feat: refactor group creation

[YheChen]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

Refactors Group creation to assign the group's id, group_name, and repo_name up front in a before_validation callback, eliminating the previous "save with nil name → assign name from id → save again" pattern.

What changed:

• Added before_validation :assign_id_and_default_names, on: :create to Group. The callback reserves the next value from groups_id_seq via nextval, then uses that id to populate id/group_name/repo_name only when the caller hasn't supplied them.
• Removed the set_repo_name after_create callback. Its job is now done by the new before_validation callback.
• Converted check_repo_uniqueness from an after_create callback that raised on conflict into a standard validate :check_repo_uniqueness, on: :create that adds an error. This avoids partway-through-creation exceptions.
• Simplified the four call sites in Assignment#add_group, Assignment#add_group_api, Student#create_group_for_working_alone_student, and Student#create_autogenerated_name_group — each collapses from a four-line save/assign/save dance into a single Group.create / Group.new call.
• Re-added the NOT NULL constraint on groups.group_name in a new migration, now that the model guarantees the column is set before save. Removed the corresponding missing_non_null_constraint exception from .active_record_doctor.rb.
• Removed three tests in group_spec.rb (validate_presence_of, validate_uniqueness_of, belong_to(:course) for group_name) that are no longer testable on :create context as the callback fills nil values before validation runs. Added a comment noting that those invariants are now enforced by the callback, the unique index, and the foreign key respectively.

Screenshots of your changes (if applicable)

Associated documentation repository pull request (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)	x
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
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Questions and Comments

(Include any questions or comments you have regarding your changes.)

[coveralls]

Coverage Report for CI Build 27021988134

Coverage increased (+0.005%) to 90.195%

Details

• Coverage increased (+0.005%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (29 of 30 lines covered, 96.67%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
app/models/group.rb	11	10	90.91%
Total (4 files)	30	29	96.67%

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	50083
Covered Lines:	46157
Line Coverage:	92.16%
Relevant Branches:	2226
Covered Branches:	1023
Branch Coverage:	45.96%
Branches in Coverage %:	Yes
Coverage Strength:	125.69 hits per line

---

💛 - Coveralls

[david-yz-liu]

use self.get_autogenerated_group_name for this and the one below

[david-yz-liu]

"up front" -> "before validation"

[david-yz-liu]

The implications of time are not great for the Changelog, as this historical context distracts from the actual change itself.

I would reword to just "Added NOT NULL constraint on groups.group_name". The PR description can provide additional context

[david-yz-liu]

The Group model still has all three of these validations (which is good). It is true that there are callbacks that ensure these validations pass, but that doesn't mean that the validations themselves aren't checked. So it's appropriate to keep the tests here.

[david-yz-liu]

Nice work, @YheChen!