PostgreSQL reload virtual columns on update via RETURNING clause

[abaldwin88]

Extends Active Record to automatically reload virtual columns on update when using PostgreSQL. This is done by issuing a single UPDATE query that includes a RETURNING clause.

Motivation

Saves an extra round trip to the database for reload and removes a developer pitfall around stale values.

Detail

Given a Post model represented by the following schema:

create_table :posts do |t|
  t.integer :upvotes_count
  t.integer :downvotes_count
  t.virtual :total_votes_count, type: :integer, as: "upvotes_count + downvotes_count", stored: true
end

total_votes_count will reflect the sum of upvotes and downvotes after update is successfully called. Prior to this change calling reload would have been necessary to obtain the value calculated by the database.

post = Post.find(1)
post.update(upvotes_count: 2, downvotes_count: 2)
# Calling `post.reload` no longer necessary
post.total_votes => 4

• At this moment MySQL and SQLite adapters do not support this behavior

Additional information

• Follow-up from PR Assign auto populated columns on Active Record object creation  #48241
• Related to PR Add ActiveRecord.reload_attributes #48434
• Closes Virtual stored columns should return new values on update #48423 when using Postgresql

CC: @nvasilevski

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[ideasasylum]

I just checked out Rails main to see how the generated columns work with previous_changes. It worked on create but doesn't on update 😞

Then I tried this branch and it works as expected 🥳

Given a virtual column like:

add_column :products, :premium, :virtual, type: :boolean, as: "price >= 1000", stored: true

I could do:

product = Product.create! user: u, price: 100
product.premium
=> false
product.update price: 2000
=> true
product.premium
=> true
product.previous_changes
=> 
{"price"=>[100, 2000],
 "updated_at"=>[Sun, 23 Jul 2023 22:22:35.416328000 UTC +00:00, Sun, 23 Jul 2023 22:22:51.945466000 UTC +00:00],
 "premium"=>[false, true]}

Yay! I can see the changes caused by the update of the premium virtual column

I hope this makes it into 7.1 as the virtual columns will solve a gnarly problem for me if the dirty tracking works for them

[excid3]

Ran into this one today as well. Would love to see this fixed.

[abaldwin88]

Rebased onto the latest from main. Added a test for dirty tracking behavior based on the example provided by @ideasasylum

[nvasilevski]

I think the biggest issue with the current implementation is changing return value of public methods which is a breaking change and normally requires a deprecation cycle

Other than that I think it's a very valuable feature to be merged and also a safe one since most of the changes happen behind the scenes and don't require any public APIs to be exposed

[abaldwin88]

Thanks for the feedback @nvasilevski. I've implemented your suggestion for keeping the signature the same expect when returning is specified. I also made a couple other small revisions based on your input, added docs, and rebased onto latest from main.

Build error from Rubocop appears to be from main. This PR does not modify action_mailer_basics.md.