test(links): add regression tests for non-default person entity key

Add tests verifying ImplicitOne2ManyLink and _resolve_links work when
the person entity has a key other than 'person' (e.g. 'individu' in
openfisca-france):

- test_implicit_one2many_with_non_default_person_key (unit)
- test_resolve_links_with_individu_key (integration)
- test_implicit_link_target_uses_actual_person_key (integration)

Author: benjello

openfisca_core/links/tests/test_implicit.py

@@ -74,3 +74,50 @@ def test_implicit_one2many(sim):
 
     counts = link.count("2024")
     assert numpy.array_equal(counts, [2, 1, 1])
+
+
+def test_implicit_one2many_with_non_default_person_key():
+    """Regression test: entity key != 'person' must not crash.
+
+    openfisca-france uses 'individu' as the person entity key.
+    Before the fix, ImplicitOne2ManyLink hardcoded target_entity_key='person',
+    causing a KeyError during link resolution.
+    """
+    individu = entities.SingleEntity("individu", "individus", "Un individu", "")
+    menage = entities.GroupEntity(
+        "menage",
+        "menages",
+        "Un ménage",
+        "",
+        roles=[{"key": "personne_de_reference"}],
+    )
+
+    tbs = taxbenefitsystems.TaxBenefitSystem([individu, menage])
+
+    class salaire(variables.Variable):
+        value_type = float
+        entity = individu
+        definition_period = periods.DateUnit.YEAR
+
+    tbs.add_variable(salaire)
+
+    sim = SimulationBuilder().build_from_dict(
+        tbs,
+        {
+            "individus": {
+                "i0": {"salaire": {"2024": 1000.0}},
+                "i1": {"salaire": {"2024": 500.0}},
+            },
+            "menages": {
+                "m0": {"personne_de_reference": ["i0", "i1"]},
+            },
+        },
+    )
+
+    # The implicit O2M link should resolve with target_entity_key='individu'
+    link = ImplicitOne2ManyLink("individus", "menage", "individu")
+    link.attach(sim.populations["menage"])
+    link.resolve(sim.populations)  # Would raise KeyError before the fix
+
+    salaires = link.sum("salaire", "2024")
+    assert numpy.array_equal(salaires, [1500.0])

openfisca_core/links/tests/test_integration.py

@@ -190,3 +190,55 @@ def test_no_links_no_problem(self):
 
         # Simulation should work fine
         assert sim.persons.count == 2
+
+
+# -- Regression test: non-default person entity key -----------------------
+
+
+class TestNonDefaultPersonKey:
+    """Verify that _resolve_links works when person entity key != 'person'.
+
+    This is a regression test for the France API crash where the person
+    entity is named 'individu' instead of 'person'.
+    """
+
+    def test_resolve_links_with_individu_key(self):
+        """Simulation.__init__ must not crash with entity key 'individu'."""
+        individu = entities.SingleEntity(
+            "individu", "individus", "Un individu", ""
+        )
+        menage = entities.GroupEntity(
+            "menage",
+            "menages",
+            "Un ménage",
+            "",
+            roles=[{"key": "personne_de_reference"}],
+        )
+        tbs = taxbenefitsystems.TaxBenefitSystem([individu, menage])
+        # This line triggered the KeyError before the fix
+        sim = SimulationBuilder().build_default_simulation(tbs, count=3)
+
+        assert sim.persons.count == 3
+        # Implicit links should be auto-generated with correct keys
+        assert "menage" in sim.persons.links  # individu → menage (M2O)
+        assert "individus" in sim.populations["menage"].links  # menage → individus (O2M)
+
+    def test_implicit_link_target_uses_actual_person_key(self):
+        """The O2M link target must be 'individu', not 'person'."""
+        individu = entities.SingleEntity(
+            "individu", "individus", "Un individu", ""
+        )
+        foyer_fiscal = entities.GroupEntity(
+            "foyer_fiscal",
+            "foyers_fiscaux",
+            "Un foyer fiscal",
+            "",
+            roles=[{"key": "declarant"}],
+        )
+        tbs = taxbenefitsystems.TaxBenefitSystem([individu, foyer_fiscal])
+        sim = SimulationBuilder().build_default_simulation(tbs, count=2)
+
+        o2m_link = sim.populations["foyer_fiscal"].links["individus"]
+        assert o2m_link.target_entity_key == "individu"
+        assert o2m_link.is_resolved
+        assert o2m_link._target_population is sim.persons