Refactor model state management.

Maintain entity state history in the model, not in the entities.
Add methods for navigating an entity's history.

examples/relate.py

@@ -13,6 +13,19 @@
 from juju.model import Model, ModelObserver
 
 
+class MyRemoveObserver(ModelObserver):
+    async def on_change(self, delta, old, new, model):
+        if delta.type == 'remove':
+            assert(new.latest() == new)
+            assert(new.next() is None)
+            assert(new.dead)
+            assert(new.current)
+            assert(new.connected)
+            assert(new.previous().dead)
+            assert(not new.previous().current)
+            assert(not new.previous().connected)
+
+
 class MyModelObserver(ModelObserver):
     async def on_change(self, delta, old, new, model):
         if model.all_units_idle():
@@ -25,6 +38,7 @@ async def run():
     model = Model()
     await model.connect_current()
 
+    model.add_observer(MyRemoveObserver())
     await model.reset(force=True)
     model.add_observer(MyModelObserver())
 

juju/delta.py

@@ -2,27 +2,24 @@
 
 
 def get_entity_delta(d):
-    _delta_types = {
-        'action': ActionDelta,
-        'application': ApplicationDelta,
-        'annotation': AnnotationDelta,
-        'machine': MachineDelta,
-        'unit': UnitDelta,
-        'relation': RelationDelta,
-    }
-
     return _delta_types[d.entity](d.deltas)
 
 
+def get_entity_class(entity_type):
+    return _delta_types[entity_type].get_entity_class()
+
+
 class EntityDelta(client.Delta):
     def get_id(self):
         return self.data['id']
 
+    @classmethod
     def get_entity_class(self):
         return None
 
 
 class ActionDelta(EntityDelta):
+    @classmethod
     def get_entity_class(self):
         from .action import Action
         return Action
@@ -32,6 +29,7 @@ class ApplicationDelta(EntityDelta):
     def get_id(self):
         return self.data['name']
 
+    @classmethod
     def get_entity_class(self):
         from .application import Application
         return Application
@@ -41,12 +39,14 @@ class AnnotationDelta(EntityDelta):
     def get_id(self):
         return self.data['tag']
 
+    @classmethod
     def get_entity_class(self):
         from .annotation import Annotation
         return Annotation
 
 
 class MachineDelta(EntityDelta):
+    @classmethod
     def get_entity_class(self):
         from .machine import Machine
         return Machine
@@ -56,12 +56,24 @@ class UnitDelta(EntityDelta):
     def get_id(self):
         return self.data['name']
 
+    @classmethod
     def get_entity_class(self):
         from .unit import Unit
         return Unit
 
 
 class RelationDelta(EntityDelta):
+    @classmethod
     def get_entity_class(self):
         from .relation import Relation
         return Relation
+
+
+_delta_types = {
+    'action': ActionDelta,
+    'application': ApplicationDelta,
+    'annotation': AnnotationDelta,
+    'machine': MachineDelta,
+    'unit': UnitDelta,
+    'relation': RelationDelta,
+}

juju/exceptions.py

@@ -0,0 +1,2 @@
+class DeadEntityException(Exception):
+    pass