calling:

User.first.active_events

回答3:

Even though your u.events isn't explicitly calling the user_events table, that table is still included in the SQL implicitly because of the necessary joins. So, you can still use that table in your find conditions:

u.events.find(:all, :conditions => ["user_events.active = ?", true])

Of course, if you plan to be doing this lookup a lot then sure, give it a separate association as Milan Novota suggests, but there's no requirement for you to do it that way

回答4:

Well, more responsibility is being put in User model than actually needed, and there is no good reason to do so.

We can first define the scope in EventUser model because where it actually belongs, like:

class EventUser < ActiveRecord::Base
  belongs_to :event
  belongs_to :user

  scope :active,   -> { where(active: true)  }
  scope :inactive, -> { where(active: false) } 
end

Now, a user could have both kind of events: active events as well as inactive events, so we can define the relationship in User model as follows:

class User < ActiveRecord::Base
  has_many :active_event_users,   -> { active },   class_name: "EventUser"
  has_many :inactive_event_users, -> { inactive }, class_name: "EventUser"

  has_many :inactive_events, through: :inactive_event_user,
                             class_name: "Event",
                             source: :event
  has_many :active_events,   through: :active_event_users,
                             class_name: "Event",
                             source: :event
end

The beauty in this technique is that the functionality of being an active or an inactive event belongs to EventUser model, and if in future the functionality needs to be modified, it would be modified only in one place: EventUser model, and the changes will be reflected in all other models.

来源：https://stackoverflow.com/questions/408872/rails-has-many-through-find-by-extra-attributes-in-join-model

标签

ruby-on-rails

ruby

has-many

has-many-through