Use type-based formatting and parsing

All editable values are now strings and are parsed from strings. This closely
matches the behavior of the `modify` command, for example.

beets/dbcore/db.py

@@ -466,6 +466,11 @@ class Model(object):
 
         return cls._type(key).parse(string)
 
+    def set_parse(self, key, string):
+        """Set the object's key to a value represented by a string.
+        """
+        self[key] = self._parse(key, string)
+
 
 # Database controller and supporting interfaces.
 

beetsplug/edit.py

@@ -55,14 +55,27 @@ def flatten(obj, fields):
     """Represent `obj`, a `dbcore.Model` object, as a dictionary for
     serialization. Only include the given `fields` if provided;
     otherwise, include everything.
+
+    The resulting dictionary's keys are all human-readable strings.
     """
-    d = dict(obj)
+    d = dict(obj.formatted())
     if fields:
         return {k: v for k, v in d.items() if k in fields}
     else:
         return d
 
 
+def apply(obj, data):
+    """Set the fields of a `dbcore.Model` object according to a
+    dictionary.
+
+    This is the opposite of `flatten`. The `data` dictionary should have
+    strings as values.
+    """
+    for key, value in data.items():
+        obj.set_parse(key, value)
+
+
 class EditPlugin(plugins.BeetsPlugin):
 
     def __init__(self):
@@ -233,14 +246,14 @@ class EditPlugin(plugins.BeetsPlugin):
             forbidden = False
             for key in ignore_fields:
                 if old_dict.get(key) != new_dict.get(key):
-                    self._log.warn('ignoring object where {} changed', key)
+                    self._log.warn('ignoring object whose {} changed', key)
                     forbidden = True
                     break
             if forbidden:
                 continue
 
-            obj = obj_by_id[old_dict['id']]
+            id = int(old_dict['id'])
-            obj.update(new_dict)
+            apply(obj_by_id[id], new_dict)
 
     def save_write(self, objs):
         """Save a list of updated Model objects to the database.