Record source project when adding ExCS projects

[cocomarine]

Status

• Part of https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/1724

Why not reuse remixed_from_id ?

• Although they are created by similar processes, ExCS projects added to editor-api are different from remixes.
• A lesson project copied from an ExCS source would have a non-nil remixed_from_id -> can't be teacher_project_ids with current logic in ability -> regression in teacher's ability such as teachers losing read/show_context on their students' remixes of that lesson etc.
• Api::Projects::RemixesController#index and RemixSelection both query Project.where(remixed_from_id: ), so the ExCS source project would start reporting lesson projects as its remixes, which is not correct.
• For these reasons, decided to go with new column specifically for recording source_project.

What's changed?

• Added source_project_id column
• Updated attributes_for_copy of lesson create.rb to add source project
  • Also cleared remix columns for above reasons (and for the unlikely situation where source projects have these columns populated)
• Added tests for asserting source_projects are copied during lesson copy and project remix.

[github-actions]

Test coverage

93.51% line coverage reported by SimpleCov.
Run: https://github.com/RaspberryPiFoundation/editor-api/actions/runs/33613004125

[Copilot]

Pull request overview

This PR introduces a dedicated source_project_id lineage field on Project to track Experience CS “source” projects separately from remix lineage, avoiding behavior/permission regressions tied to remixed_from_id.

Changes:

• Add projects.source_project_id with a self-referential FK (nullified on source deletion) and corresponding Project associations (source_project / derived_projects).
• Ensure lesson project creation records source_project_id and explicitly clears remix fields during ExCS-to-lesson copying.
• Add specs covering association behavior, lesson copy behavior, and remix behavior when a project already has a source_project.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
app/models/project.rb	Adds self-referential associations for source_project and derived_projects.
lib/concepts/lesson/operations/create.rb	Sets source_project_id for ExCS-based lesson project copies and clears remix fields.
db/migrate/20260828095502_add_source_project_to_projects.rb	Adds source_project reference to projects with FK nullify-on-delete.
db/schema.rb	Reflects the new column, index, and FK.
spec/models/project_spec.rb	Adds association coverage and verifies nullification when source project is destroyed.
spec/concepts/project/create_remix_spec.rb	Verifies remixes preserve source_project_id while remixed_from_id points to the immediate parent.
spec/concepts/lesson/create_spec.rb	Asserts lesson creation records source_project_id and does not inherit remix fields from the source.
spec/concepts/lesson/create_copy_spec.rb	Ensures lesson copy preserves source_project_id when copying a lesson whose project already has a source.

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[zetter-rpf]

Thanks for investigating. I want to think about this a bit more - I'm worried adding another similar column to projects might be confusing so want to think through if there any any alternatives or ways to make this clearer.

[zetter-rpf]

Great,

I've had a longer think about this, the thing I don't like about the existing code is how the permissions rely on remixed_from_id to determine what students and teachers should do with a project. I think if a project has been remixed should be a separate concern to if a student or teacher should be able to see it. Currently checking remixes does add complexity to the abilities as we're doing additional database loads for this.

But, even if we changed the abilities, I don't think it would be right to populate the remixed_from_id as part of this - remixing is a different action to what we're doing here (as you said in the description).