Rewrite ExpressionObject

The main difference between this version and the old is that the following
three $pipelines will produce the same output (SERVER-6468):

{'a.b': 1} // classic inclusion
{a: {b: 1} // nested inclusion
{'a.b': '$a.b'} // dotted FieldPathExpression
{a: {b: '$a.b'} // nested FieldPathExpression

Additionally support for exclusion of fields other than _id has been
removed for 2.2 (SERVER-6361)

Other related bugs:
SERVER-6177 better error for {a:1, 'a.b':1}
SERVER-6181 support for computed _id
SERVER-6184 support {'a.b': 1, a:{c:1}} in either order

Remaining TODOs:
Re-enable the isSimple() method
See if this can be used more directly by $group to support nested fields (SERVER-6198)

Disabled test for SERVER-5369 because it depends on ability to exclude
arbitrary fields.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/field_path.h b/src/mongo/db/pipeline/field_path.h
index f6f59ad3295..efbda98a55d 100755
--- a/src/mongo/db/pipeline/field_path.h
+++ b/src/mongo/db/pipeline/field_path.h
@@ -83,6 +83,9 @@ namespace mongo {
 
         static const char prefix[];
 
+        /** a FieldPath like this but missing the first element (useful for recursion) */
+        FieldPath tail() const;
+
     private:
         vector<string> vFieldName;
     };