How to mock ember cli mirage for json api while insert and delete

i am using mirage for mock server with ember application

while doing insert and delete am getting this error

Assertion Failed: An adapter cannot assign a new id to a record that already has an id. <employee-demo@model:employee::ember565:4> had id: 4 and you tried to update it with null. This likely happened because your server returned data in response to a find or update that had a different id than the one you sent.

I have following code in config.js of mirage

this.get(‘/employees’, function(store, request) { return { data: store.employees.map(attrs => ( {type: ‘employees’, id: attrs.id, attributes: attrs } )) };

  });

  this.get('/employees/:id',function(db, request) {
    return {
        data: db.employees.map(attrs => (
            {type: 'employees', id: attrs.id, attributes: attrs }
        ))
    };
  });

  this.post('/employees', function(store, request) {
      let attrs = JSON.parse(request.requestBody).employee;
      let employee = store.employees.insert(attrs);

      console.log(employee);

      /*return {
        data: employee
      };*/

      return {
          data: store.employees.map(attrs => (
              {type: 'employees', id: attrs.id, attributes: attrs }
          ))
      };
  });
  this.del('/employees/:id',function(store, request) {
    //console.log(attrs.id);
    var id = request.params.id;
    console.log(id);
    store.employees.remove(id);


    return {
        data: store.employees.map(attrs => (
            {type: 'employees', attributes: attrs }
        ))
    };


  });