Reload record into memory

Lets say we are creating a simple blog. The blog has posts and each post can have many comments:

class Post < ActiveRecord::Base
  has_many :comments
end
 
class Comment < ActiveRecord::Base
  belongs_to :post
end

Lets also say that on the posts index page, all posts should be listed, and for each post the number of comments. To make this efficient lets add counter cache in posts for the number of comments. First a comments_count:integer column must be added in the posts table, and then the comments model has to be updated like this:

class Comment < ActiveRecord::Base
  belongs_to :post, :counter_cache => true
end

Now to the interesting part. Lets create a test for it:

context "a post" do
  setup { @post = Factory(:post) }
 
  context "gets a new comment" do
    setup { @comment = Factory(:comment, :post => @post) }
 
    should_change("the post comments count", :by => 1) { @post.comments_count }
   end
end

So, If we have a post. And then create a comment to that post. Then the post comments_count should increase by one. This seems like a good test. But it will fail because the comments_count was not expected to be zero, but was. Why is that? Testing this in the console will make the comments_count update correct.

Thinking about it, it’s kinda obvious why it fails. This is what happens in the test: A post is created and then fetched from the database into memory. Then the same thing happens with a comment. When the comment record is saved, it makes sure that the post comments_count is updated in the database. Thats right, in the database. Above I said that a post was read into memory. So the post the test is working with, is in memory. But the comment only updates the post in the database.

So what needs to be done is to update the post from the database, before checking the comments_count. Thankfully there’s a nice method in ActiveRecord that helps with this, called reload. Lets update the test to use reload, and then run it again. And what do you know, it passes. Here’s the working test:

context "a post" do
  setup { @post = Factory(:post) }
 
  context "gets a new comment" do
    setup { @comment = Factory(:comment, :post => @post) }
 
    should_change("the post comments count", :by => 1) { @post.reload.comments_count }
  end
end

So whenever you update a record from another. Make sure to reload the record from the database to get the changes in to memory.