feat: Developers can now more easily override the mapping from SQL column type to JSON schema

docs/classes/singer_sdk.connectors.sql.SQLToJSONSchema.rst

@@ -0,0 +1,8 @@
+singer_sdk.connectors.sql.SQLToJSONSchema
+=========================================
+
+.. currentmodule:: singer_sdk.connectors.sql
+
+.. autoclass:: SQLToJSONSchema
+    :members:
+    :special-members: __init__, __call__

docs/conf.py

@@ -158,6 +158,10 @@
         "https://json-schema.org/understanding-json-schema/reference/%s",
         "%s",
     ),
+    "column_type": (
+        "https://docs.sqlalchemy.org/en/20/core/type_basics.html#sqlalchemy.types.%s",
+        "%s",
+    ),
 }
 
 # -- Options for intersphinx -----------------------------------------------------------

docs/guides/index.md

@@ -9,4 +9,5 @@ porting
 pagination-classes
 custom-clis
 config-schema
+sql-tap
 ```

docs/guides/sql-tap.md

@@ -0,0 +1,54 @@
+# Building SQL taps
+
+## Default type mapping
+
+The Singer SDK automatically handles the most common SQLAlchemy column types, using [`functools.singledispatchmethod`](inv:python:py:class:#functools.singledispatchmethod) to process each type. See the [`SQLToJSONSchema`](connectors.sql.SQLToJSONSchema) reference documentation for details.
+
+## Custom type mapping
+
+If the class above doesn't cover all the types supported by the SQLAlchemy dialect in your tap, you can subclass it and override or extend with a new method for the type you need to support:
+
+```python
+import functools
+
+from sqlalchemy import Numeric
+from singer_sdk import typing as th
+from singer_sdk.connectors import SQLConnector
+from singer_sdk.connectors.sql import SQLToJSONSchema
+
+from my_sqlalchemy_dialect import VectorType
+
+
+class CustomSQLToJSONSchema(SQLToJSONSchema):
+    @SQLToJSONSchema.to_jsonschema.register
+    def custom_number_to_jsonschema(self, column_type: Numeric):
+        """Override the default mapping for NUMERIC columns.
+
+        For example, a scale of 4 translates to a multipleOf 0.0001.
+        """
+        return {"type": ["number"], "multipleOf": 10**-column_type.scale}
+
+    @SQLToJSONSchema.to_jsonschema.register(VectorType)
+    def vector_to_json_schema(self, column_type):
+        """Custom vector to JSON schema."""
+        return th.ArrayType(th.NumberType()).to_dict()
+```
+
+````{tip}
+You can also use a type annotation to specify the type of the column when registering a new method:
+
+```python
+@SQLToJSONSchema.to_jsonschema.register
+def vector_to_json_schema(self, column_type: VectorType):
+    return th.ArrayType(th.NumberType()).to_dict()
+```
+````
+
+Then, you need to use your custom type mapping in your connector:
+
+```python
+class MyConnector(SQLConnector):
+    @functools.cached_property
+    def type_mapping(self):
+        return CustomSQLToJSONSchema()
+```

docs/reference.rst

@@ -143,3 +143,12 @@ Batch
 
     batch.BaseBatcher
     batch.JSONLinesBatcher
+
+Other
+-----
+
+.. autosummary::
+    :toctree: classes
+    :template: class.rst
+
+    connectors.sql.SQLToJSONSchema

singer_sdk/connectors/sql.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import functools
 import logging
 import sys
 import typing as t
@@ -109,6 +110,83 @@ def prepare_part(self, part: str) -> str:  # noqa: PLR6301
         return part
 
 
+class SQLToJSONSchema:
+    """SQLAlchemy to JSON Schema type mapping helper.
+
+    This class provides a mapping from SQLAlchemy types to JSON Schema types.
+    """
+
+    @functools.singledispatchmethod
+    def to_jsonschema(self, column_type: sa.types.TypeEngine) -> dict:  # noqa: ARG002, D102, PLR6301
+        return th.StringType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def datetime_to_jsonschema(self, column_type: sa.types.DateTime) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic datetime type.
+
+        Args:
+            column_type (:column_type:`DateTime`): The column type.
+        """
+        return th.DateTimeType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def date_to_jsonschema(self, column_type: sa.types.Date) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a date type.
+
+        Args:
+            column_type (:column_type:`Date`): The column type.
+        """
+        return th.DateType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def time_to_jsonschema(self, column_type: sa.types.Time) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a time type.
+
+        Args:
+            column_type (:column_type:`Time`): The column type.
+        """
+        return th.TimeType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def integer_to_jsonschema(self, column_type: sa.types.Integer) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a an integer type.
+
+        Args:
+            column_type (:column_type:`Integer`): The column type.
+        """
+        return th.IntegerType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def float_to_jsonschema(self, column_type: sa.types.Numeric) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic number type.
+
+        Args:
+            column_type (:column_type:`Numeric`): The column type.
+        """
+        return th.NumberType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def string_to_jsonschema(self, column_type: sa.types.String) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a generic string type.
+
+        Args:
+            column_type (:column_type:`String`): The column type.
+        """
+        # TODO: Enable support for maxLength.
+        # if sa_type.length:
+        #     return StringType(max_length=sa_type.length).type_dict  # noqa: ERA001
+        return th.StringType.type_dict  # type: ignore[no-any-return]
+
+    @to_jsonschema.register
+    def boolean_to_jsonschema(self, column_type: sa.types.Boolean) -> dict:  # noqa: ARG002, PLR6301
+        """Return a JSON Schema representation of a boolean type.
+
+        Args:
+            column_type (:column_type:`Boolean`): The column type.
+        """
+        return th.BooleanType.type_dict  # type: ignore[no-any-return]
+
+
 class SQLConnector:  # noqa: PLR0904
     """Base class for SQLAlchemy-based connectors.
 
@@ -162,6 +240,17 @@ def logger(self) -> logging.Logger:
         """
         return logging.getLogger("sqlconnector")
 
+    @functools.cached_property
+    def type_mapping(self) -> SQLToJSONSchema:
+        """Return the type mapper object.
+
+        Override this method to provide a custom mapping for your SQL dialect.
+
+        Returns:
+            The type mapper object.
+        """
+        return SQLToJSONSchema()
+
     @contextmanager
     def _connect(self) -> t.Iterator[sa.engine.Connection]:
         with self._engine.connect().execution_options(stream_results=True) as conn:
@@ -266,8 +355,8 @@ def get_sqlalchemy_url(self, config: dict[str, t.Any]) -> str:  # noqa: PLR6301
 
         return t.cast(str, config["sqlalchemy_url"])
 
-    @staticmethod
     def to_jsonschema_type(
+        self,
         sql_type: (
             str  # noqa: ANN401
             | sa.types.TypeEngine
@@ -293,10 +382,25 @@ def to_jsonschema_type(
         Returns:
             The JSON Schema representation of the provided type.
         """
-        if isinstance(sql_type, (str, sa.types.TypeEngine)):
+        if isinstance(sql_type, sa.types.TypeEngine):
+            return self.type_mapping.to_jsonschema(sql_type)
+
+        if isinstance(sql_type, str):  # pragma: no cover
+            warnings.warn(
+                "Passing string types to `to_jsonschema_type` is deprecated. "
+                "Please pass a SQLAlchemy type object instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
             return th.to_jsonschema_type(sql_type)
 
-        if isinstance(sql_type, type):
+        if isinstance(sql_type, type):  # pragma: no cover
+            warnings.warn(
+                "Passing type classes to `to_jsonschema_type` is deprecated. "
+                "Please pass a SQLAlchemy type object instead.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
             if issubclass(sql_type, sa.types.TypeEngine):
                 return th.to_jsonschema_type(sql_type)
 

singer_sdk/typing.py

@@ -53,6 +53,7 @@
 from __future__ import annotations
 
 import json
+import sys
 import typing as t
 
 import sqlalchemy as sa
@@ -65,9 +66,13 @@
     get_datelike_property_type,
 )
 
-if t.TYPE_CHECKING:
-    import sys
+if sys.version_info < (3, 13):
+    from typing_extensions import deprecated
+else:
+    from typing import deprecated  # noqa: ICN003 # pragma: no cover
+
 
+if t.TYPE_CHECKING:
     from jsonschema.protocols import Validator
 
     if sys.version_info >= (3, 10):
@@ -1086,6 +1091,10 @@ def __iter__(self) -> t.Iterator[Property]:
         return self.wrapped.values().__iter__()
 
 
+@deprecated(
+    "Use `SQLToJSONSchema` instead.",
+    category=DeprecationWarning,
+)
 def to_jsonschema_type(
     from_type: str | sa.types.TypeEngine | type[sa.types.TypeEngine],
 ) -> dict:
@@ -1119,9 +1128,9 @@ def to_jsonschema_type(
         "bool": BooleanType.type_dict,
         "variant": StringType.type_dict,
     }
-    if isinstance(from_type, str):
+    if isinstance(from_type, str):  # pragma: no cover
         type_name = from_type
-    elif isinstance(from_type, sa.types.TypeEngine):
+    elif isinstance(from_type, sa.types.TypeEngine):  # pragma: no cover
         type_name = type(from_type).__name__
     elif issubclass(from_type, sa.types.TypeEngine):
         type_name = from_type.__name__

tests/core/test_connector_sql.py

@@ -11,7 +11,7 @@
 
 from samples.sample_duckdb import DuckDBConnector
 from singer_sdk.connectors import SQLConnector
-from singer_sdk.connectors.sql import FullyQualifiedName
+from singer_sdk.connectors.sql import FullyQualifiedName, SQLToJSONSchema
 from singer_sdk.exceptions import ConfigValidationError
 
 if t.TYPE_CHECKING:
@@ -22,6 +22,10 @@ def stringify(in_dict):
     return {k: str(v) for k, v in in_dict.items()}
 
 
+class MyType(sa.types.TypeDecorator):
+    impl = sa.types.LargeBinary
+
+
 class TestConnectorSQL:  # noqa: PLR0904
     """Test the SQLConnector class."""
 
@@ -392,3 +396,82 @@ def prepare_part(self, part: str) -> str:
 def test_fully_qualified_name_empty_error():
     with pytest.raises(ValueError, match="Could not generate fully qualified name"):
         FullyQualifiedName()
+
+
+@pytest.mark.parametrize(
+    "sql_type, expected_jsonschema_type",
+    [
+        pytest.param(sa.types.VARCHAR(), {"type": ["string"]}, id="varchar"),
+        pytest.param(
+            sa.types.VARCHAR(length=127),
+            {"type": ["string"], "maxLength": 127},
+            marks=pytest.mark.xfail,
+            id="varchar-length",
+        ),
+        pytest.param(sa.types.TEXT(), {"type": ["string"]}, id="text"),
+        pytest.param(sa.types.INTEGER(), {"type": ["integer"]}, id="integer"),
+        pytest.param(sa.types.BOOLEAN(), {"type": ["boolean"]}, id="boolean"),
+        pytest.param(sa.types.DECIMAL(), {"type": ["number"]}, id="decimal"),
+        pytest.param(sa.types.FLOAT(), {"type": ["number"]}, id="float"),
+        pytest.param(sa.types.REAL(), {"type": ["number"]}, id="real"),
+        pytest.param(sa.types.NUMERIC(), {"type": ["number"]}, id="numeric"),
+        pytest.param(
+            sa.types.DATE(),
+            {"type": ["string"], "format": "date"},
+            id="date",
+        ),
+        pytest.param(
+            sa.types.DATETIME(),
+            {"type": ["string"], "format": "date-time"},
+            id="datetime",
+        ),
+        pytest.param(
+            sa.types.TIMESTAMP(),
+            {"type": ["string"], "format": "date-time"},
+            id="timestamp",
+        ),
+        pytest.param(
+            sa.types.TIME(),
+            {"type": ["string"], "format": "time"},
+            id="time",
+        ),
+        pytest.param(
+            sa.types.BLOB(),
+            {"type": ["string"]},
+            id="unknown",
+        ),
+    ],
+)
+def test_sql_to_json_schema_map(
+    sql_type: sa.types.TypeEngine,
+    expected_jsonschema_type: dict,
+):
+    m = SQLToJSONSchema()
+    assert m.to_jsonschema(sql_type) == expected_jsonschema_type
+
+
+def test_custom_type():
+    class MyMap(SQLToJSONSchema):
+        @SQLToJSONSchema.to_jsonschema.register
+        def custom_number_to_jsonschema(self, column_type: sa.types.NUMERIC) -> dict:
+            """Custom number to JSON schema.
+
+            For example, a scale of 4 translates to a multipleOf 0.0001.
+            """
+            return {"type": ["number"], "multipleOf": 10**-column_type.scale}
+
+        @SQLToJSONSchema.to_jsonschema.register(MyType)
+        def my_type_to_jsonschema(self, column_type) -> dict:  # noqa: ARG002
+            return {"type": ["string"], "contentEncoding": "base64"}
+
+    m = MyMap()
+
+    assert m.to_jsonschema(MyType()) == {
+        "type": ["string"],
+        "contentEncoding": "base64",
+    }
+    assert m.to_jsonschema(sa.types.NUMERIC(scale=2)) == {
+        "type": ["number"],
+        "multipleOf": 0.01,
+    }
+    assert m.to_jsonschema(sa.types.BOOLEAN()) == {"type": ["boolean"]}