MongoDB – how to query for a nested item inside a collection?

Use dot notation (e.g. advertisers.name) to query and retrieve fields from nested objects:

db.agencies.find({
 "advertisers.created_at": {
   $gte: start,
   $lt: end
  }
 },
{
 _id: 1,
  program_ids: 1,
  "advertisers.name": 1
 }
}).limit(1).toArray();

Reference: Retrieving a Subset of Fields
and Dot Notation