Observed behavior: If multiple pipeline stages are specified in a single object (eg. { $project:

}, the last one is used and earlier ones are dropped.

Expected behavior: Only one pipeline stage spec is allowed in a pipeline spec object. Otherwise a parse error is triggered.

Currently multiple pipeline stages are properly specified as separate objects within an array (eg. [ { $project:

} ] ).

Test

c = db.c;
c.drop();

c.save( { a:1 } );
// Runs as a $project.
printjson( c.aggregate( { $project:{ z:'$a' } } ) );
// Runs as a $sort.
printjson( c.aggregate( { $project:{ z:'$a' }, $sort:{ a:1 } } ) );
// Runs as a $group.
printjson( c.aggregate( { $project:{ z:'$a' }, $sort:{ a:1 }, $group:{ _id:'$a' } } ) );