fix: omit unfetchable list blobs from column discovery

nodejs/__test__/table.test.ts

@@ -2500,7 +2500,7 @@ describe("when dealing with blob columns", () => {
     expect(bytes[0]!.equals(payload)).toBe(true);
   });
 
-  it("creates and adds list blob columns", async () => {
+  it("creates list blobs without advertising unfetchable elements", async () => {
     const db = await connect(tmpDir.name);
     const schema = new Schema([
       new Field("id", new Int64(), true),
@@ -2519,7 +2519,16 @@ describe("when dealing with blob columns", () => {
       { id: 3n, images: [gamma, null] },
       { id: 4n, images: [] },
     ]);
-    expect(await table.blobColumns()).toEqual(["images.image"]);
+    expect(await table.blobColumns()).toEqual([]);
+    const [first] = await table.query().withRowId().toArray();
+    for (const column of ["images", "images.image"]) {
+      await expect(
+        table.fetchBlobs(column, [first._rowid as bigint]),
+      ).rejects.toThrow(/blobs inside lists cannot be fetched/);
+      await expect(
+        table.fetchBlobFiles(column, [first._rowid as bigint]),
+      ).rejects.toThrow(/blobs inside lists cannot be fetched/);
+    }
     const rows = await table.query().toArray();
     const byId = new Map(rows.map((row) => [Number(row.id), row]));
     expect(descriptorSizes(byId.get(1)!.images)).toEqual([
@@ -2565,6 +2574,7 @@ describe("when dealing with blob columns", () => {
         ],
       },
     ]);
+    expect(await table.blobColumns()).toEqual([]);
     const rows = await table.query().toArray();
     const byId = new Map(rows.map((row) => [Number(row.id), row]));
     expect(

nodejs/lancedb/blob.ts

@@ -42,6 +42,8 @@ export type BlobOptions = {
  *
  * Query results are descriptors, not payload bytes. Use {@link Table.fetchBlobs}
  * or {@link Table.fetchBlobFiles} to read bytes.
+ * Blobs inside lists can be written and queried as descriptors, but cannot
+ * currently be fetched because a row ID does not identify a list element.
  *
  * @example
  * ```ts

nodejs/lancedb/table.ts

@@ -516,7 +516,8 @@ export abstract class Table {
   abstract takeRowIds(rowIds: readonly (bigint | number)[]): TakeQuery;
 
   /**
-   * Blob v2 columns, including nested dotted paths.
+   * Blob v2 columns addressable by row ID, including struct-nested dotted paths.
+   * Blobs inside lists are omitted because a row ID does not identify an element.
    */
   abstract blobColumns(): Promise<string[]>;
 
@@ -526,6 +527,7 @@ export abstract class Table {
    * Reads the table's current checkout. IDs from another version can fail after
    * compaction unless stable row ids are enabled. Results keep input order and
    * duplicates. Null blobs are `null`. Empty blobs are empty buffers.
+   * Blobs inside lists cannot be fetched by row ID.
    */
   abstract fetchBlobs(
     column: string,