title: Unique index in mongoDB 3.2 ignoring null values
body: <p>I want to add the unique index to a field ignoring null values in the unique indexed field and ignoring the documents that are filtered based on partialFilterExpression.</p>
  
  <p>The problem is Sparse indexes can't be used with the Partial index. </p>
  
  <p>Also, adding unique indexes, adds the null value to the index key field and hence the documents can't be ignored based on $exist criteria in the PartialFilterExpression.</p>
  
  <p>Is it possible in MongoDB 3.2 to get around this situation?</p>