Writing scopes for many-to-many relationships

Question

I've got an application with 3 models: Renters, Departments, Criteria.

# app/models/department.rb

class Department < ActiveRecord::Base
  attr_accessible :name, :abbr

  has_many :renter_departments
  has_many :renters, :through => :renter_departments
end

and

# app/models/criterium.rb

class Criterium < ActiveRecord::Base
  attr_accessible :name

  has_many :renter_criteria
  has_many :renters, :through => :renter_criteria
end

I'm trying to make two scopes in the Renter model that will let me find renters by department and by criterium.

Here's the code I have:

# app/models/renter.rb

class Renter < ActiveRecord::Base

  # Relationships
  has_many :renter_departments
  has_many :renter_criteria
  has_many :departments, :through => :renter_departments
  has_many :criteria, :through => :renter_criteria

  # Scopes
  scope :from_department, lambda { |abbr| joins(:departments).where("abbr = ?", abbr) }
  scope :has_criterium, lambda { |criterium| joins(:criteria).where("name = ?", criterium) }

end

Using

Renter.from_department("SCS")

or

Renter.has_criterium("Economic considerations")

works on its own. However, when I try to chain them,

Renter.from_department("SCS").has_criterium("Economic considerations")

I get an error that says:

  Renter Load (0.3ms)  SELECT "renters".* FROM "renters" INNER JOIN "renter_departments" ON "renter_departments"."renter_id" = "renters"."id" INNER JOIN "departments" ON "departments"."id" = "renter_departments"."department_id" INNER JOIN "renter_criteria" ON "renter_criteria"."renter_id" = "renters"."id" INNER JOIN "criteria" ON "criteria"."id" = "renter_criteria"."criterium_id" WHERE (abbr = 'SCS') AND (name = 'Economic considerations')
SQLite3::SQLException: ambiguous column name: name: SELECT "renters".* FROM "renters" INNER JOIN "renter_departments" ON "renter_departments"."renter_id" = "renters"."id" INNER JOIN "departments" ON "departments"."id" = "renter_departments"."department_id" INNER JOIN "renter_criteria" ON "renter_criteria"."renter_id" = "renters"."id" INNER JOIN "criteria" ON "criteria"."id" = "renter_criteria"."criterium_id" WHERE (abbr = 'SCS') AND (name = 'Economic considerations')
ActiveRecord::StatementInvalid: SQLite3::SQLException: ambiguous column name: name: SELECT "renters".* FROM "renters" INNER JOIN "renter_departments" ON "renter_departments"."renter_id" = "renters"."id" INNER JOIN "departments" ON "departments"."id" = "renter_departments"."department_id" INNER JOIN "renter_criteria" ON "renter_criteria"."renter_id" = "renters"."id" INNER JOIN "criteria" ON "criteria"."id" = "renter_criteria"."criterium_id" WHERE (abbr = 'SCS') AND (name = 'Economic considerations')

Additionally,

>> Renter.from_department("SCS").class
=> ActiveRecord::Relation
>> Renter.has_criterium("Economic considerations").class
=> ActiveRecord::Relation

Any ideas on what I'm doing wrong?

Answer 1

You need to reference the table name in your scopes:

scope :from_department, lambda { |abbr| joins(:departments).where("departments.abbr = ?", abbr) }
scope :has_criterium, lambda { |criterium| joins(:criteria).where("criteria.name = ?", criterium) }

Answer 2

There are multiple name columns among the tables you're joining on. Make the WHERE clause use renters.name or departments.name or whichever table has the appropriate name column.