title: Using mongoimport to import a CSV file is it possible to import NULLs?
body: <p>I'm trying to import from CSV into mongodb 3.4 using mongoimport, and I would like for empty columns to be imported as null values for the fields.</p>
  
  <p>I was under the impression from the mongoimport documentation, that if --ignoreBlanks was not specified that I would get the behavior I wanted. </p>
  
  <blockquote>
    <p>--ignoreBlanks</p>
  
  <pre><code>Ignores empty fields in csv and tsv exports. If not 
  specified, mongoimport creates fields without values in imported
  documents.
  </code></pre>
  </blockquote>
  
  <p>However, when I try to load this sample data without --ignoreblanks:</p>
  
  <pre><code>field_1.string(),field_2.int32(),field_3.string()
  A,5,B
  C,,D
  E,7,F
  </code></pre>
  
  <p>then I get an error on any field that is not a string.</p>
  
  <pre><code>mongoimport --collection null_test --type csv --headerline --columnsHaveTypes --file null_test.csv --verbose 
  
  2017-08-08T16:55:42.989+0000    filesize: 67 bytes
  2017-08-08T16:55:42.989+0000    using fields: field_1,field_2,field_3
  2017-08-08T16:55:43.001+0000    connected to: localhost:27017
  2017-08-08T16:55:43.001+0000    ns: DEV.null_test
  2017-08-08T16:55:43.001+0000    connected to node type: standalone
  2017-08-08T16:55:43.001+0000    using write concern: w='1', j=false, fsync=false, wtimeout=0
  2017-08-08T16:55:43.001+0000    using write concern: w='1', j=false, fsync=false, wtimeout=0
  2017-08-08T16:55:43.001+0000    Failed: type coercion failure in document #1 for column 'field_2', could not parse token '' to type int32
  2017-08-08T16:55:43.001+0000    imported 0 documents
  </code></pre>
  
  <p>For fields that are strings it loads an empty string rather than a null.</p>
  
  <p>What am I doing wrong here? Is it possible to load fields as NULL using mongoimport with CSV or TSV files?</p>
  
  <p>For what it's worth, if I use mongoimport to import a json file with NULLs, it imports them just fine as actual NULLs.</p>
  
  <pre><code>[
      {
          "field1": "A",
          "field2": null,
          "field3": "C"
      },
      {
          "field1": 1,
          "field2": 5,
          "field3": null
      }
  ]
  </code></pre>