MongoDB aggregation framework match OR

In this particular case, where you are $or-ing the same field, the $in operator would be a better choice, also because it increases readability:

$match: { 
  'author': { 
    $in: ['dave','john'] 
  } 
}

According to the MongoDB docs, using $in is recommended in this case:

$or versus $in

When using $or with <expressions> that are equality checks for the value of the same field, use the $in operator instead of the $or operator.

https://docs.mongodb.com/manual/reference/operator/query/or/#or-versus-in

$match: { $or: [{ author: 'dave' }, { author: 'john' }] }

Like so, since the $match operator just takes what you would normally put into the find() function