In-memory order updater

[benjaminwil]

Summary

This pull request proposes a replacement to the default Spree::OrderUpdater that has new and improved functionality.

We don't expect this to be the default order updater implementation in the next minor version of Solidus, but we would like to propose it as the default for the next major version of Solidus.

Note: The commits on this pull request have a long list of co-authors, as the Super Good team is approaching this as a collaborative mob programming exercise.

Below we'll add some more context about exactly what benefits the in-memory order updater brings to Solidus.

Increased performance for order updates

By default, the new order updater make fewer write calls to the database. This means that order- and checkout-related controller actions can resolve faster, with fewer round trips to the database server whenever an order is updated (which is a very often).

Order update simulation

By passing persist: false, to the in-memory order updater, an order can be recalculated and rolled back to its last state without any database writes. This enables¹ stores to let administrators and customers preview changes to orders before accepting those changes.

We know that many stores add custom features and enhancements to the order model and the order updater, so we have also added a database write query monitor that warns developers if their application code is making unintended writes when persist: false has been set.

Test performance, deeper test coverage

Because order updates require fewer round trips, this also benefits the Solidus test suite: the main side effect is that any order scaffolding and factories should just be faster.

This pull request includes additional test coverage against the current order updater as well as test coverage for the new in-memory order updater. This includes integration test coverage against both solidus_promotions and solidus_legacy_promotions.

Anatomy of this pull request

Obviously, this is a massive change. We appreciate your careful review, and if you're confused about any of these changes please leave your comments and questions. We touch a lot code across the Solidus meta gem, but all of our changes are in service of:

1. Ensuring all of the order dependencies (line items, shipments, adjustments, and so on) can be trusted to not generate database write unless directed to via the order updater. So, many commits concern persistence toggling, attribute assignment, and the representation of records currently loaded in memory.
2. Ensuring there is integrated order test coverage across all the parts of Solidus that result in order updates.
3. Ensuring database writes can be logged as warnings for both Solidus developers and application developers.

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

Footnotes

1. By enables we mean that application developers can build features that preview order changes. But as of this implementation, no part of solidus_admin, solidus_backend, or the Solidus starter frontend make use of this functionality. ↩

[jarednorman]

Just making a note that we are waiting on Alistair to rebase #6026 against this.

@AlistairNorman could you just make sure that gets done at some point before the 20th? That way we can have it for our next session.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.53%. Comparing base (765d4fd) to head (2a95c04).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5872      +/-   ##
==========================================
+ Coverage   89.46%   89.53%   +0.07%     
==========================================
  Files         977      980       +3     
  Lines       20397    20543     +146     
==========================================
+ Hits        18248    18394     +146     
  Misses       2149     2149              

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