fixes for album artist inference

- Inference must be enabled explicitly with the "infer_aa" flag. It does not
  happen transparently.
- Infer both artist and artist ID.
- Fixed a bug where only the database row was using the inferred data, not the
  returned data structure.
- Added tests.

beets/library.py

@@ -1080,18 +1080,25 @@ class Library(BaseLibrary):
         if record:
             return Album(self, dict(record))
 
-    def add_album(self, items):
+    def add_album(self, items, infer_aa=False):
         """Create a new album in the database with metadata derived
         from its items. The items are added to the database if they
-        don't yet have an ID. Returns an Album object.
+        don't yet have an ID. Returns an Album object. If the
+        infer_aa flag is set, then the album artist field will be
+        guessed from artist fields when not present.
         """
         # Set the metadata from the first item.
         #fixme: check for consensus?
         item_values = dict(
             (key, getattr(items[0], key)) for key in ALBUM_KEYS_ITEM)
-        if not item_values['albumartist']:
-            item_values['albumartist'] = getattr(items[0], 'artist')
-
+        if infer_aa:
+            namemap = {
+                'albumartist': 'artist',
+                'mb_albumartistid': 'mb_artistid',
+            }
+            for field, itemfield in namemap.iteritems():
+                if not item_values[field]:
+                    item_values[field] = getattr(items[0], itemfield)
 
         sql = 'INSERT INTO albums (%s) VALUES (%s)' % \
               (', '.join(ALBUM_KEYS_ITEM),
@@ -1104,7 +1111,7 @@ class Library(BaseLibrary):
         record = {}
         for key in ALBUM_KEYS:
             if key in ALBUM_KEYS_ITEM:
-                record[key] = getattr(items[0], key)
+                record[key] = item_values[key]
             else:
                 # Non-item fields default to None.
                 record[key] = None

test/test_db.py

@@ -445,6 +445,27 @@ class AlbumInfoTest(unittest.TestCase):
         self.assertEqual(ai.album, self.i.album)
         self.assertEqual(ai.year, self.i.year)
 
+    def test_infer_aa_gets_artist_and_id(self):
+        i = item()
+        i.albumartist = ''
+        i.mb_albumartistid = ''
+        ai = self.lib.add_album((i,), infer_aa=True)
+        self.assertEqual(ai.albumartist, i.artist)
+        self.assertEqual(ai.mb_albumartistid, i.mb_artistid)
+
+    def test_no_infer_aa_sets_blank_artist_and_id(self):
+        i = item()
+        i.albumartist = ''
+        i.mb_albumartistid = ''
+        ai = self.lib.add_album((i,), infer_aa=False)
+        self.assertEqual(ai.albumartist, '')
+        self.assertEqual(ai.mb_albumartistid, '')
+
+    def test_infer_aa_lets_album_values_override(self):
+        ai = self.lib.add_album((self.i,), infer_aa=True)
+        self.assertEqual(ai.albumartist, self.i.albumartist)
+        self.assertEqual(ai.mb_albumartistid, self.i.mb_albumartistid)
+
     def test_albuminfo_stores_art(self):
         ai = self.lib.get_album(self.i)
         ai.artpath = '/my/great/art'