title: PyMongo Aggregation "AttributeError: 'dict' object has no attribute '_txn_read_preference'"
body: <p>I'm sure there is an error in my code since I'm a newby to pyMongo, but I'll give it a go.  The data in MongoDB is 167k+ and is as follows:</p>
  <pre><code>{'overall': 5.0,
   'reviewText': {'ago': 1,
                  'buy': 2,
                  'daughter': 1,
                  'holiday': 1,
                  'love': 2,
                  'niece': 1,
                  'one': 2,
                  'still': 1,
                  'today': 1,
                  'use': 1,
                  'year': 1},
   'reviewerName': 'dcrm'}
  </code></pre>
  <p>I would like to get a tally of terms used within that <code>reviewText</code> field for all 5.0 ratings.  I have run the following code and I get the error that follows.  Any insight?</p>
  <pre class="lang-rb prettyprint-override"><code>#1 Find the top 20 most common words found in 1-star reviews.
  
  aggr = [{&quot;$unwind&quot;: &quot;$reviewText&quot;}, 
          {&quot;$group&quot;: { &quot;_id&quot;: &quot;$reviewText&quot;, &quot;word_freq&quot;: {&quot;$sum&quot;:1}}}, 
          {&quot;$sort&quot;: {&quot;word_freq&quot;: -1}},
          {&quot;$limit&quot;: 20},
          {&quot;$project&quot;: {&quot;overall&quot;:&quot;$overall&quot;, &quot;word_freq&quot;:1}}]
  disk_use = { 'allowDiskUse': True }
  findings = list(collection.aggregate(aggr, disk_use))
  
  for item in findings:
      p(item)
  </code></pre>
  <p>As you can see, I came across the 'allDiskUse' component since I seemed to exceed the 100MB threshold.  But the error that I get is:</p>
  <pre class="lang-rb prettyprint-override"><code>AttributeError: 'dict' object has no attribute '_txn_read_preference'
  </code></pre>