one to many relationship unit test

add a test suite to check a relationship implementation works
this adds a constraint on esper because this implementation use a same component instance (called group) for many entities

tests/test_relationships.py

@@ -0,0 +1,68 @@
+"""
+check a one-to-many relationship implementation is working
+this test suite implementation uses the theme of ownership
+"""
+
+import esper
+
+
+class Inventory:
+    """the entity belongs to the inventory
+    every entity in the same group must use the same instance of Inventory"""
+
+
+class Hold:
+    """the inventory belongs to this entity"""
+
+    def __init__(self, invent: Inventory) -> None:
+        self.invent = invent
+
+
+def test_query_owned() -> None:
+    invent = Inventory()
+
+    owner = esper.create_entity(Hold(invent))
+    expected = [esper.create_entity(invent), esper.create_entity(invent)]
+
+    esper.create_entity(Inventory())
+
+    # we want to get all owned entities by owner
+    # step 1 : get Hold component of $owner
+    h = esper.try_component(owner, Hold)
+    assert h is not None
+    # step 2 : iterates entities with an Inventory component
+    result = []
+    for owned, invent in esper.get_component(Inventory):
+        # step 3 : skip the entities with another Inventory
+        if invent is not h.invent:
+            continue
+        # we would get entities owned by $owner
+        result.append(owned)
+
+    # check result
+    assert result == expected
+
+
+def test_get_owner() -> None:
+    invent = Inventory()
+
+    expected = [esper.create_entity(Hold(invent))]
+    owned = esper.create_entity(invent)
+
+    esper.create_entity(Hold(Inventory()))
+
+    # we want to get the owner of $owned
+    # step 1 : get Inventory component of $owned
+    invent_ = esper.try_component(owned, Inventory)
+    assert invent_ is not None
+    # step 2 : iterates entities with a Hold component
+    result = []
+    for owner, h in esper.get_component(Hold):
+        # step 3 : skip entities that own an other Inventory
+        if h.invent is not invent_:
+            continue
+        # we would get the entity that owns $owned
+        result.append(owner)
+
+    # check result
+    assert result == expected