fix: unblock generation when no from-scratch-generator is configured

src/data_designer/engine/compiler.py

@@ -5,9 +5,10 @@
 
 import logging
 
-from data_designer.config.column_configs import SeedDatasetColumnConfig
+from data_designer.config.column_configs import SamplerColumnConfig, SeedDatasetColumnConfig
 from data_designer.config.data_designer_config import DataDesignerConfig
 from data_designer.config.errors import InvalidConfigError
+from data_designer.config.sampler_params import UUIDSamplerParams
 from data_designer.engine.resources.resource_provider import ResourceProvider
 from data_designer.engine.resources.seed_reader import SeedReader
 from data_designer.engine.validation import ViolationLevel, rich_print_violations, validate_data_designer_config
@@ -17,6 +18,7 @@
 
 def compile_data_designer_config(config: DataDesignerConfig, resource_provider: ResourceProvider) -> DataDesignerConfig:
     _resolve_and_add_seed_columns(config, resource_provider.seed_reader)
+    _add_internal_row_id_column_if_needed(config)
     _validate(config)
     return config
 
@@ -41,6 +43,35 @@ def _resolve_and_add_seed_columns(config: DataDesignerConfig, seed_reader: SeedR
     config.columns.extend([SeedDatasetColumnConfig(name=col_name) for col_name in seed_col_names])
 
 
+def _add_internal_row_id_column_if_needed(config: DataDesignerConfig) -> None:
+    """Adds a UUID sampler column named '_internal_row_id' (set to drop) if needed to enable generation.
+
+    Generation requires either:
+    - At least one sampler column (which can generate data from scratch), OR
+    - A seed dataset (which provides initial data rows)
+
+    If neither exists, a UUID sampler column '_internal_row_id' is automatically added and marked for drop
+    to enable the generation process to start.
+
+    Args:
+        config: The DataDesigner configuration to potentially modify.
+    """
+    has_sampler_column = any(isinstance(col, SamplerColumnConfig) for col in config.columns)
+    has_seed_dataset_column = any(isinstance(col, SeedDatasetColumnConfig) for col in config.columns)
+
+    if not has_sampler_column and not has_seed_dataset_column:
+        logger.warning(
+            "🔔 No sampler column or seed dataset detected. Adding UUID column '_internal_row_id' (marked for drop) to enable generation."
+        )
+        id_column = SamplerColumnConfig(
+            name="_internal_row_id",
+            sampler_type="uuid",
+            params=UUIDSamplerParams(),
+            drop=True,
+        )
+        config.columns.insert(0, id_column)
+
+
 def _validate(config: DataDesignerConfig) -> None:
     allowed_references = _get_allowed_references(config)
     violations = validate_data_designer_config(

tests/engine/dataset_builders/test_column_wise_builder.py

@@ -13,6 +13,7 @@
 from data_designer.config.dataset_builders import BuildStage
 from data_designer.config.processors import DropColumnsProcessorConfig
 from data_designer.config.run_config import RunConfig
+from data_designer.config.sampler_params import SamplerType, UUIDSamplerParams
 from data_designer.engine.column_generators.generators.base import GenerationStrategy
 from data_designer.engine.dataset_builders.column_wise_builder import (
     MAX_CONCURRENCY_PER_NON_LLM_GENERATOR,
@@ -31,6 +32,7 @@
 @pytest.fixture
 def stub_test_column_configs():
     return [
+        SamplerColumnConfig(name="some_id", sampler_type=SamplerType.UUID, params=UUIDSamplerParams()),
         LLMTextColumnConfig(name="test_column", prompt="Test prompt", model_alias="test_model"),
         LLMTextColumnConfig(name="column_to_drop", prompt="Test prompt", model_alias="test_model"),
     ]
@@ -92,8 +94,7 @@ def test_column_wise_dataset_builder_creation(stub_resource_provider, stub_test_
         data_designer_config=stub_test_config_builder.build(),
         resource_provider=stub_resource_provider,
     )
-
-    assert len(builder._column_configs) == 2
+    assert len(builder._column_configs) == 3
     assert builder._resource_provider == stub_resource_provider
     assert isinstance(builder._registry, DataDesignerRegistry)
 
@@ -136,26 +137,33 @@ def test_column_wise_dataset_builder_batch_manager_initialization(stub_column_wi
 def test_column_wise_dataset_builder_single_column_configs_property(
     stub_resource_provider, stub_model_configs, config_type, expected_single_configs
 ):
+    config_builder = DataDesignerConfigBuilder(model_configs=stub_model_configs)
+
     if config_type == "single":
-        single_config = LLMTextColumnConfig(name="test_column", prompt="Test prompt", model_alias="test_model")
-        config_builder = DataDesignerConfigBuilder(model_configs=stub_model_configs)
+        # Add an LLM text column - these don't get grouped into MultiColumnConfigs
+        single_config = expected_single_configs[0]
         config_builder.add_column(single_config)
+
         builder = ColumnWiseDatasetBuilder(
             data_designer_config=config_builder.build(),
             resource_provider=stub_resource_provider,
         )
-        assert builder.single_column_configs == [single_config]
+
+        # Since there's no sampler, _internal_row_id is auto-added, plus the LLM column
+        configs = builder.single_column_configs
+        assert len(configs) == 2
+        assert configs[0].name == "_internal_row_id"
+        assert configs[1] == single_config
+
     else:
-        sampler_config = SamplerColumnConfig(
-            name="sampler_col", sampler_type="category", params={"values": ["A", "B", "C"]}
-        )
-        config_builder = DataDesignerConfigBuilder(model_configs=stub_model_configs)
+        sampler_config = expected_single_configs[0]
         config_builder.add_column(sampler_config)
+
         builder = ColumnWiseDatasetBuilder(
             data_designer_config=config_builder.build(),
             resource_provider=stub_resource_provider,
         )
-        assert builder.single_column_configs == [sampler_config]
+        assert builder.single_column_configs == expected_single_configs
 
 
 def test_column_wise_dataset_builder_build_method_basic_flow(

tests/engine/test_compiler.py

@@ -5,10 +5,10 @@
 
 import pytest
 
-from data_designer.config.column_configs import SamplerColumnConfig
+from data_designer.config.column_configs import ExpressionColumnConfig, SamplerColumnConfig
 from data_designer.config.config_builder import DataDesignerConfigBuilder
 from data_designer.config.errors import InvalidConfigError
-from data_designer.config.sampler_params import CategorySamplerParams, SamplerType
+from data_designer.config.sampler_params import CategorySamplerParams, SamplerType, UUIDSamplerParams
 from data_designer.config.seed_source import HuggingFaceSeedSource
 from data_designer.engine.compiler import compile_data_designer_config
 from data_designer.engine.resources.resource_provider import ResourceProvider
@@ -78,3 +78,69 @@ def test_validation_errors(resource_provider: ResourceProvider):
             compile_data_designer_config(builder.build(), resource_provider)
 
     assert "validation errors" in str(excinfo)
+
+
+def test_adds_id_column_when_no_sampler_and_no_seed_dataset(stub_resource_provider: ResourceProvider):
+    """Test that a UUID '_internal_row_id' column is automatically added when there's no sampler column or seed dataset."""
+    builder = DataDesignerConfigBuilder()
+    builder.add_column(
+        ExpressionColumnConfig(
+            name="derived_value",
+            expr="'constant_value'",
+        )
+    )
+    stub_resource_provider.seed_reader = None
+
+    config = compile_data_designer_config(builder.build(), stub_resource_provider)
+
+    assert len(config.columns) == 2
+    assert config.columns[0].name == "_internal_row_id"
+    assert isinstance(config.columns[0], SamplerColumnConfig)
+    assert config.columns[0].sampler_type == "uuid"
+    assert isinstance(config.columns[0].params, UUIDSamplerParams)
+    assert config.columns[0].drop is True
+
+
+def test_does_not_add_id_column_when_sampler_exists(stub_resource_provider: ResourceProvider):
+    """Test that no '_internal_row_id' column is added when a sampler column already exists."""
+    builder = DataDesignerConfigBuilder()
+    builder.add_column(
+        SamplerColumnConfig(
+            name="category",
+            sampler_type=SamplerType.CATEGORY,
+            params=CategorySamplerParams(values=["a", "b", "c"]),
+        )
+    )
+    builder.add_column(
+        ExpressionColumnConfig(
+            name="derived_value",
+            expr="{{ category }}_suffix",
+        )
+    )
+    stub_resource_provider.seed_reader = None
+
+    config = compile_data_designer_config(builder.build(), stub_resource_provider)
+
+    assert len(config.columns) == 2
+    assert config.columns[0].name == "category"
+    assert config.columns[1].name == "derived_value"
+    assert not any(col.name == "_internal_row_id" for col in config.columns)
+
+
+def test_does_not_add_id_column_when_seed_dataset_exists(resource_provider: ResourceProvider):
+    """Test that no '_internal_row_id' column is added when a seed dataset is configured."""
+    builder = DataDesignerConfigBuilder()
+    builder.add_column(
+        ExpressionColumnConfig(
+            name="derived_value",
+            expr="{{ city }}_derived",
+        )
+    )
+    builder.with_seed_dataset(HuggingFaceSeedSource(path="hf://datasets/test/data.csv"))
+
+    config = compile_data_designer_config(builder.build(), resource_provider)
+
+    # Should have the expression column + 2 seed columns (city, country) from the fixture
+    assert len(config.columns) == 3
+    assert config.columns[0].name == "derived_value"
+    assert not any(col.name == "_internal_row_id" for col in config.columns)