Fix ActiveStorage::Blob inverse association:

[Edouard-chin]

Motivation / Background

This is a fix needed to unblock #50284, because Active Storage relies on a Active Record bug.

Detail

The problem is best understood with a small snippet:

blob = ActiveStorage::Blob.new

ActiveStorage::Attachment.new(blob: blob)
ActiveStorage::Attachment.new(blob: blob)

# Currently:
p blob.attachments #=> #<ActiveRecord::Associations::CollectionProxy []>

# Once the Active Record bug is fixed:
p blob.attachments #=> #<ActiveRecord::Associations::CollectionProxy [#<ActiveStorage::Attachment id: nil, name: nil, record_type: nil, record_id: nil, blob_id: nil, created_at: nil>, #<ActiveStorage::Attachment id: nil, name: nil, record_type: nil, record_id: nil, blob_id: nil, created_at: nil>]>

# Saving the blob would result in trying to create 2 attachments which
# fails because of unique constraints.

Additional information

The real code path that does what the snippet above does is located here:

rails/activestorage/lib/active_storage/attached/many.rb

Line 52 in 9c3ffab

record.public_send("#{name}=", blobs + attachables.flatten)

It's basically doing this:

user.images.attach "photo1.png"
# Initialize a Blob record and an Attachment

user.images.attach "photo2.png"
# Get the Blob from above, create another Attachment
# Initialize a new Blob record and an new Attachment

# Rinse and repeat every time `attach` is called

Solution

• Explicitly set the inverse_of, so that it behaves as if Infer :inverse_of for has_many ..., through: #50284 is shipped
• Don't build a new attachment for blob already having one.

Tests

I didn't add tests, the test suite is already well covered, adding the inverse_of
without any changes breaks the test suite already. You can try by running
for instance the activestorage/test/models/attached/many_test.rb.

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.