MongoDB: aggregate $project add field with static value

Question

Can I somehow add custom field with static (not computed) value?

I want to prepare objects before send and I need to remove some fields with internal information and

Answer 1

Note that $literal was implemented in Mongo 2.6. So now you can simply write:

db.test.aggregate(
   {$project: {_id: 0, data: 1, entity_id: {$literal: 54}}})

See $literal.