Fix up decimal migration_diff

[dereuromark]

Fixes #659

targeting 5.x if we cannot get it shimmed in 4.x for both builtin and phinx.

In CakePHP schema: 'length' = precision, 'precision' = scale
In migrations: 'precision' = precision, 'scale' = scale

I wonder if we want to change this at some point
The code here has some TODOs:

// TODO deprecate precision/scale and align with cakephp/database in 5.x
// TODO this can be cleaned up when we stop using phinx data structures for column definitions

maybe we will also want to look into cakephp/phinx#2383 changes and whats relevant for us.

[Copilot]

Pull Request Overview

This PR fixes decimal column handling in migration diffs by correctly mapping CakePHP schema attributes to migration attributes. The main issue (referenced as #659) is that CakePHP's schema representation uses 'length' for precision and 'precision' for scale, while migrations use 'precision' and 'scale' respectively.

• Added logic to correctly map decimal column attributes between CakePHP schema and migration formats
• Added comprehensive test coverage for decimal column changes
• Enhanced test setup/teardown to handle decimal change test scenarios

Reviewed Changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_app/config/MigrationsDiffDecimalChange/.gitkeep	Empty directory marker for decimal change test migrations
tests/comparisons/Diff/decimalChange/schema-dump-test_comparisons_mysql.lock	Schema dump for decimal change test baseline
tests/comparisons/Diff/decimalChange/mysql/the_diff_decimal_change_mysql.php	Expected migration output for decimal column changes
tests/comparisons/Diff/decimalChange/initial_decimal_change_mysql.php	Initial migration creating test table with decimal column
tests/TestCase/Command/BakeMigrationDiffCommandTest.php	Test case for decimal change scenario with cleanup logic
src/Command/BakeMigrationDiffCommand.php	Core fix mapping CakePHP schema attributes to migration attributes for decimal columns

Comments suppressed due to low confidence (1)

tests/TestCase/Command/BakeMigrationDiffCommandTest.php:420

• The pattern 'TheDiff$scenario' will not match DecimalChange migrations which use the 'Initial' prefix instead of 'TheDiff'. This will cause the test to fail when trying to find the generated migration file. The pattern should use the $classPrefix variable defined earlier in the method: "{$classPrefix}{$scenario}".

        $generatedMigration = $this->getGeneratedMigrationName($destinationConfigDir, "*TheDiff$scenario*");

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[markstory]

Why is this wrong? The precision was 5 before.

[dereuromark]

Could this be a quirk in the 5.next core layer?

1. Initial migration creates: DECIMAL(5,2) with precision => 5, scale => 2
2. After running the initial migration, CakePHP's schema reflection reads the column back from MySQL
3. When CakePHP reads a DECIMAL(5,2) column, it appears to be returning the total length/precision as 10 instead of 5

According to claude possible reasons "Why this happens":
This is likely due to how CakePHP's database schema layer interprets decimal columns from MySQL. MySQL stores DECIMAL(5,2) as having:

• M = 5 (total digits including scale)
• D = 2 (scale/decimal places)

But when CakePHP reads this back, it may be applying a default precision value of 10, or calculating the total column width differently (possibly including sign, decimal point, etc.).

[dereuromark]

The weird thing:
Core has existing tests here

        [
            'DECIMAL(11,2)',
            ['type' => 'decimal', 'length' => 11, 'precision' => 2, 'unsigned' => false],
        ],

They seem to pass fine

[dereuromark]

I am out of ideas whats going on. once its green locally, its red on CI :(

[dereuromark]

@markstory I might have finally found the missing piece.. :)

The down might still be a bit weird, but I think it fixes the critical part it seems.