Exterminate! (or Destroying Associated Records in Rails)

thecodepixi profile image Emily A. Pixi Updated on 4 min read

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

Let's say, for example, that you're building a Rails app that has users, allows those users to have blogs, and allows users to make posts, associated with their own blogs.

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

    class User 
        has_many :blogs
        has_many :blog_posts, through: :blogs 

    class Blog
        belongs_to :user 
        has_many :blog_posts 

    class BlogPost
        belongs_to :blog 
        belongs_to :user 

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

Now, 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 their data is deleted, there are a few simple things we can do to make this happen "auto-magically"...

First, we can use an ActiveRecord method dependent, and set it to destroy to the end of our has_many association between User and their blogs:

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

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

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 blog_posts are still in the database. That's not good! We don't need those anymore. They're just taking up space, since the user and blog they belonged to have been deleted.

So, how do we ensure that their blog_posts get deleted as well?

Thinking about our database 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 their 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 any 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 

Now, if we delete a 'User', the following things would happen:

our user#destroy action will probably look like this:

    def destroy 
        @user = User.find_by(id: <some user id>)

        # probably a redirect would happen down here  

and the resulting 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


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

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])

      # probably a redirect would happen down here  

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


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satyapatidar profile image
Satya Patidar

Thanks Emily for the great post!

I am using dependent: :destroy approach in my Heroku Rails application and it seems to work well for small set of dependent records. But when the associated (has_many) is a bigger set and it's nested too (associated association can have has_many association and so on), it takes too long to destroy all those associations and on Heroku we have only 30 seconds request timeout, so eventually request times out.

Do you have any suggestion for this scenario?

Thanks in advance!

leifika profile image
Leif Johansen

I was struggling to understand this and your explanation was great and really helped!
Thank You!

kharper profile image
Kyla Harper

Great post, thank you for writing it!

thecodepixi profile image
Emily A. Pixi Author

Thanks Kyla! Glad you enjoyed it!