Rails-在关系模型中查找匹配项

Question

Let's say we have these two models:

class Attendee < ActiveRecord::Base
   has_many :dances
   has_many :partners, through: :events
end

class Dance < ActiveRecord::Base
   belongs_to :attendee
   belongs_to :partner, class_name: Attendee
end

I'd like to implement a method, for example Dance.matches_of(attendee) that returns the dances of an attendee only if his partners have a dance where he is the partner. How would you implement an efficient solution?

EDIT:

Sorry because I couldn't explain my question well, I'll try to clarify it (I changed the Event class name to Dance ). If an user wants to dance with another one, he/she will create an object Dance , specifying the partner. So, let's say that Bob would like to dance with Sarah, the database table dances would look like this (I'll use usernames instead of ids to make it clearer, hopefully):

|    id    |    attendee_id    |    partner_id    |
---------------------------------------------------
|    1     |       bob         |       sarah      |

So, Dance.matches_of(bob) would return nothing, as there is no one that he wants to dance with who also wants him as a dancing partner (poor Bob). After a while Sarah thinks that maybe Bob is not such a bad guy, and he deserves a chance to hit the floor. Now, the dances table looks like this:

|    id    |    attendee_id    |    partner_id    |
---------------------------------------------------
|    1     |       bob         |       sarah      |
---------------------------------------------------
|    2     |      sarah        |        bob       |

Dance.matches_of(bob) now returns the dance with id 1, because it's the record that shows Bob's interest on dancing with Sarah AND Sarah wants to dance with him as well ( Dance.matches_of(sarah) would retrieve the second record). If Bob wanted to dance with Anna too, and so did she, the method would retrieve two records and Bob would be the happiest guy at the party, as he'd have two dancing partners.

I hope this explanation is clearer, but I'm aware that if it's so hard to be explained and understood, maybe the approach I'm following is not correct, so I'm open to suggestions.

EDIT 2:

The solution I came up with:

def self.matches_of(attendee) 
    attendee.dances.select { |dance| dance.partner.partners.include? attendee }
end

But I don't expect it to be much efficient anyway.

Answer 1

Given your models, you can add a matches_of scope to the Dance model:

class Attendee < ActiveRecord::Base
  has_many :dances
  has_many :partners, class_name: 'Attendee', through: :dances
end

class Dance < ActiveRecord::Base
  belongs_to :attendee
  belongs_to :partner, class_name: 'Attendee'

  scope :matches_of, -> (attendee) {
    where(partner_id: attendee.id, attendee_id: attendee.partner_ids)
  }
end

Given dances like

|    id    |    attendee_id    |    partner_id    |
---------------------------------------------------
|    1     |       bob         |       sarah      |

matches_of will return []

bob = Attendee.find 1
Dance.matches_of(bob) 
# returns []

Given dances like

|    id    |    attendee_id    |    partner_id    |
---------------------------------------------------
|    1     |       bob         |       sarah      |
---------------------------------------------------
|    2     |      sarah        |        bob       |

matches_of will return Dance #2

Dance.matches_of(bob) 
# returns [#<Dance id: 2, attendee_id: 2, partner_id: 1>]

Answer 2

I can think of a method like this, if there is a partner_id relating to an id in attendees table:

def partner_events
    Event.where('partner_id = ? and attendee_id in (?)', id, events.map(&:partner_id))
end

Answer 3

def matches_of(attendee) 
  partner_ids = Event.pluck(:attendee_id).where(:partern_id = > attendee.id).uniq
  Event.where('attendee_id = ? and partner_id in (?)', attendee.id, partner_ids)
end