refactor(dfns): reshape LAK period data

autotest/dfns/test_migrate.py

@@ -319,6 +319,20 @@ def test_migrate_lak_connectiondata_shape(dfn_dir, tmp_path, v1_shape):
     assert component.dims["nlakeconn"].value == "sum(packagedata.nlakeconn)"
 
 
+@pytest.mark.parametrize(
+    ("name", "setting"), [("gwf-lak", "laksetting"), ("prt-prp", "releasesetting")]
+)
+def test_migrate_period_union_element(dfn_dir, name, setting):
+    """A period list of only a union is a list of the union: PRP's, and LAK's
+    once its lake or outlet number moves into the arms."""
+    component = _migrate_dev3(dfn_dir, name)
+    item = component.blocks["period"].fields["perioddata"].item
+    assert isinstance(item, v2.Union) and item.name == setting
+    if name == "gwf-lak":
+        assert item.arms["stage"].fields["lakeno"].fk == "packagedata.ifno"
+        assert item.arms["rate"].fields["outletno"].fk == "outlets.outletno"
+
+
 def test_migrate_auxiliary_stays_self_sizing(dfn_dir):
     component = _migrate_dev3(dfn_dir, "gwf-chd")
     assert component.blocks["options"].fields["auxiliary"].shape == []

docs/md/sfr-lak-structure.md

@@ -87,7 +87,9 @@ explicitly in `_fix_lak_relations` (`migrate_to_v2_0_0_dev2.py`):
 copy, pushed inside each `laksetting` union arm with a name and `fk`
 appropriate to that arm's category: `lakeno` / `fk: packagedata.ifno` for
 the 8 lake-setting arms, `outletno` / `fk: outlets.outletno` for the 5
-outlet-setting arms. (`outlets.outletno` is also newly marked `pk`, since
+outlet-setting arms. That leaves `laksetting` the only field of the period
+list's element, so the element becomes the union itself, as PRP's
+`releasesetting` is. (`outlets.outletno` is also newly marked `pk`, since
 nothing else previously pointed at it — needed for the new `outletno` fk to
 resolve.)
 

modflow_devtools/dfns/migrate_to_v2_0_0_dev2.py

@@ -1328,7 +1328,8 @@ def _fix_lak_relations(name: str, blocks: dict[str, v2.Block]) -> dict[str, v2.B
       fields nested inside the corresponding `laksetting` union arms, each
       carrying a correct, non-conditional `fk` (a single `fk` on the shared
       `number` field can't be correct for both arm categories at once, since
-      the target block depends on which arm is present).
+      the target block depends on which arm is present). The period list's
+      element is then `laksetting` itself.
     - Marks `outlets.outletno` as `pk` (the new `outletno` fk needs one).
     - Marks `outlets.lakein`/`outlets.lakeout` as `fk: packagedata.ifno` —
       unambiguous, but named differently from `ifno` so the general
@@ -1381,12 +1382,12 @@ def _retarget(
             new_arms[arm_name] = arm
 
     new_setting = setting_field.model_copy(update={"arms": new_arms})
-    new_item = item.model_copy(
-        update={
-            "fields": {fn: f for fn, f in item.fields.items() if fn != "number"}
-            | {"laksetting": new_setting}
-        }
-    )
+    fields = {fn: f for fn, f in item.fields.items() if fn != "number"} | {
+        "laksetting": new_setting
+    }
+    # With `number` in its arms, a record of only the union is the union: the
+    # list's element is the union itself, as PRP's `releasesetting` is.
+    new_item = new_setting if len(fields) == 1 else item.model_copy(update={"fields": fields})
     new_perioddata = perioddata.model_copy(update={"item": new_item})
     new_period = period.model_copy(
         update={"fields": {**period.fields, "perioddata": new_perioddata}}