fix(node): sanitize Map fields across Arrow versions (#3650)

## Summary

- reconstruct foreign Arrow Map schemas from their single sanitized
entries field
- reject malformed Map types with anything other than one child
- preserve the complete Map schema and `keysSorted` value through
empty-table creation and IPC round trips across Arrow 15–18

## Testing

- `./node_modules/.bin/jest --runInBand __test__/arrow.test.ts
__test__/sanitize.test.ts`
- `pnpm lint`
- `pnpm build`
- `pnpm run docs`

Fixes #2337

nodejs/__test__/arrow.test.ts

@@ -52,6 +52,7 @@ describe.each([arrow15, arrow16, arrow17, arrow18])(
       Float64,
       Struct,
       List,
+      Map_,
       Int16,
       Int32,
       Int64,
@@ -69,6 +70,30 @@ describe.each([arrow15, arrow16, arrow17, arrow18])(
     type Schema = ApacheArrow["Schema"];
     type Table = ApacheArrow["Table"];
 
+    function expectValidMapField(
+      // biome-ignore lint/suspicious/noExplicitAny: Arrow Field types vary across supported versions
+      field: any,
+    ): void {
+      expect(DataType.isMap(field.type)).toBe(true);
+      expect(field.type.keysSorted).toBe(true);
+      expect(field.type.children).toHaveLength(1);
+
+      const entries = field.type.children[0];
+      expect(entries.name).toBe("entries");
+      expect(entries.nullable).toBe(false);
+      expect(DataType.isStruct(entries.type)).toBe(true);
+      expect(entries.type.children).toHaveLength(2);
+
+      const [key, value] = entries.type.children;
+      expect([key.name, value.name]).toEqual(["key", "value"]);
+      expect(key.nullable).toBe(false);
+      expect(DataType.isUtf8(key.type)).toBe(true);
+      expect(value.nullable).toBe(true);
+      expect(DataType.isInt(value.type)).toBe(true);
+      expect(value.type.bitWidth).toBe(32);
+      expect(value.type.isSigned).toBe(true);
+    }
+
     // Helper method to verify various ways to create a table
     async function checkTableCreation(
       tableCreationMethod: (
@@ -938,6 +963,34 @@ describe.each([arrow15, arrow16, arrow17, arrow18])(
           false,
         );
       });
+
+      it("will make an empty table with a Map field", async function () {
+        const schema = new Schema([
+          new Field(
+            "attributes",
+            new Map_(
+              new Field(
+                "entries",
+                new Struct([
+                  new Field("key", new Utf8(), false),
+                  new Field("value", new Int32(), true),
+                ]),
+                false,
+              ),
+              true,
+            ),
+          ),
+        ]);
+
+        const table = makeEmptyTable(schema);
+
+        expectValidMapField(table.schema.fields[0]);
+
+        const buffer = await fromTableToBuffer(table);
+        const roundTripped = tableFromIPC(buffer);
+
+        expectValidMapField(roundTripped.schema.fields[0]);
+      });
     });
 
     describe("when using two versions of arrow", function () {

nodejs/__test__/sanitize.test.ts

@@ -2,7 +2,7 @@
 // SPDX-FileCopyrightText: Copyright The LanceDB Authors
 
 import * as arrow from "../lancedb/arrow";
-import { sanitizeField, sanitizeType } from "../lancedb/sanitize";
+import { sanitizeField, sanitizeMap, sanitizeType } from "../lancedb/sanitize";
 
 describe("sanitize", function () {
   describe("sanitizeType function", function () {
@@ -181,4 +181,15 @@ describe("sanitize", function () {
       );
     });
   });
+
+  describe("sanitizeMap function", function () {
+    it.each([
+      ["no children", []],
+      ["two children", [{}, {}]],
+    ])("should reject a Map type with %s", function (_, children) {
+      expect(() => sanitizeMap({ children, keysSorted: false })).toThrow(
+        "Expected a Map type to have exactly one child",
+      );
+    });
+  });
 });

nodejs/lancedb/sanitize.ts

@@ -288,12 +288,11 @@ export function sanitizeMap(typeLike: object) {
   if (!("keysSorted" in typeLike) || typeof typeLike.keysSorted !== "boolean") {
     throw Error("Expected a Map type to have a `keysSorted` property");
   }
+  if (typeLike.children.length !== 1) {
+    throw Error("Expected a Map type to have exactly one child");
+  }
 
-  return new Map_(
-    // biome-ignore lint/suspicious/noExplicitAny: skip
-    typeLike.children.map((field) => sanitizeField(field)) as any,
-    typeLike.keysSorted,
-  );
+  return new Map_(sanitizeField(typeLike.children[0]), typeLike.keysSorted);
 }
 
 export function sanitizeDuration(typeLike: object) {