It ist wrong that MissingCollectionTermsMixin extends `MissingTerms…

…Mixin` because the `super()` call then leads to the wrong method (aka the one on `MissingTermsMixin` instead of the one on the class where the mix-in is used on.

src/z3c/form/term.py

@@ -131,9 +131,8 @@ def __init__(self, context, request, form, field, vocabulary, widget):
         self.terms = vocabulary
 
 
-class MissingTermsMixin(object):
-    """This can be used in case previous values/tokens get missing
-    from the vocabulary and you still need to display/keep the values"""
+class MissingTermsBase(object):
+    """Base class for MissingTermsMixin classes."""
 
     def _canQueryCurrentValue(self):
         return (interfaces.IContextAware.providedBy(self.widget) and
@@ -144,17 +143,6 @@ def _queryCurrentValue(self):
             (self.widget.context, self.field),
             interfaces.IDataManager).query()
 
-    def getTerm(self, value):
-        try:
-            return super(MissingTermsMixin, self).getTerm(value)
-        except LookupError:
-            if self._canQueryCurrentValue():
-                curValue = self._queryCurrentValue()
-                if curValue == value:
-                    return self._makeMissingTerm(value)
-
-            raise
-
     def _makeToken(self, value):
         """create a unique valid ASCII token"""
         return util.createCSSId(util.toUnicode(value))
@@ -166,6 +154,21 @@ def _makeMissingTerm(self, value):
             value, self._makeToken(value),
             title=_(u'Missing: ${value}', mapping=dict(value=uvalue)))
 
+
+class MissingTermsMixin(MissingTermsBase):
+    """This can be used in case previous values/tokens get missing
+    from the vocabulary and you still need to display/keep the values"""
+
+    def getTerm(self, value):
+        try:
+            return super(MissingTermsMixin, self).getTerm(value)
+        except LookupError:
+            if self._canQueryCurrentValue():
+                curValue = self._queryCurrentValue()
+                if curValue == value:
+                    return self._makeMissingTerm(value)
+            raise
+
     def getTermByToken(self, token):
         try:
             return super(MissingTermsMixin, self).getTermByToken(token)
@@ -266,7 +269,7 @@ def __init__(self, context, request, form, field, vocabulary, widget):
         self.terms = vocabulary
 
 
-class MissingCollectionTermsMixin(MissingTermsMixin):
+class MissingCollectionTermsMixin(MissingTermsBase):
     """`MissingTermsMixin` adapted to collections."""
 
     def getTerm(self, value):