Bug Description

Deleting a flow raises a psycopg.errors.ForeignKeyViolation IntegrityError when the flow has associated
execution traces (records in the trace and span tables). The deletion fails inside cascade_delete_flow
and surfaces as a 500 error in the ASGI application.

Error log:

Unable to cascade delete flow: 21e3cc42-cada-492b-8183-5fca183f65cd
IntegrityError: (psycopg.errors.ForeignKeyViolation) update or delete on table "trace"
violates foreign key constraint "span_trace_id_fkey" on table "span"
DETAIL: Key (id)=(e306f1a8-94d5-47a6-8206-579ed2c82203) is still referenced from table "span".

There are two compounding root causes:

Root Cause 1 — Missing ON DELETE CASCADE on span.trace_id FK at the database level

The migration 3478f0bd6ccb_add_trace_and_span_tables.py creates the span table's FK to trace
without ondelete="CASCADE":

# span → trace: MISSING ondelete="CASCADE"
sa.ForeignKeyConstraint(["trace_id"], ["trace.id"]),

# trace → flow: correctly has CASCADE
sa.ForeignKeyConstraint(["flow_id"], ["flow.id"], ondelete="CASCADE"),

When PostgreSQL cascades the flow → trace delete (which works), it attempts to delete trace rows next,
but fails because span records still reference them and the FK has no cascade rule.

Root Cause 2 — cascade_delete_flow never explicitly deletes trace or span records

cascade_delete_flow in src/backend/base/langflow/api/utils/core.py uses raw SQL delete() statements
and relies entirely on the database-level FK cascade from flow → trace → span. However:

• Raw SQL bypasses SQLAlchemy ORM cascade (cascade="all, delete-orphan" defined on
  TraceTable.spans), so the ORM safety net never fires.
• The function explicitly deletes MessageTable, TransactionTable, VertexBuildTable, and
  FlowVersion before deleting Flow, but never handles SpanTable or TraceTable,
  leaving them entirely dependent on the broken DB-level cascade chain.

Proposed Fix Options

Option A is immediately safe for both SQLite and PostgreSQL and requires no schema migration.
Option B is the more robust long-term solution and can be applied alongside Option A for defence in depth.

Affected Files

Reproduction

1. Run Langflow against a PostgreSQL database.
2. Apply all migrations up to and including 59a272d6669a — trace.flow_id will have CASCADE,
   but span.trace_id will not.
3. Execute at least one flow that generates traces (creates rows in trace and span).
4. Delete that flow via the UI or API: DELETE /api/v1/flows/{flow_id}.
5. Observe the 500 error and ForeignKeyViolation in the application logs.

Expected behavior

Deleting a flow should atomically remove all child records in dependency order without any
IntegrityError:

span → trace → flow

This is consistent with how other child tables (MessageTable, TransactionTable, etc.) are
already handled by cascade_delete_flow.

Who can help?

No response

Operating System

Openshift

Langflow Version

1.8.1

Python Version

3.12

Screenshot

No response

Flow File

No response