auto-increment using loopback.js and MongoDB - mongodb

Question

i want to increase mongodb document number automatically using loopback.
I made function in mongo
function getNextSequence(name) {
var ret = db.counters.findAndModify(
{
query: { _id: name },
update: { $inc: { seq: 1 } },
new: true
}
);
return ret.seq;
}
db.tweet.insert(
{
"_id" : getNextSequence("userid"),
"content": "test",
"date": "1",
"ownerUsername": "1",
"ownerId": "1"
}
)
It is working in mongo shell.
However when I insert using loopback.js browser (http://localhost:3000/explorer/), It is not working.
400 error(SytaxError) code is showing.
I can not use mongo function in loopback rest API ?
I think problem is quotes in this line getNextSequence("userid"),

Answer 1

Create a collection counters with properties value and collection
{
"name": "counters",
"base": "PersistedModel",
"idInjection": true,
"options": {
"validateUpsert": true
},
"properties": {
"type": "number",
"collection": "string"
},
"validations": [],
"relations": {},
"acls": [
{
"accessType": "*",
"principalType": "ROLE",
"principalId": "$everyone",
"permission": "ALLOW"
}
],
"methods": []
}
Now supposing your auto-increment collection name tweets.
Insert this value to counters.
{
"value" : 0,
"collection" : "tweet"
}
Now common/models/tweet.js
tweet.observe('before save', function (ctx, next) {
var app = ctx.Model.app;
//Apply this hooks for save operation only..
if(ctx.isNewInstance){
//suppose my datasource name is mongodb
var mongoDb = app.dataSources.mongodb;
var mongoConnector = app.dataSources.mongodb.connector;
mongoConnector.collection("counters").findAndModify({collection: 'tweet'}, [['_id','asc']], {$inc: { value: 1 }}, {new: true}, function(err, sequence) {
if(err) {
throw err;
} else {
// Do what I need to do with new incremented value sequence.value
//Save the tweet id with autoincrement..
ctx.instance.id = sequence.value.value;
next();
} //else
});
} //ctx.isNewInstance
else{
next();
}
}); //Observe before save..

Answer 2

I would love to add 1 more point to Robins Answer,you can add upsert:true so that it automatically creates the document if it doesn't exist
tweet.observe('before save', function (ctx, next) {
var app = ctx.Model.app;
//Apply this hooks for save operation only..
if(ctx.isNewInstance){
//suppose my datasource name is mongodb
var mongoDb = app.dataSources.mongodb;
var mongoConnector = app.dataSources.mongodb.connector;
mongoConnector.collection("counters").findAndModify({collection: 'tweet'}, [['_id','asc']], {$inc: { value: 1 }}, {new: true,upsert:true}, function(err, sequence) {
if(err) {
throw err;
} else {
// Do what I need to do with new incremented value sequence.value
//Save the tweet id with autoincrement..
ctx.instance.id = sequence.value.value;
next();
} //else
});
} //ctx.isNewInstance
else{
next();
}
}); //Observe before save..

Answer 3

You can do something like in this example for loopback 4
let last_record = await this.testRepository.findOne({order: ['id DESC']});
if(last_record) invoice.id = last_record.id+1;
This will generate your model with the property:
#property({
type: 'number',
id: true,
default: 1,
generated: false
})
id: number;
Hopefully, this helps, please write me if there is any other code. Thanks

Answer 4

If you want to use MongoDB operators directly in loopback methods you need to enable the option "allowExtendedOperators", you can do so on a per model basis or at the data source level (will apply to all models using the data source).
datasources.json:
"MongoDs": {
"host": "127.0.0.1",
"port": 27017,
"url": "mongodb://localUser:MYPASSWORD!#127.0.0.1:27017/test-database",
"database": "test-database",
"password": "MYPASSWORD!",
"name": "MongoDs",
"user": "localUser",
"useNewUrlParser": true,
"connector": "mongodb",
"allowExtendedOperators": true
},