fix(api): match missing-column fallbacks on the DBAPI error, not the statement

str(ProgrammingError) embeds the full SQL, which contains every column
name — the substring check matched any error and could take the wrong
retry branch. Parse the missing column from exc.orig instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/data_loader.py

@@ -4,6 +4,7 @@ Provides efficient queries with caching.
 """
 
 import logging
+import re
 
 import pandas as pd
 import numpy as np
@@ -291,13 +292,28 @@ _GIAS_CODE_COLUMN_NAMES = (
     "admissions_policy_code",
 )
 
+_MISSING_COLUMN_RE = re.compile(r'column "?(?:s\.)?(\w+)"? does not exist')
+
+
+def _missing_column_name(exc: Exception) -> Optional[str]:
+    """Name of the missing column from a psycopg2 UndefinedColumn error.
+
+    Inspects exc.orig (the DBAPI error), whose message names only the
+    offending column — str(exc) also embeds the full SQL statement, which
+    contains every column name and therefore must not be matched against.
+    """
+    orig = getattr(exc, "orig", None)
+    match = _MISSING_COLUMN_RE.search(str(orig) if orig is not None else str(exc))
+    return match.group(1) if match else None
+
 
 def load_school_data_as_dataframe() -> pd.DataFrame:
     """Load all school + KS2 data as a pandas DataFrame."""
     try:
         df = pd.read_sql(_MAIN_QUERY, engine)
     except sqlalchemy.exc.ProgrammingError as exc:
-        if any(col in str(exc) for col in _GIAS_CODE_COLUMN_NAMES):
+        missing = _missing_column_name(exc)
+        if missing in _GIAS_CODE_COLUMN_NAMES:
             logging.getLogger(__name__).warning(
                 "marts predate the GIAS code migration — falling back to "
                 "legacy name-column query: %s",
@@ -308,7 +324,7 @@ def load_school_data_as_dataframe() -> pd.DataFrame:
             except Exception as exc2:
                 print(f"Warning: Could not load school data from marts: {exc2}")
                 return pd.DataFrame()
-        elif "has_sixth_form" in str(exc):
+        elif missing == "has_sixth_form":
             logging.getLogger(__name__).warning(
                 "marts.dim_school is missing has_sixth_form (pipeline hasn't "
                 "rebuilt the mart yet on this DB) — retrying without it: %s",

backend/tests/test_gias_translation.py

@@ -4,7 +4,7 @@ rest of the backend sees must carry today's name strings."""
 import numpy as np
 import pandas as pd
 
-from backend.data_loader import translate_gias_code_columns
+from backend.data_loader import _missing_column_name, translate_gias_code_columns
 from backend.gias_codes import ESTABLISHMENT_STATUS, PHASE_OF_EDUCATION
 
 
@@ -44,6 +44,39 @@ def test_missing_code_columns_are_a_noop():
     assert out.iloc[0]["status"] == "Open"
 
 
+def _fake_exc(orig_message):
+    """A stand-in for sqlalchemy.exc.ProgrammingError: str(exc) embeds the
+    full SQL statement (deliberately containing every column name below, to
+    prove the matcher doesn't fall back to it), while .orig carries the real
+    DBAPI error message naming only the offending column."""
+    exc = Exception(
+        "SELECT s.phase_code, s.school_type_code, s.religious_character_code, "
+        "s.status_code, s.admissions_policy_code, s.has_sixth_form FROM ... "
+        f"[SQL: ...] (Background on this error at: https://...)"
+    )
+    exc.orig = Exception(orig_message) if orig_message is not None else None
+    return exc
+
+
+def test_missing_column_name_quoted():
+    assert _missing_column_name(_fake_exc('column "phase_code" does not exist')) == "phase_code"
+
+
+def test_missing_column_name_unquoted():
+    assert _missing_column_name(_fake_exc("column phase_code does not exist")) == "phase_code"
+
+
+def test_missing_column_name_table_prefixed():
+    assert (
+        _missing_column_name(_fake_exc("column s.has_sixth_form does not exist"))
+        == "has_sixth_form"
+    )
+
+
+def test_missing_column_name_no_match_returns_none():
+    assert _missing_column_name(_fake_exc("relation \"marts.dim_school\" does not exist")) is None
+
+
 def test_load_school_data_survives_premigration_marts(monkeypatch):
     """Real prod state until the nightly pipeline first rebuilds the mart with
     the GIAS code columns: marts.dim_school still has the old name columns
@@ -75,9 +108,12 @@ def test_load_school_data_survives_premigration_marts(monkeypatch):
         calls.append(query)
         if len(calls) == 1:
             raise sqlalchemy.exc.ProgrammingError(
-                "(psycopg2.errors.UndefinedColumn) column s.phase_code does not exist",
-                None,
-                None,
+                statement=str(data_loader._MAIN_QUERY),
+                params=None,
+                orig=Exception(
+                    "(psycopg2.errors.UndefinedColumn) column s.phase_code "
+                    "does not exist\nLINE 5:         s.phase_code,"
+                ),
             )
         return good_df.copy()
 

backend/tests/test_sixth_form_flag.py

@@ -148,10 +148,14 @@ def test_load_school_data_survives_missing_has_sixth_form_column(monkeypatch):
     def fake_read_sql(query, con):
         calls.append(query)
         if len(calls) == 1:
+            # The statement text still contains phase_code, school_type_code,
+            # etc. (it's the full _MAIN_QUERY SELECT list) — that's exactly
+            # the collision this test guards against: matching must be done
+            # against exc.orig (the DBAPI error), not str(exc)/the statement.
             raise sqlalchemy.exc.ProgrammingError(
-                "SELECT ...",
-                None,
-                Exception(
+                statement=str(data_loader._MAIN_QUERY),
+                params=None,
+                orig=Exception(
                     "(psycopg2.errors.UndefinedColumn) column s.has_sixth_form "
                     "does not exist"
                 ),