Increment a field in model using remote method in loopback? - mysql

Question

There is a field called counter in a model and whenever if I call a custom remote method like
Request URL
http://loopback:3000/api/models/increment_counter
Request body
EMPTY
Response
{
"counter" : [Value after Increment]
}
Currently to increment First i have to get the counter value from db and increment it one and update the couter value, This involves two queries, Is it possible to do it in a single NodeAPI call like the below mysql query.I am currently using Mysql as DB.
mysql_query("
UPDATE model
SET counter = counter + 1
WHERE model_id = '1'
");
Thank you

Answer 1

Given the MySQL syntax you want, you seem to need an atomic counter.
database transactions
With the MySQL connector, you can use database transactions. It is supported by the MySQL connector. It's a bit overkill for just atomic counters but it will get the job done.
Here is an example, inside a custom remote method
MyModel.someRemoteMethodForIncrementing = function(callback) {
// Start the transaction
MyModel.beginTransaction({
isolationLevel: MyModel.Transaction.READ_COMMITTED
}, function(err, tx) {
// Retrieve the current counter value as part of the transaction
MyModel.findById(id, {
transaction: tx
}, function(err, data) {
if (err) {
console.log(err);
return tx.rollback(function(err) {
callback(err);
});
}
// Increment the counter as part of the transaction
var inc = data.counter + 1;
MyModel.updateAttributes({
counter: inc
}, {
transaction: tx
}, function(err, newData) {
if (err) {
console.log(err);
return tx.rollback(function(err) {
callback(err);
});
}
// Commit the transaction to make it happen
tx.commit(function(err) {
if (err) return callback(err);
// Counter should have been incremented
callback();
});
});
});
});
};
I haven't tested it but it should work, let me know
extented operators
In the future, you will be able to use the $inc (increment) extended operator but so far it's only supported by MongoDB connector, not MySQL.
Just for reference, here is the syntax (works only with MongoDB)
PUT api/Model/:id?filter={"$inc":{"name":propertyToIncrement, "count":incrementAmount}}
There is an ongoing PR that I am trying to get landed to get MySQL support, but there are many things to be done before it can get merged.

Answer 2

Yes you can do it in a single loopback remote method call. Assume you are sending an Id in your request
yourModel.remoteMethod = function(data, cb){
yourModel.findById(data.id, function(err, object){
if(err){
cb(err, null);
}
object.counter += 1;
object.save(function(saveErr, afterSaveObj){
cb(saveErr, afterSaveObj.counter);
})
});
}
here cb is a callback which loopback passes to your remoteMethod.