Cannot assign to read only property '_id' - node.js

Question

get Error Cannot assign to read only property '_id' when insert data in mongodb using node js
Cannot assign to read only property '_id' of
{"Diagnosis":"vvvvv",
"Treatment":[{
"name":"treatment1",
"SubTreatment":[{
"name":"subtreatment1",
"SubTreatment2" : [{
"name":"subtreatmentexp1",
"$$hashKey":"object:27","Name":"vvvvv"
}],
"$$hashKey":"object:22","Name":"vvvvv"
}],
"$$hashKey":"object:12",
"Name":"vvvvv"
}
]}
please help me to solve this error
my code is
app.post('/post', function (req, res) {
var diagnosis;
var obj = req.headers['data'];
var temp = JSON.parse(obj)
console.log("Received body data:");
console.log(obj);
MongoClient.connect(url, function (err, db) {
assert.equal(null, err);
insertDocument(temp, db, function () {
db.close();
});
});
var insertDocument = function (obj, db, callback) {
try {
console.log(obj);
db.collection('diagnosis').remove();
db.collection('diagnosis').insertOne(obj, function (err, result) {
assert.equal(err, null);
console.log("Inserted a document into the Diagnosis collection.");
callback();
});
res.send("Inserted a document into the Diagnosis collection.");
} catch (err) {
console.log(err);
}
};
})
my function to call API is
$scope.post = function () {
var diagnosis = $scope.Treatments;
var httpRequest = $http({
method: 'POST',
url: '/post',
headers: {
'Content-Type': 'application/json; charset=utf-8',
'data': JSON.stringify($scope.Treatments)
},
}).success(function (data, status) {
});
};
Treatments array....
$scope.Treatments =
{
Diagnosis: '',
Treatment: [
{
name: 'treatment1',
SubTreatment: [
{
name: 'subtreatment1',
SubTreatment2: [{ name: 'subtreatmentexp1' }]
}
],
}
]
};
Generated JSON to insert data is
{
"Diagnosis": "sss",
"Treatment": [
{
"name": "treatment1",
"SubTreatment": [
{
"name": "subtreatment1",
"SubTreatment2": [
{
"name": "subtreatmentexp1",
"$$hashKey": "object:27",
"Name": "sss"
}
],
"$$hashKey": "object:22",
"Name": "sss"
}
],
"$$hashKey": "object:12",
"Name": "sss"
}
]
}