title: MongoDB: find non exisiting or specific value
body: <p>I know about <code>$exists</code> in MongoDB but I don't know how to combine it with an OR logic within find().</p>
  
  <p>I want to find all transactions where the <code>base_currency</code> field is either not existing or has a specific value. At the same time <code>trade_currency</code> must have a specific value. Here's what I tried but doesn't work.</p>
  
  <pre><code>txs = db.transactions.find({
      'base_currency': { $or: [{ $exists: true }, { $eq: base_currency }]},
      'trade_currency': currency
  }).sort([('datetime_closed', 1)])
  </code></pre>