Find where associated records exist

问题

How can I select only those Employees who have associated Tag records? In other words, only select employee records that have one or more tag records associated with them.

class Employee < ActiveRecord::Base
  has_and_belongs_to_many :tags
end

class Tag < ActiveRecord::Base
  has_and_belongs_to_many :employees
end

The query below (which is wrong) will give you guys an idea of what I'm trying to do.

Employee.includes(:tags).where("tags.id != nil")

回答1:

You can use .joins

Employee.joins(:tags)

The SQL this generates contains and INNER JOIN on the tags table, omitting employees table records who have no associated tags record.

来源：https://stackoverflow.com/questions/12078819/find-where-associated-records-exist