Merge pull request #1380 from quantopian/show-correct-symbol

BUG: always show the most recent symbol in Asset objects

tests/test_assets.py

@@ -568,18 +568,24 @@ class AssetFinderTestCase(WithTradingCalendars, ZiplineTestCase):
 
         for asof in pd.date_range('2014-01-01', '2014-01-05', tz='utc'):
             # from 01 through 05 sid 0 held 'A'
+            A_result = finder.lookup_symbol('A', asof)
             assert_equal(
-                finder.lookup_symbol('A', asof),
+                A_result,
                 finder.retrieve_asset(0),
                 msg=str(asof),
             )
+            # the symbol should always be the last symbol
+            assert_equal(A_result.symbol, 'B')
 
             # from 01 through 05 sid 1 held 'C'
+            C_result = finder.lookup_symbol('C', asof)
             assert_equal(
-                finder.lookup_symbol('C', asof),
+                C_result,
                 finder.retrieve_asset(1),
                 msg=str(asof),
             )
+            # the symbol should always be the last symbol
+            assert_equal(C_result.symbol, 'A')
 
         # no one held 'B' before 06
         with self.assertRaises(SymbolNotFound):
@@ -596,20 +602,24 @@ class AssetFinderTestCase(WithTradingCalendars, ZiplineTestCase):
             # from 06 through 10 sid 0 held 'B'
             # we test through the 11th because sid 1 is the last to hold 'B'
             # so it should ffill
+            B_result = finder.lookup_symbol('B', asof)
             assert_equal(
-                finder.lookup_symbol('B', asof),
+                B_result,
                 finder.retrieve_asset(0),
                 msg=str(asof),
             )
+            assert_equal(B_result.symbol, 'B')
 
             # from 06 through 10 sid 1 held 'A'
             # we test through the 11th because sid 1 is the last to hold 'A'
             # so it should ffill
+            A_result = finder.lookup_symbol('A', asof)
             assert_equal(
-                finder.lookup_symbol('A', asof),
+                A_result,
                 finder.retrieve_asset(1),
                 msg=str(asof),
             )
+            assert_equal(A_result.symbol, 'A')
 
     def test_lookup_symbol(self):
 

zipline/assets/assets.py

@@ -440,22 +440,50 @@ class AssetFinder(object):
     def _select_asset_by_symbol(asset_tbl, symbol):
         return sa.select([asset_tbl]).where(asset_tbl.c.symbol == symbol)
 
-    def _lookup_most_recent_symbols(self, sids):
+    def _select_most_recent_symbols_chunk(self, sid_group):
+        """Retrieve the most recent symbol for a set of sids.
+
+        Parameters
+        ----------
+        sid_group : iterable[int]
+            The sids to lookup. The length of this sequence must be less than
+            or equal to SQLITE_MAX_VARIABLE_NUMBER because the sids will be
+            passed in as sql bind params.
+
+        Returns
+        -------
+        sel : Selectable
+            The sqlalchemy selectable that will query for the most recent
+            symbol for each sid.
+
+        Notes
+        -----
+        This is implemented as an inner select of the columns of interest
+        ordered by the end date of the (sid, symbol) mapping. We then group
+        that inner select on the sid with no aggregations to select the last
+        row per group which gives us the most recently active symbol for all
+        of the sids.
+        """
         symbol_cols = self.equity_symbol_mappings.c
+        inner = sa.select(
+            (symbol_cols.sid,) +
+            tuple(map(
+                op.getitem(symbol_cols),
+                symbol_columns,
+            )),
+        ).where(
+            symbol_cols.sid.in_(map(int, sid_group)),
+        ).order_by(
+            symbol_cols.end_date.asc(),
+        )
+        return sa.select(inner.c).group_by(inner.c.sid)
+
+    def _lookup_most_recent_symbols(self, sids):
         symbols = {
             row.sid: {c: row[c] for c in symbol_columns}
             for row in concat(
                 self.engine.execute(
-                    sa.select(
-                        (symbol_cols.sid,) +
-                        tuple(map(op.getitem(symbol_cols), symbol_columns)),
-                    ).where(
-                        symbol_cols.sid.in_(map(int, sid_group)),
-                    ).order_by(
-                        symbol_cols.end_date.desc(),
-                    ).group_by(
-                        symbol_cols.sid,
-                    )
+                    self._select_most_recent_symbols_chunk(sid_group),
                 ).fetchall()
                 for sid_group in partition_all(
                     SQLITE_MAX_VARIABLE_NUMBER,