title: duplicate key error index in mongoengine
body: <p>I have a <code>Page</code> model as follow and the <code>page_id</code> field isn't <code>unique</code> but I can't create an instance from <code>Page</code> and this error rises 
  <br>
          <code>OperationError: Could not save document (insertDocument :: caused by ::11000 E11000 duplicate key error index: shopify.page.$pageId_1  dup key: { : null })</code>
  I have no idea that why the duplicate key error rises when the <code>page_id</code> field isn't <code>unique</code>
  <br><br>
  The <code>Site</code> model : 
  <br></p>
  
  <pre><code>class Page(Document):
  
      # page identity
      page_id = StringField()
      store = ReferenceField('Store')
      is_product = BooleanField(default = False)
      is_homepage = BooleanField(default = False)
      product = ReferenceField('Product')
      requests = ListField(EmbeddedDocumentField('Request'))
  
      # page stat
      visitors = IntField(default = 0)
      views = IntField(default = 0)
      past_days = ListField(EmbeddedDocumentField('DayStat'))
  </code></pre>