Implement Diagram.trace() for upstream restriction propagation

[dimitri-yatsenko]

Summary

Implement Diagram.trace(table_expr) as the upstream mirror of Diagram.cascade() — walking the FK graph upward from a restricted table expression, propagating the key restriction through every ancestor with OR convergence.

Context

Discussion: #1232
Related: #242 (farfetch), PR #1407 (graph-driven cascade)

PR #1407 delivered downstream graph operations (Diagram.cascade() and Diagram.restrict()). The upstream direction is missing — users cannot programmatically walk the FK graph upward to find which ancestor entities contributed to a given result.

Design

Diagram.trace(table_expr) constructs a restricted upstream diagram:

trace = dj.Diagram.trace(MyChild & key)
trace.counts()          # entity counts per ancestor table
trace[Session]          # pre-restricted Session QueryExpression
trace[Session].fetch1("session_date")
trace[ExtractTraces].to_arrays("trace")

• Direction: upstream (child → ancestors)
• Convergence: OR — an ancestor entity is included if reachable through any FK path
• trace[TableClass]: returns the ancestor table pre-restricted through the FK join path
• trace.counts(): entity counts per ancestor table
• trace.heading(): all attributes across ancestor tables, grouped by table

Convergence rules

Method	Direction	Convergence	Question
cascade	downstream	OR	What's affected if this is deleted?
restrict	downstream	AND	What satisfies all these conditions?
trace	upstream	OR	What contributed to this result?

Restriction propagation (inverse of cascade)

For edge Parent→Child with attr_map, trace reverses the propagation: project child FK values back onto parent primary keys.

Implementation notes

• Reuses existing Diagram infrastructure: dependency graph, attr_map handling, topological ordering, cross-schema discovery from Implement graph-driven cascade delete and restrict on Diagram #1407
• trace[TableClass] via __getitem__ — only table classes accepted
• Returns standard QueryExpression objects — all existing query operators (join, aggr, proj, restrict) and fetch methods work unchanged

[CBroz1]

• trace[TableClass] via __getitem__ — only table classes accepted

I think it would make sense to also allow a string here, and return a FreeTable object