v2: validate mappings

https://petab.readthedocs.io/en/latest/v2/documentation_data_format.html#mapping-table:
> The petabEntityId may be the same as the modelEntityId, but it must not be used to alias an entity that already has a valid PEtab identifier. This restriction is to avoid unnecessary complexity in the PEtab problem files.

Author: dweindl

petab/v2/core.py

@@ -846,6 +846,22 @@ class Mapping(BaseModel):
         populate_by_name=True, extra="allow", validate_assignment=True
     )
 
+    @model_validator(mode="after")
+    def _validate(self) -> Self:
+        if (
+            self.model_id
+            and self.model_id != self.petab_id
+            and is_valid_identifier(self.model_id)
+        ):
+            raise ValueError(
+                "Aliasing of entities that already have a valid identifier "
+                "is not allowed. Simplify your PEtab problem by removing the "
+                f"mapping entry for `{self.petab_id} -> {self.model_id}`, "
+                f"and replacing all occurrences of `{self.petab_id}` with "
+                f"`{self.model_id}`."
+            )
+        return self
+
 
 class MappingTable(BaseTable[Mapping]):
     """PEtab mapping table."""

tests/v2/test_core.py

@@ -836,3 +836,33 @@ def test_get_output_parameters():
     assert petab_problem.get_output_parameters(
         observable=False, noise=True
     ) == ["p1", "p3", "p5"]
+
+
+def test_mapping_validation():
+    """Test that invalid mapping entries raise errors."""
+
+    # alias invalid model entity ID
+    Mapping(
+        petab_id="valid_id",
+        model_id=" 1_invalid",
+    )
+
+    with pytest.raises(ValidationError, match="Invalid ID"):
+        # invalid petab entity ID
+        Mapping(
+            petab_id="1_invalid",
+            model_id="valid_id",
+        )
+
+    with pytest.raises(ValidationError, match="Aliasing.*not allowed"):
+        # unnecessary aliasing is forbidden
+        Mapping(
+            petab_id="forbidden_alias_of_valid_id",
+            model_id="valid_id",
+        )
+
+    # missing model_id is valid (annotation-only entry)
+    Mapping(petab_id="valid_id", name="some name")
+
+    # identity mapping is valid
+    Mapping(petab_id="valid_id", model_id="valid_id", name="some name")