查找大于特定关联x的所有对象

Question

我有很多Farms,每个农场都有很多animals.我需要找到每个有超过5只动物的农场.

我需要一些与此类似的东西......:

Farm.where(animals.count > 5)  

更新/回答:

Farm.joins(:animals).group("farm_id").having("count(farm_id) > 5")

Answer 1

尝试:

Farm.joins(:animals).group("farm.id").having("count(animals.id) > ?",5)

参考:https://stackoverflow.com/a/9370734/429758

Answer 2

考虑在Farm - > Animal上实现counter_cache

class Farm < ActiveRecord::Base
  has_many :animals
end

class Animal < ActiveRecord::Base
  belongs_to :farm, counter_cache: true
end

不要忘记向表中添加animals_count(整数)farms.

class AddAnimalCounterCacheToFarm < ActiveRecord::Migration
  def up
    add_column :farms, :animals_count, :integer
    # if you need to populate for existing data
    Farm.reset_column_information
    Farm.find_each |farm|
      farm.update_attribute :animals_count, farm.animals.length
    end
  end

  def down
    remove_column :farms, :animals_count
  end
end

寻找有5个或更多动物的农场

Farm.where("farms.animals_count >= 5")