Steps

Step 1: Teach the Topic model about Votes

Edit app/models/topic.rb so that it looks like this:

class Topic < ActiveRecord::Base
  has_many :votes, dependent: :destroy
end

Step 2: Teach the Vote model about Topics

Edit app/models/vote.rb so that it looks like this:

class Vote < ActiveRecord::Base
  belongs_to :topic
end

Step 3: Play around with Topics and Votes in the Rails console

First, make sure you've made at least one topic on the site.

Next, open a Rails console in a terminal window:

rails c

Expected result:

$ rails c
Loading development environment (Rails 4.0.0)
1.9.3-p194 :001 >

At the console, try the following things

See how many topics exist:

Topic.count

Save the first topic into a variable:

my_topic = Topic.first

my_topic here could have been any variable name, but we'll stick with my_topic for consistency.

Change the title of that topic to something else:

my_topic.update_attributes(title: 'Edited in the console')

Add a vote to that topic:

my_topic.votes.create

See how many votes that topic has:

my_topic.votes.count

Remove a vote from that topic:

my_topic.votes.first.destroy

Note that the things you can do to Model classes (like Topic and Vote), differ from the things you can do to Model instances (like my_topic, here). my_topic.votes is an association, and here behaves mostly like a model class.

Model class / association methods

• Topic.first
• Topic.last
• Topic.all
• Topic.count
• Topic.find_by_id(5)
• Topic.destroy_all
• my_topic.votes.count
• my_topic.votes.create
• my_topic.votes.destroy_all

Model instance methods

• my_topic.title
• my_topic.title = 'New title'
• my_topic.update_attributes(title: 'New title')
• my_topic.save
• my_topic.save!
• my_topic.destroy
• my_topic.votes.first.destroy

An exhaustive list of things you can do to models and associations can be found in the Active Record Query Interface RailsGuide.