test: improve coverage of Model around adding relationships

Author: yt-ms

src/structurizr/model/model.py

@@ -348,7 +348,6 @@ def add_relationship(
             Relationship
 
         """
-
         if relationship is None:
             relationship = Relationship(**kwargs)
         # Check

tests/unit/model/test_model.py

@@ -0,0 +1,64 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+"""Ensure the expected behaviour of the model."""
+
+
+import pytest
+
+from structurizr.model import Model
+
+
+@pytest.fixture(scope="function")
+def empty_model() -> Model:
+    """Provide an empty Model on demand for test cases to use."""
+    return Model()
+
+
+def test_model_get_relationship_by_id(empty_model: Model):
+    """Test retrieving relationships by their IDs."""
+    sys1 = empty_model.add_software_system(name="sys1")
+    sys2 = empty_model.add_software_system(name="sys2")
+    r = empty_model.add_relationship(source=sys1, destination=sys2, id="r1")
+    assert empty_model.get_relationship("r1") is r
+
+
+def test_model_add_relationship_twice_ignored(empty_model: Model):
+    """Make sure that adding an existing relationship to the Model makes no difference."""
+    sys1 = empty_model.add_software_system(name="sys1")
+    sys2 = empty_model.add_software_system(name="sys2")
+    r = empty_model.add_relationship(source=sys1, destination=sys2)
+    assert list(empty_model.get_relationships()) == [r]
+    empty_model.add_relationship(r)
+    assert list(empty_model.get_relationships()) == [r]
+
+
+def test_model_cannot_add_relationship_with_same_id_as_existing(empty_model: Model):
+    """Ensure you can't add two relationships with the same ID."""
+    sys1 = empty_model.add_software_system(name="sys1")
+    sys2 = empty_model.add_software_system(name="sys2")
+    r1 = empty_model.add_relationship(source=sys1, destination=sys2, id="r1")
+    with pytest.raises(
+        ValueError, match="Relationship.* has the same ID as Relationship.*"
+    ):
+        r2 = empty_model.add_relationship(source=sys1, destination=sys2, id="r1")
+
+
+def test_model_cannot_add_relationship_with_same_id_as_element(empty_model: Model):
+    """Ensure you can't add a relationship with the same ID as an element."""
+    sys1 = empty_model.add_software_system(name="sys1")
+    sys2 = empty_model.add_software_system(name="sys2")
+    with pytest.raises(
+        ValueError, match="Relationship.* has the same ID as SoftwareSystem.*"
+    ):
+        empty_model.add_relationship(source=sys1, destination=sys2, id=sys1.id)