Question

我有两个模型，如Doctor＆amp;患者。

Doctor model has many patients
Patients belongs to one doctor.

每位医生都有名为fees(integer datatype)的栏目，用于存储医生费用。每位医生都有10美元，20美元等固定费用。

现在我想计算所有患者的总费用。例如，如果存在3名患者，其中3名医生收费10,20和20。分别为30。然后所有患者的总费用将是10 + 20 + 30 = 60.他们是否可以在没有在rails代码中循环的情况下执行sql？

class Doctor < ActiveRecord::Base
  has many :patients
end

class Patient < ActiveRecord::Base
  belongs_to: doctor
end

Answer 1

执行以下操作： -

# get first all doctors
doctors = Doctor.all
total_fees = doctors.inject(0) do |sum, doctor|
  # doctor.patients.count will give how many patients visited 
  # that specific doctor. With that count, I am multiplying that specific
  # doctor's fees, which is giving each doctors earnings from all his 
  # patients. # inject block then summing all doctors earnings and returned back.
  sum += doctor.fees * doctor.patients.size
end
total_fees
# => 60

如果你喜欢一个班轮 -

doctors.inject(0) { |sum, doctor| sum + doctor.fees * doctor.patients.size }

再次阅读帖子后，我想出了以下解决方案（与上面相同，但没有循环）： -

Doctor.joins(:patients).select("sum(doctors.fees) as total")[0].total # => 60

这是另一种方式，效率更高： -

Doctor.joins(:patients).sum("doctors.fees") # => 60

如何从子模型中总结父模型记录

1 个答案: