title: MongoDB: use MongoImport with csv to update single field only
body: <p>I am trying to update a single field in each document in my collection using a csv and Mongoimport with –upsert included.
  However the process removes all other fields in the document.</p>
  
  <p>I have a Books Collection with documents like:</p>
  
  <pre><code>{
  
      "_id" : "knOIv8ZUUK", 
      "Price" : 2.2, 
      "Title" : "Rats Ahoy"
  }
  
  {
  
      "_id" : "okYEGuWznv", 
      "Price" : 3.3, 
      "Title" : "Friendly Fish"
  }
  </code></pre>
  
  <p>a csv file:</p>
  
  <pre><code>_id,Price
  
  knOIv8ZUUK,2.2
  
  okYEGuWznv,3.3
  </code></pre>
  
  <p>And import using:</p>
  
  <pre><code>mongoimport  --db local  --collection Books --upsert  --type csv  
               --headerline  --file c:\import\newPrice
  </code></pre>
  
  <p>With results deleting the Title field</p>
  
  <pre><code>{ 
  
      "_id" : "knOIv8ZUUK", 
      "Price" : 2.2
  }
  
  { 
  
      "_id" : "okYEGuWznv", 
      "Price" : 3.3
  }
  </code></pre>
  
  <p>I, incorrectly, thought Upsert would just update an imported field.
  So is there another process I can use to update just 1 field in large number of documents?
  thanks</p>