Only load last observation of map data by default

ax/core/observation.py

@@ -511,7 +511,7 @@ def observations_from_data(
         statuses_to_include_map_metric = NON_ABANDONED_STATUSES
     is_map_data = isinstance(data, MapData)
     map_keys = []
-    if is_map_data:
+    if is_map_data and len(assert_is_instance(data, MapData).map_df) > 0:
         data = assert_is_instance(data, MapData)
         map_keys.extend(data.map_keys)
         if latest_rows_per_group is not None:

ax/modelbridge/base.py

@@ -112,6 +112,7 @@ def __init__(
         fit_tracking_metrics: bool = True,
         fit_on_init: bool = True,
         fit_only_completed_map_metrics: bool = True,
+        latest_rows_per_group: int | None = 1,
     ) -> None:
         """
         Applies transforms and fits model.
@@ -159,6 +160,11 @@ def __init__(
             fit_only_completed_map_metrics: Whether to fit a model to map metrics only
                 when the trial is completed. This is useful for applications like
                 modeling partially completed learning curves in AutoML.
+            latest_rows_per_group: If specified and data is an instance of MapData,
+                uses MapData.latest() with `rows_per_group=latest_rows_per_group` to
+                retrieve the most recent rows for each group. Useful in cases where
+                learning curves are frequently updated, preventing an excessive
+                number of Observation objects.
         """
         t_fit_start = time.monotonic()
         transforms = transforms or []
@@ -188,6 +194,7 @@ def __init__(
         self._fit_abandoned = fit_abandoned
         self._fit_tracking_metrics = fit_tracking_metrics
         self._fit_only_completed_map_metrics = fit_only_completed_map_metrics
+        self._latest_rows_per_group = latest_rows_per_group
         self.outcomes: list[str] = []
         self._experiment_has_immutable_search_space_and_opt_config: bool = (
             experiment is not None and experiment.immutable_search_space_and_opt_config
@@ -302,7 +309,7 @@ def _prepare_observations(
         return observations_from_data(
             experiment=experiment,
             data=data,
-            latest_rows_per_group=None,
+            latest_rows_per_group=self._latest_rows_per_group,
             statuses_to_include=self.statuses_to_fit,
             statuses_to_include_map_metric=self.statuses_to_fit_map_metric,
             map_keys_as_parameters=map_keys_as_parameters,

ax/modelbridge/tests/test_base_modelbridge.py

@@ -1062,8 +1062,8 @@ def test_fit_only_completed_map_metrics(
         )
         _, kwargs = mock_observations_from_data.call_args
         self.assertTrue(kwargs["map_keys_as_parameters"])
-        # assert `latest_rows_per_group` is not specified or is None
-        self.assertIsNone(kwargs.get("latest_rows_per_group"))
+        # assert `latest_rows_per_group` is not specified or is 1
+        self.assertEqual(kwargs.get("latest_rows_per_group"), 1)
         mock_observations_from_data.reset_mock()
 
         # calling without map data calls observations_from_data with

ax/modelbridge/torch.py

@@ -120,6 +120,7 @@ def __init__(
         fit_on_init: bool = True,
         default_model_gen_options: TConfig | None = None,
         fit_only_completed_map_metrics: bool = True,
+        latest_rows_per_group: int | None = 1,
     ) -> None:
         # This warning is being added while we are on 0.4.3, so it will be
         # released in 0.4.4 or 0.5.0. The `torch_dtype` argument can be removed
@@ -163,6 +164,7 @@ def __init__(
             fit_tracking_metrics=fit_tracking_metrics,
             fit_on_init=fit_on_init,
             fit_only_completed_map_metrics=fit_only_completed_map_metrics,
+            latest_rows_per_group=latest_rows_per_group,
         )
 
     def feature_importances(self, metric_name: str) -> dict[str, float]: