fix: omit unfetchable list blobs from column discovery (#4312)

nodejs/__test__/table.test.ts

@@ -3041,7 +3041,7 @@ 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),
@@ -3060,7 +3060,16 @@ 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([
@@ -3106,6 +3115,7 @@ 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

@@ -81,6 +81,8 @@ 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

@@ -526,7 +526,8 @@ 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[]>;
 
@@ -538,6 +539,8 @@ export abstract class Table {
    * duplicates. Null blobs are `null`. Empty blobs are empty buffers.
    * One missing or deleted row ID rejects the entire batch, including on remote
    * tables; no partial results are returned.
+   * Blobs inside lists cannot be fetched by row ID.
+   *
    * Remote servers limit each request to 1024 row IDs and 64 MiB of blob bytes.
    * The client splits requests automatically and reads an individual larger
    * blob through the Range route. This method still materializes all bytes in