How to show useful error messages from a database

问题:

Using Phonegap you can set a function to be called back if the whole database transaction or the individual SQL statement errors. I'd like to know how to get more information about the error.

I have one generic error-handling function, and lots of different SELECTs or INSERTs that may trigger it. How can I tell which one was at fault? It is not always obvious from the error message.

My code so far is...

function get_rows(tx) {
   tx.executeSql("SELECT * FROM Blah", [], lovely_success, statement_error);
}
function add_row(tx) {
   tx.executeSql("INSERT INTO Blah (1, 2, 3)", [], carry_on, statement_error);
}
function statement_error(tx, error) {
   alert(error.code + ' / ' + error.message);
}

From various examples I see the error callback will be passed a transaction object and an error object. I read that .code can have the following values:

• UNKNOWN_ERR = 0
• DATABASE_ERR = 1
• VERSION_ERR = 2
• TOO_LARGE_ERR = 3
• QUOTA_ERR = 4
• SYNTAX_ERR = 5
• CONSTRAINT_ERR = 6
• TIMEOUT_ERR = 7

Are there any other properties/methods of the error object?
What are the properties/methods of the transaction object at this point?

I can't seem to find a good online reference for this. Certainly not on the Phonegap website!

回答1:

http://docs.phonegap.com/en/2.2.0/cordova_storage_storage.md.html#SQLError

The SQLError object is thrown when an error occurs when manipulating a database.

This object has two properties:

• code (one of your described constants)
• message: A description of the error.

So in your code error.message will give you a nice description of what went wrong in which transaction.

I think that's exactly what you want, isn't it?

Best regards, F481

回答2:

Transaction object

The only thing you can do with the transaction object is call its .executeSql() method, as far as I can ascertain. I cannot find any properties of this object.

Error object

The error object has a .code property which contains a number. You can either check the numerical value (see my original question above) or use something like:
if (error.code == error.DATABASE_ERR) alert('nasty database error')

The .message property is a string and may return something like this:

• could not prepare statement (1 near "wibble": syntax error)
• could not prepare statement (1 no such table: MyyTable)
• could not prepare statement (1 table MyTable has no column named MyColunm)
• could not execute statement (19 constraint failed)

Other messages are possible! This is just the few I spotted when debugging in Chrome. I notice in Phonegap the messages are briefer: "no such table: MyyTable"

There are two sets of success/error callbacks

Also note that there is another database error callback on the initial call to .transaction(). Your function will only be returned an error object (no transaction object).

The error's .code will be zero and the .message will be "the statement callback raised an exception or statement error callback did not return false".

So remember to have your statement callbacks (function mentioned inside .executeSql such as my statement_error in the code example of my original question) return true or false depending on whether you want your transaction error callback (second function mentioned inside .transaction) to be hit. The 'success' callback you specified (third one inside .transaction) will be run if you return true (or don't return anything).