SERVER-14833 {_id:-1} index builds should no-op if _id index exists

jstests/index_id_desc.js

@@ -0,0 +1,38 @@
+// Test creation of an index with key pattern {_id: -1}.  It is expected that a request for creation
+// of a {_id: -1} index is treated as if it were a request for creation of a {_id: 1} index.
+// SERVER-14833.
+
+var coll = db.index_id_desc;
+var indexes;
+var res;
+
+// Test ensureIndex({_id: -1}) on a nonexistent collection.
+coll.drop();
+res = coll.ensureIndex({_id: -1});
+assert.isnull(res);
+indexes = coll.getIndexes();
+assert.eq(1, indexes.length);
+assert.eq("_id_", indexes[0].name);
+assert.eq({_id: 1}, indexes[0].key);
+
+// Test ensureIndex({_id: -1}) on a normal empty collection.
+coll.drop();
+assert.commandWorked(coll.runCommand("create"));
+assert.eq(1, coll.getIndexes().length);
+res = coll.ensureIndex({_id: -1});
+assert.isnull(res);
+indexes = coll.getIndexes();
+assert.eq(1, indexes.length);
+assert.eq("_id_", indexes[0].name);
+assert.eq({_id: 1}, indexes[0].key);
+
+// Test ensureIndex({_id: -1}) on an empty collection with no _id index.
+coll.drop();
+assert.commandWorked(coll.runCommand("create", {autoIndexId: false}));
+assert.eq(0, coll.getIndexes().length);
+res = coll.ensureIndex({_id: -1});
+assert.isnull(res);
+indexes = coll.getIndexes();
+assert.eq(1, indexes.length);
+assert.eq("_id_", indexes[0].name);
+assert.eq({_id: 1}, indexes[0].key);

src/mongo/db/index.cpp

@@ -360,6 +360,14 @@ namespace mongo {
             uasserted(12504, s);
         }
 
+        /* this is because we want key patterns like { _id : 1 } and { _id : -1 } to
+           all be treated as the same pattern.
+        */
+        if ( IndexDetails::isIdIndexPattern(key) ) {
+            key = id_obj;
+            name = "_id_";
+        }
+
         sourceCollection = nsdetails(sourceNS);
         if( sourceCollection == 0 ) {
             // try to create it
@@ -393,9 +401,6 @@ namespace mongo {
             uasserted(12505,s);
         }
 
-        /* this is because we want key patterns like { _id : 1 } and { _id : <someobjid> } to
-           all be treated as the same pattern.
-        */
         if ( IndexDetails::isIdIndexPattern(key) ) {
             //if( !god ) {
             //ensureHaveIdIndex( sourceNS.c_str(), mayInterrupt );
@@ -439,14 +444,8 @@ namespace mongo {
             }
             // idea is to put things we use a lot earlier
             b.append("v", v);
-            if ( IndexDetails::isIdIndexPattern(o["key"].Obj()) ) {
-                b.append("name", "_id_");
-                b.append("key", id_obj);
-            }
-            else {
-                b.append( o["name"] );
-                b.append(o["key"]);
-            }
+            b.append("name", name);
+            b.append("key", key);
             if( o["unique"].trueValue() )
                 b.appendBool("unique", true); // normalize to bool true in case was int 1 or something...
             b.append(o["ns"]);