How to organize WebOS SQL code that depends on previous results? - database

Question

I want to do a simple WebOS Mojo app that adds dates to a database, but before that it needs to check few conditions by interrogating the database. Given the asynchronous mode of accessing the database in WebOS I'm wondering what is the most efficient and short way of writing such code.
For example I need to make sure the new date is not already in the database. Then I need to get the closest date so that I can compute the difference in days and throw an error if the difference is too small. Then I need to insert the new date and then compute an average.
This would be easy to do in a synchronous way of accessing the db, but I don't really like the idea of writing parts of the code in multiple success handlers of the different sql statements executed. Is there a more elegant solution?

Answer 1

You can use inline callbacks to the HTML 5 relational database functions:
function createProject(project, onSuccess) {
if (project.projectId)
throw new StorageError("project already exists");
if (project.path)
throw new StorageError("project already has a path");
project.projectId = ++Tracker.maxLocalId * 4096;
project.path = calcNextProjectPath();
project.normalize();
Tracker.db.transaction(
function (transaction) {
transaction.executeSql(
"INSERT OR ROLLBACK INTO item (dbId, path, hasChildren, kind, summaryText, place, work, responsible, responsibleClass) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)",
[project.projectId, project.path, project.hasChildren, project.kind, project.title, project.place, project.work, project.responsible, project.responsibleClass],
function (transaction, resultSet) {
Mojo.Log.info("DB: inserted new project item", resultSet.insertId, project.title);
transaction.executeSql(
"INSERT OR ROLLBACK INTO project (projectId, accountId, title, backendKind, backendStatus, backendLastChanged, lastSuccessfulDown) \
VALUES (?, ?, ?, ?, ?, ?, ?)",
[project.projectId, project.accountId, project.title, project.backendKind, project.backendStatus, project.backendLastChanged, project.lastSuccessfulDown],
function (transaction, resultSet) {
Warn.logInfo("created new project", "projectId=" + resultSet.insertId, project.title);
if (onSuccess)
onSuccess();
},
function (transaction, sqlError) {
Warn.bannerError("create", $L("Quit and reset your phone."), "project row insertion failed: ", sqlError.message, sqlError.code, project);
return true; // abort whole transaction
}
);
},
function (transaction, sqlError) { // failure of insert project item
if (sqlError.code === 1 && sqlError.message === "constraint failed" && Mojo.appInfo.id !== "com.outlinetracker.outlinetracker") {
upgradeAlert(true);
} else {
Warn.bannerError("create", $L("Quit and reset your phone."), "project item insertion failed: ", sqlError.message, sqlError.code);
}
return true; // abort whole transaction
}
);
}, // end transaction function
function (sqlError) { // seems to only be called for exceptions in callbacks
Warn.logError($L("Quit and reset your phone."), "transaction 'insert project' failed", sqlError.message, sqlError.code);
}
); // end transaction call
}