Always create a sample size of 1 no matter how small the scale.

sdv/sampling/hierarchical_sampler.py

@@ -1,5 +1,6 @@
 """Hierarchical Samplers."""
 import logging
+import warnings
 
 import pandas as pd
 
@@ -273,13 +274,24 @@ def _sample(self, scale=1.0):
         # DFS to sample roots and then their children
         non_root_parents = set(self.metadata._get_parent_map().keys())
         root_parents = set(self.metadata.tables.keys()) - non_root_parents
+        send_min_sample_warning = False
         for table in root_parents:
             num_rows = round(self._table_sizes[table] * scale)
+            if num_rows <= 0:
+                send_min_sample_warning = True
+                num_rows = 1
             synthesizer = self._table_synthesizers[table]
             LOGGER.info(f'Sampling {num_rows} rows from table {table}')
             sampled_data[table] = self._sample_rows(synthesizer, num_rows)
             self._sample_children(table_name=table, sampled_data=sampled_data, scale=scale)
 
+        if send_min_sample_warning:
+            warn_msg = (
+                "The 'scale' parameter it too small. Some tables may have 1 row."
+                ' For better quality data, please choose a larger scale.'
+            )
+            warnings.warn(warn_msg)
+
         added_relationships = set()
         for relationship in self.metadata.relationships:
             parent_name = relationship['parent_table_name']

tests/integration/multi_table/test_hma.py

@@ -1839,3 +1839,27 @@ def test_disjointed_tables():
     # Assert
     for table in real_data:
         assert list(real_data[table].columns) == list(disjoin_synthetic_data[table].columns)
+
+
+def test_small_sample():
+    """Test that the sample function still works with a small scale"""
+    # Setup
+    data, metadata = download_demo(
+        modality='multi_table',
+        dataset_name='fake_hotels'
+    )
+    synthesizer = HMASynthesizer(metadata)
+    synthesizer.fit(data)
+
+    # Run and Assert
+    warn_msg = re.escape(
+        "The 'scale' parameter it too small. Some tables may have 1 row."
+        ' For better quality data, please choose a larger scale.'
+    )
+    with pytest.warns(Warning, match=warn_msg):
+        synthetic_data = synthesizer.sample(scale=0.01)
+
+    assert (len(synthetic_data['hotels']) == 1)
+    assert (len(synthetic_data['guests']) >= len(data['guests']) * .01)
+    assert synthetic_data['hotels'].columns.tolist() == data['hotels'].columns.tolist()
+    assert synthetic_data['guests'].columns.tolist() == data['guests'].columns.tolist()