ActiveStorage: Avoiding the problem of N+1 queries


18 October 2023

ActiveStorage: Avoiding the problem of N+1 queries The N+1 problem is common in database-oriented programming and occurs when an additional query is required for each relationship between objects, generating an excess of queries. This problem also affects Active Storage, which creates records in the database through polymorphic relationships.

Suppose we have a model called Solution, where each solution must have an attached icon in the form of an image. Something like this:

class Solution < ApplicationRecord
    has_one_attached :icon

The definition of the model is simple and we should have no problems querying it. Whether with Solution.first, Solution.limit(10) or another query related to our model, it should normally generate only one request to the database. However, the complication arises when trying to generate a URL for each of the icons associated with our model. At this point, ActiveStorage requires querying the additional tables active_storage_attachments and active_storage_blobs. If this data has not been previously loaded, this is when the N+1 problem occurs.

To display the problem we will use the following code:

icon_urls ={|s| s.icon.url }


The solution will always be to preload the data. But how?


ActiveRecord’s answer to this common performance problem is the includes method:

icon_urls = Solution.all.includes(icon_attachment: :blob).map{|s| s.icon.url }

And that’s it, includes defines whether to load the data via preload (separate queries) or eager_load (all in one query).

ActiveStorage scopes

Now, ActiveStorage is aware of this problem and that’s why they also offer a solution on their side: A scope to make your life with Rails even easier.

Every time we use has_one_attached in our model we get this scope added with the name with_attached_#{attachment_name}. For better understanding, in our case it would look like this:

icon_urls ={|s| s.icon.url }

This performs the same action as includes, but our query is simpler to read.

These same methods are also available for when variants and/or has_many_attachments are used.

Enjoy programming

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