Use Ruby's select method on a Rails relation and update it

Question

I have an ActiveRecord relation of a user's previous "votes"...

@previous_votes = current_user.votes

I need to filter these down to votes only on the current "challenge", so Ruby's select method seemed like the best way to do that...

@previous_votes = current_user.votes.select { |v| v.entry.challenge_id == Entry.find(params[:entry_id]).challenge_id }

But I also need to update the attributes of these records, and the select method turns my relation into an array which can't be updated or saved!

@previous_votes.update_all :ignore => false
# ...
# undefined method `update_all' for #<Array:0x007fed7949a0c0>

How can I filter down my relation like the select method is doing, but not lose the ability to update/save it the items with ActiveRecord?

Poking around the Google it seems like named_scope's appear in all the answers for similar questions, but I can't figure out it they can specifically accomplish what I'm after.

Answer 1

The problem is that select is not an SQL method. It fetches all records and filters them on the Ruby side. Here is a simplified example:

votes = Vote.scoped
votes.select{ |v| v.active? }
# SQL: select * from votes
# Ruby: all.select{ |v| v.active? }

Since update_all is an SQL method you can't use it on a Ruby array. You can stick to performing all operations in Ruby or move some (all) of them into SQL.

votes = Vote.scoped
votes.select{ |v| v.active? }
# N SQL operations (N - number of votes)
votes.each{ |vote| vote.update_attribute :ignore, false }
# or in 1 SQL operation
Vote.where(id: votes.map(&:id)).update_all(ignore: false)

If you don't actually use fetched votes it would be faster to perform the whole select & update on SQL side:

Vote.where(active: true).update_all(ignore: false)

While the previous examples work fine with your select, this one requires you to rewrite it in terms of SQL. If you have set up all relationships in Rails models you can do it roughly like this:

entry = Entry.find(params[:entry_id])
current_user.votes.joins(:challenges).merge(entry.challenge.votes)
# requires following associations:
# Challenge.has_many :votes
# User.has_many :votes
# Vote.has_many :challenges

And Rails will construct the appropriate SQL for you. But you can always fall back to writing the SQL by hand if something doesn't work.

Answer 2

A nice approach this is to use scopes. In your case, you can set this up the scope as follows:

class Vote < ActiveRecord::Base
  scope :for_challenge, lambda do |challenge_id|
    joins(:entry).where("entry.challenge_id = ?", challenge_id)
  end
end

Then your code for getting current votes will look like:

challenge_id = Entry.find(params[:entry_id]).challenge_id
@previous_votes = current_user.votes.for_challenge(challenge_id)