A list column backfilled with a dictionary

Reported as a 500 on a live instance, immediately after it updated, and read
off its journal rather than guessed at:

  ValueError: Attribute 'reasoning_efforts' does not accept objects of
              type <class 'dict'>

`Mapped[list[str]]` is not Optional, so the column is NOT NULL, so SQLite
demands a default for the rows that already exist. `_literal_default` chose one
by asking `column.type.python_type` -- and `MutableList.as_mutable(JSON)`
returns the *same* JSON type object with a listener attached rather than
subclassing it, so `python_type` is `dict` for both flavours. Every existing row
got '{}' in a list column, and MutableList refuses a dict while *loading*: not a
wrong value sitting quietly, an exception on every read of the table.

Model.reasoning_efforts was the first list-shaped JSON column this project had
ever added to a table that already had rows, so the flaw had been harmless since
the runner was written. 1.2.0 stepped on it.

The shape now comes from the column's Python-side default -- `default=list`
against `default=dict` -- which is the only thing that can tell the two apart.
And `repair_json_shapes` puts right what was already written, on start,
converging like ensure_fts beside it, narrow enough that a legitimate {} in a
dict column survives.

Why 1981 tests missed it: conftest builds a fresh database, where the column is
created from the model with its real default. The backfill only runs on a
database that already exists, so the suite had never once exercised the path
that broke. The new tests corrupt a row exactly as the migration did and assert
it loads again.

Verified against a backup of the reporting instance's own database: the load
raises before, eleven rows are repaired, all eleven models load after.

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

CHANGELOG.md

@@ -16,6 +16,23 @@ for 1.0.0 have something to be assembled from.
 
 ## Unreleased
 
+## 1.3.1
+
+- Fixed: **updating to 1.2.0 or later broke every page that lists models**, with
+  a 500 and nothing but the error page to show for it. The per-model reasoning
+  effort list added in 1.2.0 was the first list-shaped setting this application
+  had ever added to a table that already had rows in it, and the code that fills
+  in such a column on existing rows could not tell a list from a dictionary — so
+  it wrote the wrong kind of empty value into every model, and reading one back
+  raised rather than returning nothing.
+
+  A fresh install was never affected, which is exactly why it was not caught:
+  the column is only filled in that way on a database that already existed.
+
+  This release both stops it happening and **puts right the rows already
+  written**, on start, with nothing to run by hand. If your instance is showing
+  the error page, updating is the whole fix.
+
 ## 1.3.0
 
 - **A model's reasoning efforts can now be detected rather than known.** There

src/lembas/__init__.py

@@ -1,3 +1,3 @@
 """LLeMbas - a Middle-earth themed web UI for OpenAI-compatible LLM endpoints."""
 
-__version__ = "1.3.0"
+__version__ = "1.3.1"

src/lembas/db/migrations.py

@@ -39,6 +39,29 @@ log = logging.getLogger(__name__)
 MANUAL_STEPS: list[str] = []
 
 
+def _default_shape(column: Column) -> type | None:
+    """`list` or `dict`, from the column's own Python-side default.
+
+    `default=list` and `default=dict` are how the two JSON flavours are
+    declared, and SQLAlchemy keeps the callable. Calling it is cheap and is the
+    only way to tell a MutableList column from a MutableDict one -- see the note
+    in `_literal_default`.
+    """
+    default = column.default
+    if default is None or not getattr(default, "is_callable", False):
+        return None
+    try:
+        # SQLAlchemy wraps a zero-argument callable to take a context.
+        produced = default.arg(None)
+    except Exception:  # noqa: BLE001 - a default we cannot call tells us nothing
+        return None
+    if isinstance(produced, list):
+        return list
+    if isinstance(produced, dict):
+        return dict
+    return None
+
+
 def _literal_default(column: Column) -> str | None:
     """A SQL literal to backfill an existing row's new column with.
 
@@ -63,8 +86,22 @@ def _literal_default(column: Column) -> str | None:
     if "JSON" in affinity:
         # MutableList columns must start as [] and MutableDict as {}; guessing
         # wrong makes the first read blow up rather than return empty.
-        python_type = getattr(column.type, "python_type", None)
-        return "'[]'" if python_type is list else "'{}'"
+        #
+        # 🚨 NOT `column.type.python_type`. `MutableList.as_mutable(JSON)`
+        # returns the *same* JSON type object with an event listener attached --
+        # it does not subclass or wrap it -- so the type cannot tell you which
+        # of the two it is, and `JSON.python_type` is `dict` for both. That read
+        # as "this is a dict column" for every list column, and the first one
+        # ever added by a migration (`Model.reasoning_efforts`, 1.2.0) arrived
+        # as `'{}'` on every existing row. `MutableList` refuses a dict, so the
+        # failure was not an empty list but a ValueError on *load* -- every page
+        # that lists models, 500, on an instance that had simply been updated.
+        #
+        # The Python-side default is the only honest signal: a JSONList column
+        # is declared `default=list` and a JSONDict one `default=dict`, and
+        # calling it says which. Anything that cannot be called or produces
+        # neither falls back to `{}`, which is what this always assumed.
+        return "'[]'" if _default_shape(column) is list else "'{}'"
     if "BOOL" in affinity:
         return "0"
     if any(token in affinity for token in ("INT", "FLOAT", "NUMERIC", "DECIMAL")):
@@ -190,6 +227,50 @@ def ensure_fts(engine: Engine) -> list[str]:
     return created
 
 
+def repair_json_shapes(engine: Engine) -> list[str]:
+    """Put right any JSON column backfilled with the wrong empty value.
+
+    `_literal_default` used to read the shape off `column.type.python_type`,
+    which is `dict` for a MutableList column as well as a MutableDict one -- so
+    the first list-shaped JSON column ever added by a migration arrived as
+    `'{}'` on every row that already existed. `MutableList` refuses a dict, and
+    refuses it while *loading*, so the symptom was not an empty list but a
+    `ValueError` and a 500 on every page that touched the table.
+
+    Converges, like `ensure_fts` beside it: it runs on every start, it is
+    idempotent, and on a database that was never damaged it does nothing. Only
+    the exact wrong value is rewritten -- `'{}'` in a column whose default
+    produces a list -- because `{}` cannot be a legitimate value there, while
+    anything else in that column might be somebody's data.
+    """
+    fixed: list[str] = []
+    inspector = inspect(engine)
+    known = set(inspector.get_table_names())
+
+    with engine.begin() as connection:
+        for table in Base.metadata.sorted_tables:
+            if table.name not in known:
+                continue
+            for column in table.columns:
+                if "JSON" not in column.type.__class__.__name__.upper():
+                    continue
+                if _default_shape(column) is not list:
+                    continue
+                result = connection.execute(
+                    text(
+                        f'UPDATE "{table.name}" SET "{column.name}" = \'[]\' '
+                        f'WHERE "{column.name}" = \'{{}}\''
+                    )
+                )
+                if result.rowcount:
+                    fixed.append(f"{table.name}.{column.name} ({result.rowcount} row(s))")
+                    log.warning(
+                        "repaired %s.%s on %d row(s): was '{}' in a list column",
+                        table.name, column.name, result.rowcount,
+                    )
+    return fixed
+
+
 def sync_schema(engine: Engine) -> list[str]:
     """Bring the database up to the declared schema. Returns what it changed."""
     import lembas.db.models  # noqa: F401  (registers every table on the metadata)
@@ -219,6 +300,14 @@ def sync_schema(engine: Engine) -> list[str]:
                 changes.append(f"add column {table.name}.{column.name}")
                 log.info("schema: %s", statement)
 
+    # Before the search indexes, and before anything can try to load a row:
+    # a column left holding the wrong empty value makes the ORM raise on read.
+    try:
+        for repair in repair_json_shapes(engine):
+            changes.append(f"repair {repair}")
+    except Exception:  # noqa: BLE001 - a repair that fails must not stop a start
+        log.exception("could not repair JSON column shapes")
+
     try:
         for index in ensure_fts(engine):
             changes.append(f"create search index {index}")

tests/test_migrations.py

@@ -230,3 +230,124 @@ def test_each_new_table_is_usable_after_the_upgrade(db, table):
 
     declared = {column.name for column in Base.metadata.tables[table].c}
     assert _columns(engine, table) == declared
+
+
+# --- A list-shaped JSON column added to a database that already had rows -----
+#
+# Reported as a 500 on a live instance the moment it updated:
+#
+#   ValueError: Attribute 'reasoning_efforts' does not accept objects
+#               of type <class 'dict'>
+#
+# `_literal_default` read the shape off `column.type.python_type`, and
+# `MutableList.as_mutable(JSON)` returns the *same* JSON type object with a
+# listener attached -- it does not subclass it -- so `python_type` is `dict` for
+# both flavours. Every existing row got `'{}'` in a list column, and MutableList
+# refuses a dict while *loading*, so every page that listed models raised.
+#
+# The suite never caught it because `conftest.py` builds a fresh database, where
+# the column is created from the model rather than backfilled by a migration.
+# These tests exercise the path that actually ran.
+def test_a_list_column_is_backfilled_with_a_list():
+    from lembas.db.migrations import _default_shape, _literal_default
+    from lembas.db.models import Model
+
+    columns = {c.name: c for c in Model.__table__.columns}
+    assert _default_shape(columns["reasoning_efforts"]) is list
+    assert _literal_default(columns["reasoning_efforts"]) == "'[]'"
+
+
+def test_a_dict_column_still_gets_a_dict():
+    from lembas.db.migrations import _literal_default
+    from lembas.db.models import Model
+
+    columns = {c.name: c for c in Model.__table__.columns}
+    assert _literal_default(columns["capabilities_json"]) == "'{}'"
+    assert _literal_default(columns["params_json"]) == "'{}'"
+
+
+def _seed_model(engine, **overrides):
+    """A real row, made the way the application makes one.
+
+    Built through the ORM rather than a hand-written INSERT: the table has
+    several NOT NULL columns and a test that enumerates them is a test that
+    breaks every time one is added, for reasons having nothing to do with what
+    it is checking.
+    """
+    from sqlalchemy.orm import Session
+
+    from lembas.db.models import Connection, Model
+
+    with Session(engine) as session:
+        connection = Connection(
+            name="local", base_url="http://127.0.0.1:1", api_key_encrypted=""
+        )
+        session.add(connection)
+        session.flush()
+        model = Model(connection_id=connection.id, model_id="bonsai", **overrides)
+        session.add(model)
+        session.commit()
+        return model.id
+
+
+def test_the_damage_already_written_is_repaired_on_start(tmp_path):
+    """The fix to `_literal_default` helps the next instance. This is the one
+    that helps the instance that has already updated."""
+    from sqlalchemy import create_engine, text
+
+    from lembas.db.migrations import repair_json_shapes, sync_schema
+
+    engine = create_engine(f"sqlite:///{tmp_path}/repair.db")
+    sync_schema(engine)
+    model_id = _seed_model(engine)
+
+    # Exactly what the broken backfill left behind on a row that predated the
+    # column: the wrong empty value, in a column that refuses it on load.
+    with engine.begin() as connection:
+        connection.execute(
+            text("UPDATE models SET reasoning_efforts = '{}' WHERE id = :id"),
+            {"id": model_id},
+        )
+
+    assert repair_json_shapes(engine)
+
+    with engine.begin() as connection:
+        stored = connection.execute(
+            text("SELECT reasoning_efforts FROM models WHERE id = :id"), {"id": model_id}
+        ).scalar()
+    assert stored == "[]"
+
+    # And the row loads again, which is the whole point -- the failure was a
+    # ValueError while reading, not a wrong value sitting harmlessly.
+    from sqlalchemy.orm import Session
+
+    from lembas.db.models import Model
+
+    with Session(engine) as session:
+        assert session.get(Model, model_id).reasoning_efforts == []
+
+    # Converges: a second run finds nothing left to do.
+    assert repair_json_shapes(engine) == []
+
+
+def test_the_repair_leaves_a_dict_column_alone(tmp_path):
+    """`{}` is a legitimate value in a MutableDict column and must survive."""
+    from sqlalchemy import create_engine, text
+
+    from lembas.db.migrations import repair_json_shapes, sync_schema
+
+    engine = create_engine(f"sqlite:///{tmp_path}/keep.db")
+    sync_schema(engine)
+    model_id = _seed_model(engine)
+
+    with engine.begin() as connection:
+        connection.execute(
+            text("UPDATE models SET capabilities_json = '{}' WHERE id = :id"),
+            {"id": model_id},
+        )
+    repair_json_shapes(engine)
+    with engine.begin() as connection:
+        stored = connection.execute(
+            text("SELECT capabilities_json FROM models WHERE id = :id"), {"id": model_id}
+        ).scalar()
+    assert stored == "{}"