Implement graph-driven cascade delete and restrict on Diagram

[dimitri-yatsenko]

Summary

Replace the error-driven cascade in Table.delete() (~200 lines) with graph-driven restriction propagation on Diagram. Table.delete() and Table.drop() now delegate to Diagram.cascade().delete() and Diagram.drop() respectively.

Resolves: #865 (applying restrictions to a Diagram), #1110 (cascade delete fails on MySQL 8 with limited privileges)

New Diagram methods

• cascade(table_expr) — OR at convergence, one-shot, for delete
• restrict(table_expr) — AND at convergence, chainable, for export
• delete() — execute cascade delete in reverse topo order
• drop() — drop tables in reverse topo order
• preview() — show affected tables and row counts without modifying data
• _from_table() — lightweight internal factory for Table.delete/Table.drop

Architecture changes

• Single Diagram class: Removed the if diagram_active: conditional. Diagram(nx.DiGraph) is always defined. Only visualization methods (draw, make_dot, make_svg, etc.) are gated on diagram_active.
• Table.delete() rewritten: ~200 lines → ~10 lines delegating to Diagram
• Table.drop() rewritten: ~35 lines → ~10 lines delegating to Diagram
• part_integrity: Moved from post-hoc check after delete to pre-check during diagram construction

Restriction propagation rules

For edge Parent→Child with attr_map:

Condition	Child restriction
Non-aliased AND parent_attrs ⊆ child.primary_key	Copy parent restriction directly
Aliased FK (fk_attrs ≠ pk_attrs)	parent.proj(**{fk: pk for fk, pk in attr_map.items()})
Non-aliased AND parent_attrs ⊄ child.primary_key	parent.proj()

Advantages over previous implementation

	Previous (error-driven)	New (graph-driven)
MySQL 8 + limited privileges	Crashes (#1110)	Works — no error parsing needed
PostgreSQL	Savepoint overhead per attempt	No errors triggered
part_integrity	Post-hoc check after delete	Pre-check before any delete
Reusability	Delete-only	Delete, drop, export, backup
Inspectability	Opaque recursive cascade	preview() shows affected data before executing

Files changed

File	Change
src/datajoint/diagram.py	Single Diagram class with cascade(), restrict(), _propagate_restrictions(), delete(), drop(), preview(), _from_table()
src/datajoint/table.py	Table.delete() and Table.drop() rewritten to delegate to Diagram. Dead cascade code removed
src/datajoint/user_tables.py	Part.drop() passes part_integrity through
docs/design/restricted-diagram-spec.md	Implementation specification

Test plan

• All 12 existing test_cascading_delete.py tests pass
• All 6 test_cascade_delete.py tests pass (3 MySQL + 3 PostgreSQL)
• All 7 test_erd.py tests pass
• Add test_diagram_operations.py for new diagram-level cascade/restrict tests

See docs/design/restricted-diagram-spec.md for the full implementation spec.

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