Refactor: Do not implicitly add system columns to $select

Co-authored-by: eric.luce <eric.luce@proofgeist.com>

Author: cursoragent

packages/fmodata/src/client/builders/query-string-builder.ts

@@ -24,19 +24,13 @@ export function buildSelectExpandQueryString(config: {
 
   // Build $select
   if (config.selectedFields && config.selectedFields.length > 0) {
-    // Add special columns if includeSpecialColumns is true and they're not already present
-    let finalSelectedFields = [...config.selectedFields];
-    if (config.includeSpecialColumns) {
-      if (!finalSelectedFields.includes("ROWID")) {
-        finalSelectedFields.push("ROWID");
-      }
-      if (!finalSelectedFields.includes("ROWMODID")) {
-        finalSelectedFields.push("ROWMODID");
-      }
-    }
-    
+    // Important: do NOT implicitly add system columns (ROWID/ROWMODID) here.
+    // - `includeSpecialColumns` controls the Prefer header + response parsing, but should not
+    //   mutate/expand an explicit `$select` (e.g. when the user calls `.select({ ... })`).
+    // - If system columns are desired with `.select()`, they must be explicitly included via
+    //   the `systemColumns` argument, which will already have added them to `selectedFields`.
     const selectString = formatSelectFields(
-      finalSelectedFields,
+      config.selectedFields,
       config.table,
       config.useEntityIds,
     );

packages/fmodata/tests/include-special-columns.test.ts

@@ -433,6 +433,34 @@ describe("includeSpecialColumns feature", () => {
     expect(firstRecord).not.toHaveProperty("ROWMODID");
   });
 
+  it("should not append ROWID/ROWMODID to explicit $select unless requested via systemColumns", () => {
+    const db = connection.database("TestDB", {
+      includeSpecialColumns: true,
+    });
+
+    // Explicit select() should remain exact (no implicit system columns)
+    const queryString = db
+      .from(contactsTO)
+      .list()
+      .select({ name: contactsTO.name })
+      .getQueryString();
+
+    expect(queryString).toContain("$select=");
+    expect(queryString).toContain("name");
+    expect(queryString).not.toContain("ROWID");
+    expect(queryString).not.toContain("ROWMODID");
+
+    // But system columns should still be selectable when explicitly requested
+    const queryStringWithSystemCols = db
+      .from(contactsTO)
+      .list()
+      .select({ name: contactsTO.name }, { ROWID: true, ROWMODID: true })
+      .getQueryString();
+
+    expect(queryStringWithSystemCols).toContain("ROWID");
+    expect(queryStringWithSystemCols).toContain("ROWMODID");
+  });
+
   it("should work with single() method", async () => {
     const db = connection.database("TestDB", {
       includeSpecialColumns: true,