How to implement a voting system in Rails?

First try stablishing the relation between the post, the user and the vote. for example i would do something like this at first:

rails generate model Vote user:references post:references

Then at the models I would set the relation:

def Vote < ApplicationRecord
   belongs_to :user
   belongs_to :post
end

def Post < ApplicationRecord
   has_many :votes
   has_many :votants, through: :votes
end

def User < ApplicationRecord
  has_many :votes
  has_many :liked_posts, through: :votes
end

having this relation will add a method called "liked_posts" to your users, which will return the posts that the user has voted for, then a "votants" method to your Post Model which will return the users that have voted for that single post.

Then you should have some sort of machanism to make a user vote for a post you can approach this in a number of ways, right now i would go with the simplest solution i think of right now:

class VotesController < ApplicationController
...
  def create
    vote = Vote.new(vote_params)
    if vote.save
      #do something
    else 
      # flash that something went wrong
    end
  end
...
  private

  def vote_params
     params.require(:vote).permit(:user_id, :post_id)
  end
end

Ok, so now you got a simple voting system, if you would like to add an upvote and downvote you should probably modify your Vote table doing something like this:

rails generate migration AddVoteTypeToVotes vote_type:integer

We'll be using an Enum type to achieve this, so going back to the votes model:

class Votes < ApplicationRecord
...
  enum selectable_vote_types: [ :upvote, :downvote ]

end

so with this changes we should be able to send 0 if the vote it's an upvote and 1 if the vote it's a downvote to our controller, so we should change the strong params inside our controller:

class VotesController < ApplicationController
...
  private

  def vote_params
    params.require(:vote).permit(:user_id, :post_id, :vote_type)
  end
end

there's one more challenge we should be facing by now, we are sending the user_id to the controller, but that's not quiet practical, so, if you are using devise for authentication you could just take the user_id from the current_user, if you are not using devise probably you should be sending the user_id through the request, or create your own current_user method.

This same thing can be implmented to the comment Model

But anyway i hope this helps.