BlogPost.md

Terminate! (or How to Destroy Associated Records in Rails)

One of the best things about Rails is ActiveRecord, and one of the best things about ActiveRecord is the ease with which you can associated different tables/database records. But what happens to a records associated data when it's deleted/destroyed?

Let's say, for example, that you're building a Rails app that has users, allows those users to make blog_posts, and allows users to leave comments on blog_posts.

In this scenario, your model structure might look something like this:

    class User 
    	has_many :blogs
    	has_many :blog_posts, through: :blogs 
    end 
    
    class Blog
    	belongs_to :user 
    	has_many :blog_posts 
    end 
    
    class blog_posts
    	belongs_to :blog 
    	belongs_to :user 
    end 

In your database your blogs table will have a foreign-key column of user_id, and your blog_posts table with have a foreign-key column of blog_id. Your blogs table becomes a join-table between users and blog_posts.

With the "magic" of active record, you'll be able to call user.blog_posts (where user is a particular instance of User), which will run an SQL query that looks something like this:

    SELECT "blog_posts".* FROM "blog_posts" INNER JOIN "blogs" ON "blog_posts"."blog_id" = "blogs"."id" WHERE "blogs"."user_id" = ? LIMIT ?  [["user_id", <user.id>], ["LIMIT", 11]]

In this scenario the User now has many blogs, which may have any number of blog_posts which are attributed to that User.

You give your users the option to destroy their account, warning them that if they delete their account their associated data will also be deleted (blogs and blog_posts).

But, is this actually true with the code we have so far?

It turns out, ActiveRecord does not default to deleting associated data when a record is deleted. This is actually a very good thing, for a lot of reasons. However, if we do want to make sure that data is deleted, there are a few things we can do to make this happen "auto-magically"...

First, we can use an ActiveRecord method dependent, set it to destroy, for user.blogs like this:

    class User 
    	has_many :blogs, dependent: :destroy 
    	has_many :comments, through: :blog_posts 
    end 

This ensures that when a users record is deleted/destroyed from our database, the associated blogs will also be identified (blogs WHERE user_id = user.id) and destroyed.

But what about their blog_posts?

If you were to now query your database for blog_posts WHERE user_id = user.id, you would see that all of their comments are still in the database. That's not good! We don't need those anymore. So, how do we ensure that their blog_posts get deleted as well?

Thinking about our data structure we know that a blog_post belongs to a blog and a blog belongs to a user. We can't directly access the blog_posts from the User model, but we know that because of the has_many through: relationship, that if a blog_post exists it is associated with a user, and the blog that the blog_post belongs to is associated to the same user.

Because of this structure, we know that if a user is deleted and their blogs are deleted, we can safely delete and blog_posts associated with that blog.

This means that we can add the same dependent: :destroy method for blog.blog_posts as we did with user.blogs:

    class Blog
    	belongs_to :user 
    	has_many :blog_posts, dependent: :destroy 
    end

Now, if we had a User we deleted, the following things would happen:

our user#destroy action will call

    def destroy 
    	@user = User.find_by(id: <some user id>)
    	@user.destroy 
    
    	# probably a redirect would happen down here  
    end 

and SQL will fire off the following:

    SELECT "blogs".* FROM "blogs" WHERE "blogs"."user_id" = ?  [["user_id", <user.id>]]
      Blog_post Load (0.2ms)  SELECT "blog_posts".* FROM "blog_posts" WHERE "blog_posts"."blog_id" = ?  [["blog_id", <blog.id>]]
      Blog_post Destroy (0.6ms)  DELETE FROM "blog_posts" WHERE "blog_posts"."id" = ?  [["id", <blog_post.id>]]
      # The above will happen for as many blog_posts exist for the associated blog 
      Blog Destroy (0.1ms)  DELETE FROM "blogs" WHERE "blogs"."id" = ?  [["id", <blog.id>]]
      User Destroy (1.2ms)  DELETE FROM "users" WHERE "users"."id" = ?  [["id", <user.id>]]

Et Voila!

You have deleted not only your user, but all of their associated user.blogs and all of those associated blog.posts, just by adding 2 extra words to your code in 2 places.

If you enjoyed this post, let me know! If there is another technique for this that you know of, I'd love to hear about it in the comments.

xx Emily / TheCodePixi

Bonus:

There is an alternative way to deal with this situation, by writing a helper method that to delete_user_data

It would look something like this:

  def delete_user_data
    @user.blog_posts.destroy_all
    @user.blogs.destroy_all 
  end 

You could then utilize this helper method inside of the user#destroy action, in your UsersController:

    def destroy 
      @user = User.find_by(id: params[:id])
      delete_user_data
      @user.destroy 
    end 

I much prefer the method using dependent: :destroy but I can imagine scenarios where the helper method may be a more appropriate solution.