关联记录上的Rails总和

Question

I have a survey model that works like so:

ResponseSets have many Responses
Responses belong_to Answer
Answer model has a "value" column.

Given a ResponseSet, I'd like the sum of the Answers that are associated with each Response.

Ie, what I'd like to be able to do, (in imaginary code) is:

response_set.responses.answers.sum('value')

However, this obviously doesn't work, I need to build a query through response_set.responses , but I don't know how.

What's the SQL-fu way to tackle this in ActiveRecord?

Answer 1

好吧，如果您使用的是Rails 3.2，则可以执行以下操作：

response_set.responses.answers.pluck(:value).inject{|sum,x| sum + x }

Answer 2

Are the answers Integers, in the sense that you're looking to find ALL numeric answers associated with a response and literally add them all up? I think you could use map and inject for something like this, depending exactly on how your models/associations are set up..

response_set.responses.answers.map(:&value).inject(:+)

Can you post your models?

Answer 3

After much trial and error I came up with this relatively simple solution, I hope this helps others in the future:

response_set.responses.joins(:answer).sum('answers.value')

To make this more convenient I just made this a method in the ResponseSet Model:

def total_value
  self.responses.joins(:answer).sum('answers.value')
end