Cannot enqueue Query after invoking quit when nesting promises - mysql

Question

I am trying to iterate through a MySQL query result and make subsequent queries in order to build out my data model. Each object requires multiple queries, therefore I am chaining promises.
The problem occurs when I nest a second set of promises.
So first I am getting a list of the objects that need to be retrieved using g.getSnapshotIds. Then I iterate through those and use the snapshotId to retrieve a full snapshot.
var gData = {};
g.getSnapshotIds(data.gId, data.userId)
.then(function(value) {
gData = value;
for ( var snapshot in value ) {
var snapshotId = value[snapshot].snapshotId;
var snapshot = {};
g.getSnapshotFull(snapshotId)
.then(function(value) {
console.log(value);
return g.getTs(snapshotId);
})
.then(function(value) {
for ( var te in value ) {
var name = value[te].t;
snapshot[name] = value[te].value;
}
console.log(snapshot);
})
.catch(function(err) {
console.log('Error:', err);
});
}
g.close();
})
.catch(function(err) {
console.log('Error:', err);
});
I am able to call g.getSnapshotFull on each ID, but when I try to move on to the next query (g.getTs(snapshotId)) it gives me the error:
Error: Cannot enqueue Query after invoking quit.
I have no idea why the MySQL connection is closing before all queries are done. Shouldn't everything inside the for loop execute sequentially before moving on?
If I comment out g.close(), I don't get the error, but the process doesn't end.
These are the relevant query methods:
class gDB {
close() {
return new Promise(function(resolve, reject) {
db.end(function(error) {
if ( error ){
reject(new Error(error));
}
// For some reason it is necessary to reestablish this
db = mysql.createConnection({
host: process.env.DBHOST,
user: process.env.DBUSER,
password: process.env.DBPASS,
database: process.env.DBNAME,
ssl: {
ca: fs.readFileSync(__dirname + '/' + process.env.DBCA)
}
});
resolve(true);
});
});
}
getSnapshotIds(gId, uId) {
return new Promise(function(resolve, reject) {
var sql = 'SELECT id AS snapshotId FROM snapshots WHERE gId=' + db.escape(gId) + ' AND uId=' + db.escape(uId) + ' ORDER BY timestamp DESC';
db.query(sql, function (error, results, fields) {
if (error) {
db.destroy();
reject(new Error(error));
} else {
resolve(results);
}
});
});
}
getSnapshotFull(snapshotId) {
return new Promise(function(resolve, reject) {
var sql = 'SELECT s.id AS snapshotId, s.timestamp, s.gId, s.uId, s.clientId FROM snapshots s INNER JOIN controls c ON s.id = c.snapshotId INNER JOIN weathers w ON s.id = w.snapshotId WHERE s.id=' + db.escape(snapshotId);
db.query(sql, function (error, results, fields) {
if (error) {
db.destroy();
reject(new Error(error));
} else {
resolve(results[0]);
}
});
});
}
getTs(snapshotId) {
return new Promise(function(resolve, reject) {
var sql = 'SELECT t.t, st.value FROM snapshots s LEFT JOIN snapshot_t st ON s.id = st.snapshotId INNER JOIN ts t ON st.tId = t.id WHERE s.id=' + db.escape(snapshotId);
db.query(sql, function (error, results, fields) {
if (error) {
db.destroy();
reject(new Error(error));
} else {
resolve(results);
}
});
});
}

Answer 1

The problem you are having is for loops are synchronous while promises are asynchronous. What is going on is you are creating a bunch of promises that are waiting for something to happen (the promise to receive data), then the for loop ends (before any of the promises finish) and you then call close. What you'll want to do is something similar to the below.
var gData = {};
g.getSnapshotIds(data.gId, data.userId)
.then(function (value) {
gData = value;
var promises = [];
for (var snapshot in value) {
var snapshotId = value[snapshot].snapshotId;
var snapshot = {};
var promise = g.getSnapshotFull(snapshotId)
.then(function (value) {
console.log(value);
return g.getTs(snapshotId);
})
.then(function (value) {
for (var te in value) {
var name = value[te].t;
snapshot[name] = value[te].value;
}
console.log(snapshot);
});
promises.push(promise);
}
return Promise.all(promises);
})
.then(function (values) {
g.close();
console.log(values);
})
.catch(function (err) {
console.log('Error:', err);
});
What solves this is saving the promise and then using Promise.all(promises) to wait for all the promises to finish. The last then block will have the results of all of the promises and that is where you can close your database connection.