fix: virtual flowpath subsetting returns all VFPs

src/icefabric/hydrofabric/subset_nhf.py

@@ -410,16 +410,17 @@ def generate_subset_virtual_only(
 ) -> dict[str, gpd.GeoDataFrame]:
     """Subset hydrofabric for a virtual-only gage (no main-network upstream).
 
-    Returns all layers that belong to the ``div_id`` the virtual flowpath
-    sits in. No graph traversal is performed because virtual-only flowpaths
-    are leaf nodes — there is no upstream virtual or main network.
+    Returns all layers and attributes that belong to the ``div_id`` the
+    virtual flowpath sits in — including ALL virtual flowpaths within the
+    divide, not just the originating one. No graph traversal is performed
+    because virtual-only flowpaths are leaf nodes — there is no upstream
+    virtual or main network.
     """
     logger.debug(f"Virtual-only subset: virtual_fp_id={virtual_fp_id}, div_id={div_id}")
     crs = source.namespace.crs
     div_ids = {int(div_id)}
-    vfp_ids = {int(virtual_fp_id)}
 
-    # Wave 1: direct div_id / virtual_fp_id filters
+    # Wave 1: direct div_id filters (parallel)
     with ThreadPoolExecutor(max_workers=6) as ex:
         f = {
             "fp": ex.submit(source.load_filtered, "flowpaths", "div_id", div_ids),
@@ -428,29 +429,36 @@ def generate_subset_virtual_only(
             "gages": ex.submit(source.load_filtered, "gages", "div_id", div_ids),
             "ref_fp": ex.submit(source.load_filtered, "reference_flowpaths", "div_id", div_ids),
             "lakes": ex.submit(source.load_filtered, "lakes", "div_id", div_ids),
-            "v_fp": ex.submit(source.load_filtered, "virtual_flowpaths", "virtual_fp_id", vfp_ids),
         }
         subset_fp = f["fp"].result()
         subset_div = f["div"].result()
         subset_wb = f["wb"].result()
         subset_gages = f["gages"].result()
         subset_ref_fp = f["ref_fp"].result()
         subset_lakes = f["lakes"].result()
-        subset_v_fp = f["v_fp"].result()
 
-    # Derive nexus IDs from the single mainstem flowpath in this divide
+    # Collect ALL virtual_fp_ids from reference_flowpaths for this divide
+    # (not just the originating one), plus the originating vfp itself to
+    # ensure it's always included.
+    all_v_fp_ids = set(subset_ref_fp["virtual_fp_id"].to_list()) | {int(virtual_fp_id)}
+    subset_v_fp = source.load_filtered("virtual_flowpaths", "virtual_fp_id", all_v_fp_ids)
+
+    # Derive nexus IDs from the mainstem flowpath(s) in this divide
     all_nex_ids = set()
     if len(subset_fp) > 0:
         all_nex_ids = set(
             subset_fp.filter(pl.col("up_nex_id").is_not_null())["up_nex_id"].cast(pl.Int64).to_list()
             + subset_fp.filter(pl.col("dn_nex_id").is_not_null())["dn_nex_id"].cast(pl.Int64).to_list()
         )
 
-    # Derive virtual_nexus from the virtual flowpath
+    # Derive virtual_nexus from ALL virtual flowpaths in this divide
     all_v_nex_ids = set()
     if len(subset_v_fp) > 0:
         all_v_nex_ids = set(
-            subset_v_fp.filter(pl.col("dn_virtual_nex_id").is_not_null())["dn_virtual_nex_id"]
+            subset_v_fp.filter(pl.col("up_virtual_nex_id").is_not_null())["up_virtual_nex_id"]
+            .cast(pl.Int64)
+            .to_list()
+            + subset_v_fp.filter(pl.col("dn_virtual_nex_id").is_not_null())["dn_virtual_nex_id"]
             .cast(pl.Int64)
             .to_list()
         )
@@ -476,12 +484,14 @@ def generate_subset_virtual_only(
     subset_nhd = source.load_filtered("nhd", "ref_id", all_ref_ids)
 
     # Null downstream pointers (outlets)
+    subset_v_fp_ids = set(subset_v_fp["virtual_fp_id"].to_list())
+
     subset_nex = subset_nex.with_columns(
         pl.when(pl.col("dn_fp_id").is_in(div_ids)).then(pl.col("dn_fp_id")).otherwise(None).alias("dn_fp_id")
     )
 
     subset_v_nex = subset_v_nex.with_columns(
-        pl.when(pl.col("dn_virtual_fp_id").is_in(vfp_ids))
+        pl.when(pl.col("dn_virtual_fp_id").is_in(subset_v_fp_ids))
         .then(pl.col("dn_virtual_fp_id"))
         .otherwise(None)
         .alias("dn_virtual_fp_id")