Prevent upvote model from being called for every comment - ruby-on-rails

Question

I have three models: User, Comment and Upvote. User-to-Comment has a one-to-many relation, Comment-to-Upvote has a one-to-many relation and User-to-Upvote has a one-to-many relation.
I want to do something similar to the upvoting done on Stackoverflow. So when you upvote/downvote the arrow will highlight and remain highlighted even if you refresh the page or come back to the page days/weeks later.
Currently I am doing this:
<% if Upvote.voted?(#user.id, comment.id) %>
<%= link_to '^', ... style: 'color: orange;'%>
<% else %>
<%= link_to '^', ... style: 'color:black;'%>
<% end %>
where the voted? method looks like this:
def self.voted?(user_id, comment_id)
find_by(comment_id: comment_id, user_id: user_id).present?
end
So if I have 10 comments on a page, this will load an upvote from my database 10 times, just to check if it exist!
There has to be a better way to go about doing this, but I think my brain stopped working, so I can't think of any.

Answer 1

Assuming you have properly set relations
# user.rb
class User
has_many :upvotes
end
we can load comments, current user and his upvotes:
# comments_controller.rb
def index
#comments = Comment.limit(10)
#user = current_user
user_upvotes_for_comments = current_user.upvotes.where(comment_id: #comments.map(&:id))
#upvoted_comments_ids = user_upvotes_for_comments.pluck(:comment_id)
end
And then change if condition in view:
# index.html.erb
<% if #upvoted_comments_ids.include?(comment.id) %>
<%= link_to '^', ... style: 'color: orange;'%>
<% else %>
<%= link_to '^', ... style: 'color:black;'%>
<% end %>
It will require only 2 DB queries. Hope it helps.

Answer 2

We can do it the following way if you want it to be handled by a single query.
Lets make sure the relations are proper
# user.rb
class User < ActiveRecord::Base
has_many :comments
has_many :upvotes
end
# comment.rb
class Comment < ActiveRecord::Base
belongs_to :user
has_many :upvotes
end
# upvote.rb
class Upvote < ActiveRecord::Base
belongs_to :user
belongs_to :comment
end
Then in the controller
def index
current_user = User.first # current_user may come from devise or any authentication logic you have.
#comments = Comment.select('comments.*, upvotes.id as upvote').joins("LEFT OUTER JOIN upvotes ON comments.id = upvotes.comment_id AND upvotes.user_id = #{current_user.id}")
end
And in view
# index.html.erb
<% #comment.each do |comment| %>
<% link_color = comment.upvote ? 'orange' : 'black' %>
<%= link_to '^', ...style: "color: #{link_color}" %>
<% end %>
# And all of your logics ;)

Answer 3

If you're limited to N comments per page then you can probably do this in two queries using the limit and offset methods to return the 1st, 2nd, ... ith set of N comments for the ith page, something like (syntax may be off, Ruby isn't my primary language)
comment_ids =
Comments.select("comment_id")
.where(user_id: user_id)
.order(post_date/comment_id/whatever)
.offset(per_page * (page_number - 1)) // assumes 1-based page index
.limit(per_page)
This gives you a list of comment_ids which you can use to query Upvote:
upvoted_comments =
Upvotes.select("comment_id")
.where(user_id: user_id, comment_id: comment_ids)
If you're sorting the comment_ids by a column that also exists in Upvote (e.g. if you're sorting by comment_id) then you can replace the Upvote set query with a range query.
Put upvoted_comments in a hash and you're good to go - if the comment_id is in the hash then it's been upvoted, else not.

Answer 4

I'm not sure this will avoid excess queries in this state but maybe you could include the upvotes when you fetch comments:
#comments = Comment.includes(:upvotes).where(foo: 'bar').limit(10)
Then in the view:
<%=
link_color = comment.upvotes.map(&:user_id).include?(#user.id) ? 'orange' : 'red'
link_to '^', ...style: "color: #{link_color}"
%>