query should only be evaluated at the end

src/c3nav/editor/wrappers.py

@@ -1,5 +1,6 @@
 import typing
 from collections import deque
+from functools import wraps
 from itertools import chain
 
 from django.db import models
@@ -9,7 +10,7 @@ from django.db.models.query_utils import DeferredAttribute
 
 
 class BaseWrapper:
-    _not_wrapped = ('_changeset', '_author', '_obj', '_changes_qs', '_initial_values', '_wrap_instances')
+    _not_wrapped = ('_changeset', '_author', '_obj', '_commands', '_wrap_instances', '_initial_values')
     _allowed_callables = ('', )
 
     def __init__(self, changeset, obj, author=None):
@@ -206,47 +207,84 @@ class ModelInstanceWrapper(BaseWrapper):
         self._changeset.add_delete(self, author=author)
 
 
-class ChangesQuerySet:
-    def __init__(self, changeset, model, author):
-        self._changeset = changeset
-        self._model = model
-        self._author = author
+class ModifiesQueryDecorator:
+    def __init__(self, test_call=False):
+        self.test_call = test_call
+
+    def __call__(self, f):
+        @wraps(f)
+        def wrapper(qs, *args, execute=False, **kwargs):
+            if execute:
+                return f(qs, *args, **kwargs)
+            if self.test_call:
+                f(qs, *args, test_call=True, **kwargs)
+            return qs._wrap_queryset(qs.get_queryset(), add_command=(f.__name__, args, kwargs))
+
+        return wrapper
+
+
+modifies_query = ModifiesQueryDecorator
+
+
+class ExecutesQueryDecorator:
+    def __init__(self, test_call=False):
+        self.test_call = test_call
+
+    def __call__(self, f):
+        @wraps(f)
+        def wrapper(qs, *args, **kwargs):
+            qs = qs.execute_commands()
+            return f(qs, *args, **kwargs)
+        return wrapper
+
+
+executes_query = ExecutesQueryDecorator
 
 
 class BaseQueryWrapper(BaseWrapper):
     _allowed_callables = ('_add_hints', '_next_is_sticky', 'get_prefetch_queryset')
 
-    def __init__(self, changeset, obj, author=None, changes_qs=None, wrap_instances=True):
+    def __init__(self, changeset, obj, author=None, commands=(), wrap_instances=True):
         super().__init__(changeset, obj, author)
-        if changes_qs is None:
-            changes_qs = ChangesQuerySet(changeset, obj.model, author)
-        self._changes_qs = changes_qs
         self._wrap_instances = wrap_instances
+        self._commands = commands
 
     def get_queryset(self):
         return self._obj
 
+    def execute_commands(self):
+        result = self
+        for name, args, kwargs in self._commands:
+            result = getattr(result, name)(*args, execute=True, **kwargs)
+        result._commands = ()
+        return result
+
     def _wrap_instance(self, instance):
         if self._wrap_instances:
             return super()._wrap_instance(instance)
         return instance
 
-    def _wrap_queryset(self, queryset, changes_qs=None, wrap_instances=None):
-        if changes_qs is None:
-            changes_qs = self._changes_qs
+    def _wrap_queryset(self, queryset, add_command=None, wrap_instances=None):
         if wrap_instances is None:
             wrap_instances = self._wrap_instances
-        return QuerySetWrapper(self._changeset, queryset, self._author, changes_qs, wrap_instances)
+        commands = self._commands
+        if add_command is not None:
+            commands += (add_command, )
+        return QuerySetWrapper(self._changeset, queryset, self._author, commands, wrap_instances)
 
+    @modifies_query()
     def all(self):
         return self._wrap_queryset(self.get_queryset().all())
 
+    @modifies_query()
     def none(self):
         return self._wrap_queryset(self.get_queryset().none())
 
+    @modifies_query()
     def select_related(self, *args, **kwargs):
         return self._wrap_queryset(self.get_queryset().select_related(*args, **kwargs))
 
+    @modifies_query()
     def prefetch_related(self, *lookups):
         new_lookups = deque()
         for lookup in lookups:
@@ -261,6 +299,7 @@ class BaseQueryWrapper(BaseWrapper):
             new_lookups.append(Prefetch(lookup, qs))
         return self._wrap_queryset(self.get_queryset().prefetch_related(*new_lookups))
 
+    @executes_query()
     def get(self, *args, **kwargs):
         results = tuple(self.filter(*args, **kwargs))
         if len(results) == 1:
@@ -269,6 +308,7 @@ class BaseQueryWrapper(BaseWrapper):
             raise self._obj.model.DoesNotExist
         raise self._obj.model.MultipleObjectsReturned
 
+    @modifies_query()
     def order_by(self, *args):
         return self._wrap_queryset(self.get_queryset().order_by(*args))
 
@@ -384,33 +424,42 @@ class BaseQueryWrapper(BaseWrapper):
             tuple(self._filter_kwarg(name, value) for name, value in kwargs.items())
         )
 
+    @modifies_query()
     def filter(self, *args, **kwargs):
         return self._wrap_queryset(self.get_queryset().filter(*self._filter(*args, **kwargs)))
 
+    @modifies_query()
     def exclude(self, *args, **kwargs):
         return self._wrap_queryset(self.get_queryset().exclude(*self._filter(*args, **kwargs)))
 
+    @executes_query()
     def count(self):
         return self.get_queryset().count()
 
+    @executes_query()
     def values_list(self, *args, flat=False):
         return self.get_queryset().values_list(*args, flat=flat)
 
+    @executes_query()
     def first(self):
         first = self.get_queryset().first()
         if first is not None:
             first = self._wrap_instance(first)
         return first
 
+    @modifies_query()
     def using(self, alias):
         return self._wrap_queryset(self.get_queryset().using(alias))
 
+    @executes_query()
     def __iter__(self):
         return iter((self._wrap_instance(instance) for instance in self.get_queryset()))
 
+    @executes_query()
     def iterator(self):
         return iter((self._wrap_instance(instance) for instance in self.get_queryset().iterator()))
 
+    @executes_query()
     def __len__(self):
         return len(self.get_queryset())