// SERVER-6181 Correctly support an expression for _id

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

c.save({a: 2});

res = c.aggregate({$project: {_id: '$a'}});
assert.eq(res.toArray(), [{_id: 2}]);

res = c.aggregate({$project: {_id: {$add: [1, '$a']}}});
assert.eq(res.toArray(), [{_id: 3}]);