model: avoid setting value on getter

* closes zenodo/rdm-project#521

invenio_accounts/models.py

@@ -100,7 +100,7 @@ class User(db.Model, Timestamp, UserMixin):
     _displayname = db.Column("displayname", db.String(255), nullable=True)
     """Case-preserving version of the username."""
 
-    _email = db.Column("email", db.String(255), unique=True)
+    email = db.Column(db.String(255), unique=True)
     """User email."""
 
     password = db.Column(db.String(255))
@@ -203,16 +203,6 @@ def username(self, username):
             self._displayname = username
             self._username = username.lower()
 
-    @hybrid_property
-    def email(self):
-        """Get email."""
-        return self._email
-
-    @email.setter
-    def email(self, email):
-        """Set lowercase email."""
-        self._email = email.lower()
-
     @hybrid_property
     def user_profile(self):
         """Get the user profile."""
@@ -239,7 +229,7 @@ def preferences(self):
         if self._preferences is None:
             return None
         elif not isinstance(self._preferences, UserPreferenceDict):
-            self._preferences = UserPreferenceDict(**self._preferences)
+            return UserPreferenceDict(**self._preferences)
 
         return self._preferences