db: message: Implement get_subject

Provide a method to get the subject of a given message.

Signed-off-by: Kieran Bingham <[email protected]>

alot/db/message.py

@@ -45,6 +45,7 @@ def __init__(self, dbman, msg, thread=None):
             self._datetime = None
         self._filename = msg.get_filename()
         self._email = None  # will be read upon first use
+        self._subject = None  # will be read upon first use
         self._attachments = None  # will be read upon first use
         self._mime_part = None  # will be read upon first use
         self._mime_tree = None  # will be read upon first use
@@ -122,6 +123,12 @@ def get_message_id(self):
         """returns messages id (str)"""
         return self._id
 
+    def get_subject(self):
+        if not self._subject:
+            mail = self.get_email()
+            self._subject = decode_header(mail.get('Subject', ''))
+        return self._subject
+
     def get_thread_id(self):
         """returns id (str) of the thread this message belongs to"""
         return self._thread_id