fix(python): coerce blob URI inputs before Arrow cast

python/python/lancedb/table.py

@@ -563,12 +563,7 @@ def _coerce_blob_list_values(
 
 
 def _coerce_value_to_blob(values: pa.Array, target_field: pa.Field) -> pa.Array:
-    if pa.types.is_null(values.type):
-        data = pa.nulls(len(values), type=pa.large_binary())
-    elif pa.types.is_large_binary(values.type):
-        data = values
-    else:
-        data = values.cast(pa.large_binary())
+    value_field = "uri" if _is_string_like(values.type) else "data"
     length = len(values)
     storage_type = target_field.type
     if isinstance(storage_type, pa.ExtensionType):
@@ -576,8 +571,11 @@ def _coerce_value_to_blob(values: pa.Array, target_field: pa.Field) -> pa.Array:
     storage_fields = list(storage_type)
     children = []
     for storage_field in storage_fields:
-        if storage_field.name == "data":
-            children.append(data)
+        if storage_field.name == value_field and not pa.types.is_null(values.type):
+            if values.type == storage_field.type:
+                children.append(values)
+            else:
+                children.append(values.cast(storage_field.type))
         else:
             children.append(pa.nulls(length, type=storage_field.type))
     storage = pa.StructArray.from_arrays(
@@ -597,7 +595,19 @@ def _physical_array_and_type(array: pa.Array) -> tuple[pa.Array, pa.DataType]:
 
 
 def _can_coerce_to_blob(data_type: pa.DataType) -> bool:
-    return _is_binary_like(data_type) or pa.types.is_null(data_type)
+    return (
+        _is_binary_like(data_type)
+        or _is_string_like(data_type)
+        or pa.types.is_null(data_type)
+    )
+
+
+def _is_string_like(data_type: pa.DataType) -> bool:
+    return (
+        pa.types.is_string(data_type)
+        or pa.types.is_large_string(data_type)
+        or pa.types.is_string_view(data_type)
+    )
 
 
 def _is_binary_like(data_type: pa.DataType) -> bool:
@@ -759,6 +769,24 @@ def _arrow_json_storage_type(input_type: pa.DataType) -> Optional[pa.DataType]:
 
 
 def _align_field(field: pa.Field, target_field: pa.Field) -> pa.Field:
+    if is_blob_v2_field(target_field):
+        source_type = field.type
+        if pa.types.is_struct(source_type):
+            if not any(child.name in ("data", "uri") for child in source_type):
+                raise ValueError(
+                    f"blob struct input for column '{field.name}' must contain "
+                    "a 'data' or 'uri' child"
+                )
+        elif not _can_coerce_to_blob(source_type) and not isinstance(
+            source_type, pa.ExtensionType
+        ):
+            raise ValueError(
+                f"cannot coerce column '{field.name}' with type {source_type} "
+                "into a blob v2 struct. expected binary bytes "
+                "(Binary, LargeBinary, BinaryView), strings "
+                "(Utf8, LargeUtf8, Utf8View), or a Struct with a "
+                "'data' or 'uri' child"
+            )
     # LanceDB exposes stored JSON columns as lance.json (JSONB-backed LargeBinary), but
     # casting the input to that storage type here merely relabels the raw JSON bytes as
     # JSONB. Lance must see arrow.json so it can perform the JSONB encoding.

python/python/tests/test_blob.py

@@ -434,6 +434,50 @@ def test_merge_insert_writes_python_bytes():
     assert blobs.to_pylist() == [b"updated", b"inserted"]
 
 
+@pytest.mark.parametrize("write", ["merge_insert", "create_table"])
+def test_uri_string_blob_write_reaches_external_base_validation(tmp_path, write):
+    blob_path = tmp_path / "payload.bin"
+    blob_path.write_bytes(b"payload")
+    schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])
+    rows = pa.table(
+        {"id": pa.array([1], pa.int64()), "image": pa.array([blob_path.as_uri()])}
+    )
+    db = lancedb.connect(tmp_path)
+
+    # These APIs do not expose add's outside-base opt-in. The URI should reach
+    # the blob policy check instead of failing at PyArrow's string-to-struct cast.
+    with pytest.raises(ValueError, match="outside registered external bases"):
+        if write == "merge_insert":
+            table = db.create_table("merge_uri", schema=schema)
+            table.merge_insert("id").when_not_matched_insert_all().execute(rows)
+        else:
+            db.create_table("create_uri", data=rows, schema=schema)
+
+
+@pytest.mark.parametrize("write", ["merge_insert", "create_table"])
+@pytest.mark.parametrize(
+    "source_type, value",
+    [
+        (pa.binary(4), b"abcd"),
+        (pa.list_(pa.uint8()), [1, 2]),
+        (pa.int64(), 42),
+    ],
+)
+def test_blob_write_rejects_unsupported_input(write, source_type, value):
+    schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])
+    rows = pa.table(
+        {"id": pa.array([1], pa.int64()), "image": pa.array([value], source_type)}
+    )
+    db = lancedb.connect("memory:///")
+
+    with pytest.raises(ValueError, match="cannot coerce column 'image' with type"):
+        if write == "merge_insert":
+            table = db.create_table("merge_invalid", schema=schema)
+            table.merge_insert("id").when_not_matched_insert_all().execute(rows)
+        else:
+            db.create_table("create_invalid", data=rows, schema=schema)
+
+
 def test_merge_insert_bytes_after_reopen_without_touching_blob_type(tmp_path):
     db = lancedb.connect(tmp_path)
     schema = pa.schema([pa.field("id", pa.int64()), lancedb.blob("image")])

python/python/tests/test_util.py

@@ -922,6 +922,39 @@ def test_cast_to_target_schema_coerces_binary_to_blob_v2():
     ]
 
 
+@pytest.mark.parametrize(
+    "string_type", [pa.string(), pa.large_string(), pa.string_view()]
+)
+def test_cast_to_target_schema_coerces_string_to_blob_v2_uri(string_type):
+    data = pa.table({"image": pa.array(["file:///payload", None], type=string_type)})
+    target = pa.schema([lancedb.blob("image")])
+
+    output = _cast_to_target_schema(data.to_reader(), target).read_all()
+
+    image = output["image"].chunk(0)
+    assert type(image.type) is lancedb.BlobType
+    assert image.storage.to_pylist() == [
+        {"data": None, "uri": "file:///payload", "position": None, "size": None},
+        None,
+    ]
+
+
+@pytest.mark.parametrize(
+    "source_type, value",
+    [
+        (pa.binary(4), b"abcd"),
+        (pa.list_(pa.uint8()), [1, 2]),
+        (pa.int64(), 42),
+    ],
+)
+def test_cast_to_target_schema_rejects_unsupported_blob_input(source_type, value):
+    data = pa.table({"image": pa.array([value], type=source_type)})
+    target = pa.schema([lancedb.blob("image")])
+
+    with pytest.raises(ValueError, match="cannot coerce column 'image' with type"):
+        _cast_to_target_schema(data.to_reader(), target)
+
+
 def test_cast_to_target_schema_coerces_binary_to_metadata_blob_struct():
     storage = lancedb.blob("image").type.storage_type
     target = pa.schema(