MongoDB Query - limit fields where name matches pattern

Question

I've read everything I can find about Projection in MongoDB. I'm hoping this is simple and I just missed it due to the overwhelming flexibility of Mongo queries.

In our MySql database, we've adopted a business practice of having "hidden" fields be prefixed with an underscore. Our application knows how to hide these fields.

Moving some data to mongo, I need to retrieve the documents, with ALL underscore prefixed fields omitted. Of course this should be done in the query rather than document manipulation after retrieval.

All the operators like $regex, $in, $all seem to apply to values. I need to build a projection that ignores an unknown number of fields based on their name. Something like:

db.coll.find({}, {"_*": 0})

Of course that doesn't work, but explains the idea.

I should note: this is necessary because the documents are editable by our application users, so I have no idea what the schema might look like. I do know our "internal" fields are prefixed with an _, and those need to be protected by omission from the editor.

Hope it's easy...

Answer 1

You can have a separate field as hidden_fields or something. See the following schema.

{_id: 'myid1', hidden_fields: {"_foo": "bar", "_foo2": "bar2"}, key1: value1 ...}

Now on the basis of above schema just do,

db.collection.find({ ... }, {hidden_fields: 1})

This will display hidden fields. Also you can have indexes on fields within sub documents so no loss in terms of performance as well.