Rails通过关联加入

Question

In Ruby on Rails, I want to find employers in the city. Lets say the models are set up this way:

City
has_many :suburbs
has_many :households, :through => suburbs
has_many :people, :through => suburbs

Suburb
has_many :households
has_many people, :through => households
belongs_to :city


Household
has_many :people
belongs_to :suburb

People
belongs_to :household
belongs_to :employer


Employer
has_many :people

I feel like I want some sort of Employer joins some_city.people but I don't know how to do this. If people belonged directly to cities, I could join Employer to people where city_id is something, but I want to find the same data without that direct join and I am a little lost.

Thank you.

Answer 1

Use nested joins

Employer.joins({:people => {:household => {:suburb => :city}}}) 

should give you the join table you're looking. If you were traversing the other direction you would use plural names

City.joins( :suburbs => {:households => {:people => :employers }})

Answer 2

You can do the join like jvans has illustrated. Or you can setup your relationships like the following:

class Employer < ActiveRecord::Base
  has_many :people
  has_many :households, through: :people
  has_many :suburbs, through: :households
  has_many :cities, through: :suburbs
end

class Person < ActiveRecord::Base
  belongs_to :household
  belongs_to :employer
end


class Household < ActiveRecord::Base
  belongs_to :suburb
  has_many :people
end

class Suburb < ActiveRecord::Base
  belongs_to :city
  has_many :households
  has_many :people, through: :households
end

class City < ActiveRecord::Base
  has_many :suburbs
  has_many :households, through: :suburbs
  has_many :people, through: :households
  has_many :employers, through: :people
end

Then you can join City from Employer , and vice-versa, directly.

For example:

Employer.joins(:cities).where("cities.name = ?", "Houston").first

SELECT "employers".* FROM "employers" 
INNER JOIN "people" ON "people"."employer_id" = "employers"."id" 
INNER JOIN "households" ON "households"."id" = "people"."household_id" 
INNER JOIN "suburbs" ON "suburbs"."id" = "households"."suburb_id" 
INNER JOIN "cities" ON "cities"."id" = "suburbs"."city_id" WHERE (cities.name = 'Houston') 
LIMIT 1