[BUG FIX] Field.igrid set by reconstruct_fields()

src/parcels/_core/field.py

@@ -98,8 +98,6 @@ def __init__(
         self.name = name
         self.model = model
 
-        self.igrid = -1  # Default the grid index to -1
-
     @property
     def data(self):
         return self.model.field_data(self.name)
@@ -108,6 +106,14 @@ def data(self):
     def grid(self):  # TODO PR: Remove in favour of referencing model grid directly
         return self.model.grid
 
+    @property
+    def igrid(self):
+        return self._igrid
+
+    @igrid.setter
+    def igrid(self, value):
+        self._igrid = value
+
     @property
     def time_interval(self):  # TODO PR: Remove in favour of referencing model time_interval directly
         if "time" not in self.model.data[self.name].dims:
@@ -215,7 +221,6 @@ def __init__(
         self.V = V
         self.W = W
         self.grid = U.grid
-        self.igrid = U.igrid
 
         if W is None:
             _assert_same_time_interval((U, V))
@@ -247,6 +252,10 @@ def interp_method(self, method: VectorInterpolator):
             raise ValueError(f"method must be a `VectorInterpolator` object. Got {type(method)=!r}")
         self._interp_method = method
 
+    @property
+    def igrid(self):
+        return self.U.igrid
+
     def eval(self, t: datetime, z, y, x, particles=None):
         """Interpolate vectorfield values in space and time.
 

src/parcels/_core/fieldset.py

@@ -106,6 +106,9 @@ def reconstruct_fields(self):
         if self.constant_model is not None:
             fields += self.constant_model.construct_fields()
         self._fields = {f.name: f for f in fields}
+        for field in self._fields.values():
+            if isinstance(field, Field):
+                field.igrid = self.gridset.index(field.grid)
 
     def __getattr__(self, name):
         """Get the field by name. If the field is not found, check if it's a context variable."""

tests/test_particleset_execute.py

@@ -16,6 +16,7 @@
     Variable,
 )
 from parcels._core.mesh import FlatMesh
+from parcels._core.spatialhash import SpatialHash
 from parcels._core.statuscodes import GridSearchingError
 from parcels._core.utils.time import timedelta_to_float
 from parcels._datasets.structured.generated import simple_UV_dataset
@@ -193,6 +194,39 @@ def SampleU(particles, fieldset):  # pragma: no cover
         pset.execute(SampleU, runtime=np.timedelta64(1, "D"), dt=np.timedelta64(1, "D"))
 
 
+def test_ei_cache_skips_spatial_hash_with_two_grids(monkeypatch):
+    """Each grid keeps its own cached ei, so stationary particles only go through the spatial hash on the first step."""
+    n_hashed = []
+    query = SpatialHash.query
+
+    def counting_query(self, y, x):
+        n_hashed.append(len(y))
+        return query(self, y, x)
+
+    monkeypatch.setattr(SpatialHash, "query", counting_query)
+
+    ds_curvilinear = datasets_structured["2d_left_rotated"][["data_g", "grid"]].rename({"data_g": "curvilinear"})
+    ds_rectilinear = datasets_structured["ds_2d_left"][["data_g", "grid"]].rename({"data_g": "rectilinear"})
+    fieldset = FieldSet.from_sgrid_conventions(ds_curvilinear, mesh=FlatMesh()) + FieldSet.from_sgrid_conventions(
+        ds_rectilinear, mesh=FlatMesh()
+    )
+
+    def SampleBothGrids(particles, fieldset):  # pragma: no cover
+        _ = fieldset.curvilinear[particles]
+        _ = fieldset.rectilinear[particles]
+
+    npart = 50
+    rng = np.random.default_rng(0)
+    pset = ParticleSet(fieldset, x=rng.uniform(1, 5, npart), y=rng.uniform(1, 5, npart), z=rng.uniform(1, 80, npart))
+    dt = np.timedelta64(1, "D")
+    pset.execute(SampleBothGrids, runtime=dt, dt=dt)
+    assert sum(n_hashed) == npart
+
+    n_hashed.clear()
+    pset.execute(SampleBothGrids, runtime=4 * dt, dt=dt)
+    assert sum(n_hashed) == 0
+
+
 def test_particleset_run_to_endtime(fieldset):
     starttime = fieldset.time_interval.left
     endtime = fieldset.time_interval.right