Type air_monitor's particulates and migrate the recorded statistics

Adding a unit to a sensor that recorded long-term statistics without one
is not cosmetic: Home Assistant raises a units_changed repair and then
*suppresses statistics generation* for that entity until a human resolves
it (sensor/recorder.py's compile path hits `continue`). Shipping the PM
device classes on their own would therefore have silently frozen the very
history the labels were meant to describe.

A v2 -> v3 config-entry migration relabels the statistics metadata first.
It rewrites only the metadata row, never the recorded values -- these
readings were always µg/m³ and only the label was missing, so nothing
needs converting, which is why this uses async_update_statistics_metadata
and not change_statistics_unit. It reads entity_ids back off the entity
registry rather than rebuilding them from descriptor keys, since a renamed
entity's statistic_id no longer follows from its key, and it is scoped by
device family: range_hood and airconditioner still declare no unit for
their identically-named sensors, so relabelling theirs would create the
exact mismatch this exists to prevent.

With the migration in place there is no longer a reason to hold the labels
back on air_monitor, so it takes them too. That board is one of the two
whose fixtures pin the grade bands the mapping rests on -- typing the
purifier and not the monitor was an inconsistency, not caution. It still
declines the shared state_class column, unchanged.

Recorder coupling is guarded: after_dependencies pulls it in when
configured, the import is local to the migration, and a setup without it
is a no-op. Freshly created entries mint v3 directly, having no history to
relabel.

Tested at both levels. The unit-level tests patch the recorder and assert
which entities are relabelled with which arguments, covering the renamed
entity, subdevice-prefixed and instanced keys, the near-miss keys
(dustbag_/dustbin_), the skipped families and the recorder-absent path.
Because a mock can only prove the call is made, not that it does what the
migration needs, a second suite drives a real in-memory recorder end to
end: statistics seeded unitless, migration run, metadata confirmed to read
µg/m³ / concentration, recorded means confirmed byte-identical, and the
units_changed issue confirmed present before and absent after.

custom_components/localthings/__init__.py

@@ -3,8 +3,12 @@
 from __future__ import annotations
 
 import logging
+import re
 
 from homeassistant.config_entries import ConfigEntry
+from homeassistant.const import (
+    CONCENTRATION_MICROGRAMS_PER_CUBIC_METER as PARTICULATE_UNIT,
+)
 from homeassistant.const import EVENT_HOMEASSISTANT_STOP
 from homeassistant.core import Event, HomeAssistant, callback
 from homeassistant.exceptions import ConfigEntryNotReady
@@ -12,7 +16,7 @@ from homeassistant.helpers import device_registry as dr
 from homeassistant.helpers import entity_registry as er
 from homeassistant.helpers.typing import ConfigType
 
-from .const import CONF_HOST, CONF_PORT, CONF_SERIAL, DOMAIN, PLATFORMS
+from .const import CONF_DEVICE_TYPE, CONF_HOST, CONF_PORT, CONF_SERIAL, DOMAIN, PLATFORMS
 from .coordinator import LocalThingsCoordinator
 from .registry.identity import resolve_serial
 from .services import async_setup_services
@@ -125,6 +129,81 @@ def _repair_placeholder_keys(hass: HomeAssistant, entry: ConfigEntry, serial: st
             )
 
 
+# Registries whose Dust/FineDust/SuperFineDust sensors gained pm10/pm25/pm1
+# and a unit in the release that introduced entry version 3. Deliberately
+# not every family reading /sensors/vs/0: range_hood and airconditioner
+# still declare no unit for their identically-named sensors, and relabelling
+# their statistics to µg/m³ would assert a unit those entities don't report
+# -- creating the very mismatch this migration exists to prevent.
+_PARTICULATE_TYPED_IN_V3 = frozenset({"air_purifier", "air_monitor"})
+
+# unique_id is f"{DOMAIN}_{serial}_{state_key}"; state_key is the descriptor
+# key, optionally carrying a subdevice prefix and a trailing instance number
+# (registry/adapter._key). Matching the tail rather than rebuilding the whole
+# id keeps this working for a renamed entity, whose entity_id -- and so its
+# statistic_id -- no longer follows from the key at all.
+_PARTICULATE_KEY_RE = re.compile(r"_(?:super_fine_dust|fine_dust|dust)\d*$")
+
+
+@callback
+def _relabel_particulate_statistics(hass: HomeAssistant, entry: ConfigEntry) -> None:
+    """Point existing particulate statistics at the unit they always were.
+
+    These sensors recorded long-term statistics with no unit, and the
+    release carrying this migration gives them µg/m³. Home Assistant treats
+    that as a unit change it can't convert and *suppresses statistics
+    generation entirely* for the entity until someone resolves the repair
+    (sensor.recorder._update_issues -> UNITS_CHANGED_ISSUE, and the matching
+    `continue` in its compile path). Silently freezing the history we just
+    finished labelling is the worst of both outcomes, so the metadata is
+    corrected up front instead.
+
+    Only the metadata row is rewritten, never the recorded values. The
+    readings were always µg/m³ concentrations (issue #325); what was missing
+    was the label, so there is nothing to convert and no way for this to
+    distort history. That is also why it uses
+    `async_update_statistics_metadata` and not `change_statistics_unit`,
+    which would scale every stored value.
+
+    A no-op when the recorder isn't loaded, when this device family isn't
+    one that gained the unit, or when the device never recorded any
+    statistics -- the underlying UPDATE simply matches no rows.
+    """
+    if entry.data.get(CONF_DEVICE_TYPE) not in _PARTICULATE_TYPED_IN_V3:
+        return
+    if "recorder" not in hass.config.components:
+        # after_dependencies pulls the recorder in when it's configured at
+        # all; a setup running without it has no statistics to relabel.
+        _LOGGER.debug("recorder not loaded, skipping statistics relabel")
+        return
+
+    from homeassistant.components.recorder.statistics import (
+        STATISTIC_UNIT_TO_UNIT_CONVERTER,
+        async_update_statistics_metadata,
+    )
+
+    # µg/m³ has a converter (MassVolumeConcentrationConverter), so its class
+    # is 'concentration', not None. Read rather than hardcoded: passing a
+    # class that disagrees with HA's own table raises, and passing none at
+    # all is deprecated and stops working in HA Core 2026.11.
+    converter = STATISTIC_UNIT_TO_UNIT_CONVERTER.get(PARTICULATE_UNIT)
+    unit_class = converter.UNIT_CLASS if converter is not None else None
+
+    ent_reg = er.async_get(hass)
+    for registry_entry in er.async_entries_for_config_entry(ent_reg, entry.entry_id):
+        if registry_entry.domain != "sensor":
+            continue
+        if not _PARTICULATE_KEY_RE.search(registry_entry.unique_id):
+            continue
+        _LOGGER.debug("relabelling statistics unit for %s", registry_entry.entity_id)
+        async_update_statistics_metadata(
+            hass,
+            registry_entry.entity_id,
+            new_unit_class=unit_class,
+            new_unit_of_measurement=PARTICULATE_UNIT,
+        )
+
+
 async def async_migrate_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     """Migrate an entry to the current version.
 
@@ -132,8 +211,11 @@ async def async_migrate_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     can key its registry entries before the first poll (issue #236), and
     repairs whatever the old placeholder-keyed registration already
     orphaned.
+
+    v2 -> v3 relabels the recorded statistics for the particulate sensors,
+    which gained a device_class/unit in the same release (issue #325).
     """
-    if entry.version > 2:
+    if entry.version > 3:
         return False  # downgrade: this release doesn't know the newer shape
 
     if entry.version == 1:
@@ -147,6 +229,11 @@ async def async_migrate_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
         _repair_placeholder_keys(hass, entry, serial)
         _LOGGER.debug("migrated entry %s to version 2 (serial=%s)", entry.entry_id, serial)
 
+    if entry.version == 2:
+        _relabel_particulate_statistics(hass, entry)
+        hass.config_entries.async_update_entry(entry, version=3)
+        _LOGGER.debug("migrated entry %s to version 3", entry.entry_id)
+
     return True
 
 

custom_components/localthings/config_flow.py

@@ -793,7 +793,10 @@ def _probe_and_validate(
 
 
 class LocalThingsConfigFlow(config_entries.ConfigFlow, domain=DOMAIN):
-    VERSION = 2
+    # v3 relabels the particulate sensors' recorded statistics; a freshly
+    # created entry has none to relabel, so it starts at the migrated
+    # version rather than walking through v2 (see async_migrate_entry).
+    VERSION = 3
 
     def __init__(self) -> None:
         self._host: str = ""

custom_components/localthings/manifest.json

@@ -1,6 +1,7 @@
 {
   "domain": "localthings",
   "name": "LocalThings",
+  "after_dependencies": ["recorder"],
   "codeowners": ["@mbillow"],
   "config_flow": true,
   "dependencies": [],

custom_components/localthings/registry/capabilities/air_monitor.py

@@ -18,15 +18,17 @@ aggregate. This board's own readings are load-bearing evidence for the
 PM mapping, though: 23 grading one step above the floor as FineDust is
 what rules out a PM10-width band for that field.
 
-Dust/FineDust/SuperFineDust nevertheless stay without an HA
-`device_class`/`unit` here, which is now a migration call rather than an
-evidence gap. The mapping confirmed for the purifier family (issue #325,
-Dust=PM10 / FineDust=PM2.5 / SuperFineDust=PM1 in μg/m³) rests on
-device-side grading that this board shares, so it would carry over. But
-these five sensors have recorded unitless long-term statistics since
-issue #210, and stamping a unit onto an existing statistic is what raises
-Home Assistant's "units changed" repair -- a deliberate follow-up, not
-something to fold into the purifier's first typed release.
+Dust/FineDust/SuperFineDust carry the same HA `device_class`/`unit` as the
+purifier family (issue #325, Dust=PM10 / FineDust=PM2.5 /
+SuperFineDust=PM1 in μg/m³). The mapping rests on device-side grading this
+board shares rather than on anything purifier-specific, so typing one
+family and not the other would have been an inconsistency, not caution.
+
+These sensors have recorded *unitless* long-term statistics since issue
+#210, though, and Home Assistant suppresses statistics generation outright
+for an entity whose unit no longer matches its recorded metadata -- so
+stamping a unit on would have silently stopped the history it was meant to
+label. __init__.py's v2->v3 entry migration relabels that metadata first.
 """
 
 from datetime import time as dt_time
@@ -36,14 +38,15 @@ from ..entities import BinarySensorDesc, SensorDesc, SwitchDesc, TimeDesc
 from .air_purifier import _AIR_QUALITY_SENSORS
 from .common import int_or_none, sensor_item_value
 
-# Extra columns (state_class, device_class, unit) are deliberately discarded
-# here: air_purifier leaves Odor/CleanLevel unstamped because they read as
+# device_class/unit are taken from the shared rows; state_class deliberately
+# is not. air_purifier leaves Odor/CleanLevel unstamped because they read as
 # graded indices on that family, while this board has stamped all five as
-# `measurement` since it was added (issue #210). Consuming state_class would
-# silently drop long-term statistics for two sensors on shipped devices, and
-# the pm10/pm25/pm1 labels are held back pending the statistics migration
-# the module docstring describes. The shared rows supply only the
-# key/icon/type here.
+# `measurement` since it was added (issue #210) -- consuming that column
+# would silently drop long-term statistics for two sensors on shipped
+# devices. The pm10/pm25/pm1 labels carry over cleanly, though: they rest on
+# device-side grading this board shares (see the module docstring), and
+# __init__.py's v2->v3 entry migration relabels the unitless statistics
+# these five have been recording so the new unit doesn't suppress them.
 SENSORS = Capability(
     href="/sensors/vs/0",
     poll_tier="warm",
@@ -54,9 +57,11 @@ SENSORS = Capability(
                 field="x.com.samsung.da.items",
                 icon=icon,
                 state_class="measurement",
+                device_class=device_class,
+                unit=unit,
                 value_fn=lambda items, t=sensor_type: sensor_item_value(items, t),
             )
-            for key, icon, sensor_type, *_ in _AIR_QUALITY_SENSORS
+            for key, icon, sensor_type, _state_class, device_class, unit in _AIR_QUALITY_SENSORS
         ),
         SensorDesc(
             key="co2",

tests/localthings/test_migration.py

@@ -38,7 +38,9 @@ async def test_migration_recovers_serial_from_unique_id(
     await hass.config_entries.async_setup(entry.entry_id)
     await hass.async_block_till_done()
 
-    assert entry.version == 2
+    # Straight through to the current version: v2 -> v3 is a statistics
+    # relabel that no-ops for a family without particulate sensors.
+    assert entry.version == 3
     assert entry.data[CONF_SERIAL] == MOCK_SERIAL
 
 
@@ -257,7 +259,7 @@ async def test_migration_rejects_a_future_entry_version(hass: HomeAssistant) ->
     written by a newer release."""
     from custom_components.localthings import async_migrate_entry
 
-    entry = MockConfigEntry(domain=DOMAIN, data=LEGACY_ENTRY_DATA, version=3)
+    entry = MockConfigEntry(domain=DOMAIN, data=LEGACY_ENTRY_DATA, version=4)
     entry.add_to_hass(hass)
 
     assert await async_migrate_entry(hass, entry) is False

tests/localthings/test_statistics_migration.py

@@ -0,0 +1,171 @@
+"""The v2 -> v3 entry migration that relabels particulate statistics.
+
+Dust/FineDust/SuperFineDust gained a pm10/pm25/pm1 device_class and a
+µg/m³ unit (issue #325) after having recorded long-term statistics with no
+unit at all. Home Assistant treats that as a unit change it cannot convert
+and *suppresses statistics generation* for the entity until a human
+resolves the repair, so the metadata is corrected during migration instead.
+
+Only the metadata row is touched, never the recorded values -- the readings
+were always µg/m³, so there is nothing to convert.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+import pytest
+from homeassistant.const import CONCENTRATION_MICROGRAMS_PER_CUBIC_METER
+from homeassistant.core import HomeAssistant
+from homeassistant.helpers import entity_registry as er
+from pytest_homeassistant_custom_component.common import MockConfigEntry
+
+from custom_components.localthings import async_migrate_entry
+from custom_components.localthings.const import CONF_DEVICE_TYPE, DOMAIN
+
+from .conftest import ENTRY_DATA, MOCK_SERIAL
+
+RELABEL = "homeassistant.components.recorder.statistics.async_update_statistics_metadata"
+
+
+@pytest.fixture(autouse=True)
+def _recorder_loaded(hass: HomeAssistant):
+    """Most tests here assume a normal install, where after_dependencies has
+    pulled the recorder in. test_is_a_no_op_without_the_recorder undoes it."""
+    hass.config.components.add("recorder")
+    return hass
+
+
+def _entry(hass: HomeAssistant, device_type: str) -> MockConfigEntry:
+    entry = MockConfigEntry(
+        domain=DOMAIN,
+        data={**ENTRY_DATA, CONF_DEVICE_TYPE: device_type},
+        unique_id=f"{DOMAIN}_{MOCK_SERIAL}",
+        version=2,
+    )
+    entry.add_to_hass(hass)
+    return entry
+
+
+def _add_sensor(hass: HomeAssistant, entry: MockConfigEntry, key: str, **kwargs):
+    return er.async_get(hass).async_get_or_create(
+        "sensor",
+        DOMAIN,
+        f"{DOMAIN}_{MOCK_SERIAL}_{key}",
+        config_entry=entry,
+        **kwargs,
+    )
+
+
+async def test_relabels_every_particulate_sensor(hass: HomeAssistant) -> None:
+    entry = _entry(hass, "air_purifier")
+    expected = {
+        _add_sensor(hass, entry, key).entity_id for key in ("dust", "fine_dust", "super_fine_dust")
+    }
+
+    with patch(RELABEL) as relabel:
+        assert await async_migrate_entry(hass, entry) is True
+
+    assert {call.args[1] for call in relabel.call_args_list} == expected
+    for call in relabel.call_args_list:
+        assert call.kwargs["new_unit_of_measurement"] == CONCENTRATION_MICROGRAMS_PER_CUBIC_METER
+        # µg/m³ has a converter, so the class must be named, not None --
+        # passing neither is deprecated and breaks in HA Core 2026.11.
+        assert call.kwargs["new_unit_class"] == "concentration"
+    assert entry.version == 3
+
+
+async def test_leaves_other_sensors_on_the_same_device_alone(hass: HomeAssistant) -> None:
+    """Odor/CleanLevel/CO2 share the resource but keep the units they had."""
+    entry = _entry(hass, "air_monitor")
+    dust = _add_sensor(hass, entry, "dust")
+    for key in ("odor", "clean_level", "co2", "dustbag_usage", "dustbin_auto_close"):
+        _add_sensor(hass, entry, key)
+
+    with patch(RELABEL) as relabel:
+        assert await async_migrate_entry(hass, entry) is True
+
+    assert [call.args[1] for call in relabel.call_args_list] == [dust.entity_id]
+
+
+async def test_skips_families_that_did_not_gain_the_unit(hass: HomeAssistant) -> None:
+    """range_hood and airconditioner still declare no unit for their
+    identically-named sensors. Relabelling their statistics would assert a
+    unit those entities don't report -- creating the very mismatch this
+    migration exists to prevent."""
+    for device_type in ("range_hood", "airconditioner"):
+        entry = _entry(hass, device_type)
+        _add_sensor(hass, entry, "dust")
+        _add_sensor(hass, entry, "fine_dust")
+
+        with patch(RELABEL) as relabel:
+            assert await async_migrate_entry(hass, entry) is True
+
+        assert relabel.call_args_list == [], device_type
+        assert entry.version == 3
+
+
+async def test_is_a_no_op_without_the_recorder(hass: HomeAssistant) -> None:
+    """A setup running without the recorder has no statistics to relabel,
+    and importing recorder internals there would fail outright."""
+    entry = _entry(hass, "air_purifier")
+    _add_sensor(hass, entry, "dust")
+
+    hass.config.components.remove("recorder")
+    with patch(RELABEL) as relabel:
+        assert await async_migrate_entry(hass, entry) is True
+
+    assert relabel.call_args_list == []
+    assert entry.version == 3
+
+
+async def test_follows_a_renamed_entity_rather_than_rebuilding_its_id(
+    hass: HomeAssistant,
+) -> None:
+    """statistic_id is the entity_id, which the user can rename. Matching the
+    unique_id tail and reading entity_id back off the registry is what keeps
+    this correct for a renamed sensor -- reconstructing an entity_id from the
+    descriptor key would relabel a statistic nobody is recording."""
+    entry = _entry(hass, "air_purifier")
+    renamed = _add_sensor(hass, entry, "dust", suggested_object_id="living_room_pm10")
+    assert renamed.entity_id == "sensor.living_room_pm10"
+
+    with patch(RELABEL) as relabel:
+        assert await async_migrate_entry(hass, entry) is True
+
+    assert [call.args[1] for call in relabel.call_args_list] == ["sensor.living_room_pm10"]
+
+
+async def test_matches_subdevice_prefixed_and_instanced_keys(hass: HomeAssistant) -> None:
+    """_key() can prefix a subdevice and append an instance number, so the
+    match is on the tail rather than the whole unique_id."""
+    entry = _entry(hass, "air_purifier")
+    ent_reg = er.async_get(hass)
+    for unique_suffix in ("indoor_0_dust", "fine_dust1", "super_fine_dust"):
+        ent_reg.async_get_or_create(
+            "sensor",
+            DOMAIN,
+            f"{DOMAIN}_{MOCK_SERIAL}_{unique_suffix}",
+            config_entry=entry,
+        )
+    # Near-misses that must not match.
+    for unique_suffix in ("dustbag_full", "dustbin_auto_close", "dust_filter_reset"):
+        ent_reg.async_get_or_create(
+            "sensor",
+            DOMAIN,
+            f"{DOMAIN}_{MOCK_SERIAL}_{unique_suffix}",
+            config_entry=entry,
+        )
+
+    with patch(RELABEL) as relabel:
+        assert await async_migrate_entry(hass, entry) is True
+
+    assert len(relabel.call_args_list) == 3
+
+
+async def test_a_fresh_entry_starts_at_the_migrated_version(hass: HomeAssistant) -> None:
+    """A newly created entry has no statistics to relabel, so the config flow
+    mints v3 directly rather than walking through the migration."""
+    from custom_components.localthings.config_flow import LocalThingsConfigFlow
+
+    assert LocalThingsConfigFlow.VERSION == 3

tests/test_air_purifier_air_quality_statistics.py

@@ -69,12 +69,15 @@ def test_metadata_comes_from_the_shared_tuples_own_columns():
         assert (row[4] is None) == (row[5] is None), row
 
 
-def test_air_monitor_keeps_stamping_every_shared_sensor():
-    """air_monitor imports _AIR_QUALITY_SENSORS and discards the extra columns
-    on purpose: that board (issue #210) has stamped all five as `measurement`
-    since it was added, and consuming state_class would silently drop
-    long-term statistics for Odor/CleanLevel there. Guards the import end to
-    end and the deliberate divergence together."""
+def test_air_monitor_takes_the_pm_labels_but_not_the_state_class():
+    """air_monitor imports _AIR_QUALITY_SENSORS and consumes device_class and
+    unit -- the mapping rests on device-side grading that board shares (issue
+    #325), so typing one family and not the other would be an inconsistency.
+
+    state_class is still discarded: that board (issue #210) has stamped all
+    five as `measurement` since it was added, and consuming the column would
+    silently drop long-term statistics for Odor/CleanLevel there. Guards the
+    import end to end and the deliberate divergence together."""
     from custom_components.localthings.registry.capabilities import air_monitor
 
     assert air_monitor.SENSORS.href == "/sensors/vs/0"
@@ -83,8 +86,14 @@ def test_air_monitor_keeps_stamping_every_shared_sensor():
             d for d in air_monitor.SENSORS.entities if d.key == key and isinstance(d, SensorDesc)
         )
         assert desc.state_class == "measurement", key
-        assert desc.device_class is None, key
-        assert desc.unit is None, key
+        assert desc.device_class == _desc(key).device_class, key
+        assert desc.unit == _desc(key).unit, key
+    # And the graded pair stays untyped on both families.
+    for key in GRADED:
+        desc = next(
+            d for d in air_monitor.SENSORS.entities if d.key == key and isinstance(d, SensorDesc)
+        )
+        assert (desc.device_class, desc.unit) == (None, None), key
 
 
 def test_every_air_quality_sensor_still_reads_a_plain_int():

tests/test_statistics_migration_end_to_end.py

@@ -0,0 +1,161 @@
+"""The v2 -> v3 statistics relabel against a real recorder, not a mock.
+
+tests/localthings/test_statistics_migration.py proves the migration calls
+HA's API with the right arguments for the right entities. It cannot prove
+that call does what the migration needs, because the recorder is patched
+out. This drives an in-memory recorder end to end: statistics recorded
+unitless, migration run, metadata inspected -- and, most importantly, the
+recorded *values* checked to be untouched, which is the claim that makes
+doing this automatically safe rather than something to ask each user about.
+
+Lives here rather than under tests/localthings/ on purpose: that package's
+autouse `enable_custom_integrations` fixture depends on `hass`, which
+starts Home Assistant before `recorder_mock` can claim its database URL.
+Nothing here loads the integration -- `async_migrate_entry` is called
+directly -- so the entry only needs the one key the v2 -> v3 step reads.
+"""
+
+from __future__ import annotations
+
+from datetime import timedelta
+from functools import partial
+from typing import cast
+
+import pytest
+from homeassistant.components.recorder.models import StatisticMeanType, StatisticMetaData
+from homeassistant.components.recorder.statistics import (
+    async_import_statistics,
+    get_metadata,
+    statistics_during_period,
+)
+from homeassistant.components.recorder.util import get_instance
+from homeassistant.const import CONCENTRATION_MICROGRAMS_PER_CUBIC_METER
+from homeassistant.core import HomeAssistant
+from homeassistant.helpers import entity_registry as er
+from homeassistant.util import dt as dt_util
+from pytest_homeassistant_custom_component.common import MockConfigEntry
+from pytest_homeassistant_custom_component.components.recorder.common import (
+    async_wait_recording_done,
+)
+
+from custom_components.localthings import async_migrate_entry
+from custom_components.localthings.const import CONF_DEVICE_TYPE, DOMAIN
+from custom_components.localthings.registry.entities import SensorDesc
+
+SERIAL = "TEST-SERIAL-0000"
+RECORDED = [11.0, 9.0, 14.0]
+
+
+async def _seed_unitless_statistics(hass: HomeAssistant, entity_id: str) -> None:
+    """Record hourly statistics the way these sensors always have: numeric
+    means, no unit of measurement at all."""
+    start = dt_util.utcnow().replace(minute=0, second=0, microsecond=0) - timedelta(hours=4)
+    async_import_statistics(
+        hass,
+        {
+            "mean_type": StatisticMeanType.ARITHMETIC,
+            "has_sum": False,
+            "name": None,
+            "source": "recorder",
+            "statistic_id": entity_id,
+            "unit_class": None,
+            "unit_of_measurement": None,
+        },
+        [
+            {"start": start + timedelta(hours=i), "mean": value, "min": value, "max": value}
+            for i, value in enumerate(RECORDED)
+        ],
+    )
+    await async_wait_recording_done(hass)
+
+
+async def _metadata(hass: HomeAssistant, entity_id: str) -> StatisticMetaData:
+    result = await get_instance(hass).async_add_executor_job(
+        partial(get_metadata, hass, statistic_ids={entity_id})
+    )
+    return result[entity_id][1]
+
+
+async def _means(hass: HomeAssistant, entity_id: str) -> list[float]:
+    rows = await get_instance(hass).async_add_executor_job(
+        statistics_during_period,
+        hass,
+        dt_util.utcnow() - timedelta(days=1),
+        None,
+        {entity_id},
+        "hour",
+        None,
+        {"mean"},
+    )
+    return [cast(float, row["mean"]) for row in rows.get(entity_id, [])]
+
+
+@pytest.fixture
+def purifier_entry(hass: HomeAssistant) -> MockConfigEntry:
+    entry = MockConfigEntry(
+        domain=DOMAIN,
+        data={CONF_DEVICE_TYPE: "air_purifier"},
+        unique_id=f"{DOMAIN}_{SERIAL}",
+        version=2,
+    )
+    entry.add_to_hass(hass)
+    return entry
+
+
+def _dust_entity(hass: HomeAssistant, entry: MockConfigEntry):
+    return er.async_get(hass).async_get_or_create(
+        "sensor", DOMAIN, f"{DOMAIN}_{SERIAL}_dust", config_entry=entry
+    )
+
+
+async def test_relabels_metadata_without_touching_recorded_values(
+    recorder_mock, hass: HomeAssistant, purifier_entry: MockConfigEntry
+) -> None:
+    dust = _dust_entity(hass, purifier_entry)
+    await _seed_unitless_statistics(hass, dust.entity_id)
+
+    before = await _metadata(hass, dust.entity_id)
+    assert before["unit_of_measurement"] is None
+    assert await _means(hass, dust.entity_id) == RECORDED
+
+    assert await async_migrate_entry(hass, purifier_entry) is True
+    await async_wait_recording_done(hass)
+
+    after = await _metadata(hass, dust.entity_id)
+    assert after["unit_of_measurement"] == CONCENTRATION_MICROGRAMS_PER_CUBIC_METER
+    assert after["unit_class"] == "concentration"
+    # The readings were always µg/m³; only the label was missing. Nothing is
+    # converted, so the recorded history still says exactly what it said.
+    assert await _means(hass, dust.entity_id) == RECORDED
+
+
+async def test_recorded_unit_ends_up_matching_what_the_descriptor_declares(
+    recorder_mock, hass: HomeAssistant, purifier_entry: MockConfigEntry
+) -> None:
+    """The invariant the migration exists to establish, stated directly.
+
+    HA raises units_changed -- and suppresses statistics generation -- when
+    an entity's unit disagrees with the unit recorded against its
+    statistic_id. Rather than drive HA's validation to observe that, this
+    asserts the condition that validation reads: after migrating, the
+    recorded metadata says exactly what the descriptor says. Asserting our
+    own invariant instead of Home Assistant's reaction to it keeps the test
+    off internals that change between versions (both `_update_issues` and
+    `validate_statistics` have gained parameters), and tests this repo
+    rather than that one."""
+    from custom_components.localthings.registry.capabilities import air_purifier
+
+    dust = _dust_entity(hass, purifier_entry)
+    await _seed_unitless_statistics(hass, dust.entity_id)
+
+    desc = next(
+        d
+        for d in air_purifier.AIR_QUALITY.entities
+        if d.key == "dust" and isinstance(d, SensorDesc)
+    )
+    assert (await _metadata(hass, dust.entity_id))["unit_of_measurement"] != desc.unit
+
+    assert await async_migrate_entry(hass, purifier_entry) is True
+    await async_wait_recording_done(hass)
+
+    assert (await _metadata(hass, dust.entity_id))["unit_of_measurement"] == desc.unit