title: Mongo db 11000 error comes , when their is no unique field define
body: <p>I have added a form data into mongoDB first time data is save but after that this error come , i dont have any field with unique type</p>
  
  <pre><code>{ [MongoError: insertDocument :: caused by :: 11000 E11000 duplicate key error index: test.employees.$employeeId_1  dup key: { : null }]
    name: 'MongoError',
    message: 'insertDocument :: caused by :: 11000 E11000 duplicate key error index: test.employees.$employeeId_1  dup key: { : null }',
    driver: true,
    code: 11000,
    index: 0,
    errmsg: 'insertDocument :: caused by :: 11000 E11000 duplicate key error index: test.employees.$employeeId_1  dup key: { : null }',
    getOperation: [Function],
    toJSON: [Function],
    toString: [Function] }
  </code></pre>
  
  <p>Here is my schema with no unique field </p>
  
  <pre><code>var employee =  
      {
          empCode : { type : String  , required : true },
          firstName : { type : String  , required : true },
          lastName : { type : String },
          email :  { type : String  },
          DOJ :{ type : Date , default: Date.now },
          DOB :{ type : Date  },
          phoneNo : { type : String },
          authentication : 
              {
                  username : { type : String },
                  password : { type : String }
              },
          designation : 
              {
                  designationID: [{type : mongoose.Schema.ObjectId , ref:'designation'}],
                  effFrom :{ type : Date , default: Date.now },
                  effTo : { type : Date }             
              },
          grade : 
              {
                  gradeID : [{type : mongoose.Schema.ObjectId, ref:'grade'}],
                  effFrom :{ type : Date , default: Date.now },
                  effTo : { type : Date }
              },
          Department :
              {
                  departmentID : [{type : mongoose.Schema.ObjectId ,ref:'department'}],
                  effFrom :{ type : Date , default: Date.now },
                  effTo : { type : Date }
              },
          manager : 
              {
                  managerID : [{type : mongoose.Schema.ObjectId , ref:'employee'}],
                  effFrom :{ type : Date , default: Date.now },
                  effTo : { type : Date }
              },
          role :
              {   
                  roleID : [{type : mongoose.Schema.ObjectId , ref:'role'}],
                  effFrom :{ type : Date , default: Date.now },
                  effTo : { type : Date }
              },
  
          status : {type:Boolean}
      }
  </code></pre>
  
  <p>i couldnt find what field is giving error , please suggest me if you can find the error it will be great help for me thanks in advance </p>