title: Update one document by _id (Invalid BSON field name _id)
body: <p>I'm trying to update an document using <code>updateOne</code> method:</p>
  
  <pre><code>UpdateResult r = coll.updateOne(
      eq("_id", id),
      this.getMapper().mapToMongoDocumentEntry(entity)
  );
  </code></pre>
  
  <p>Nevertheless, I'm getting an exception telling me:</p>
  
  <blockquote>
    <p>Invalid BSON field name _id</p>
  </blockquote>
  
  <p><code>mapToMongoDocumentEntity</code> returns a <code>Document</code> like: </p>
  
  <pre><code>Document{
    _id=588b0d7108980f004323ca73,
    username=user,
    password=.---,
    cname=----,
    sname=,
    mail=mail,
    creation=Fri Jan 27 09:05:52      UTC 2017,
    validation=null
  }
  </code></pre>
  
  <p><code>mapToMongoDocumentEntry</code> code:</p>
  
  <pre><code>public Document mapToMongoDocumentEntry(User entity) {
      Document result = new Document();
  
      if (entity.getId() != null)
          result.put(UserEntityMongoDocumentMapper.FIELD_ID, new ObjectId(entity.getId()));
  
      result.put(UserEntityMongoDocumentMapper.FIELD_USER, entity.getUser());
      result.put(UserEntityMongoDocumentMapper.FIELD_PASSWORD, entity.getPasswd());
      result.put(UserEntityMongoDocumentMapper.FIELD_COMMONNAME, entity.getCname());
      result.put(UserEntityMongoDocumentMapper.FIELD_SURNAME, entity.getSname());
      result.put(UserEntityMongoDocumentMapper.FIELD_MAIL, entity.getMail());
      result.put(UserEntityMongoDocumentMapper.FIELD_CREATION, entity.getCreation());
      result.put(UserEntityMongoDocumentMapper.FIELD_VALIDATION, entity.getValidation());
  
      return result;
  }
  </code></pre>
  
  <p>Any ideas?</p>