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.

Answer 2

edit as of 2.6 the $literal expression exists so you don't have to use the workaround now.

Original answer: I know this may sound really dumb, but you can use a "no-op" expression to "compute" what you need.

Example:

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

There was a proposed $literal operator for exactly this use case but it hasn't been implemented yet, you can vote for it here.