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A Cocoa / Objective-C wrapper around SQLite

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README.markdown

FMDB

This is an Objective-C wrapper around SQLite: http://sqlite.org/

The FMDB Mailing List:

http://groups.google.com/group/fmdb

Read the SQLite FAQ:

http://www.sqlite.org/faq.html

Since FMDB is built on top of SQLite, you're going to want to read this page top to bottom at least once. And while you're there, make sure to bookmark the SQLite Documentation page: http://www.sqlite.org/docs.html

Automatic Reference Counting (ARC) or Manual Memory Management?

You can use either style in your Cocoa project. FMDB Will figure out which you are using at compile time and do the right thing.

Usage

There are three main classes in FMDB:

  1. FMDatabase - Represents a single SQLite database. Used for executing SQL statements.
  2. FMResultSet - Represents the results of executing a query on an FMDatabase.
  3. FMDatabaseQueue - If you're wanting to perform queries and updates on multiple threads, you'll want to use this class. It's described in the "Thread Safety" section below.

Database Creation

An FMDatabase is created with a path to a SQLite database file. This path can be one of these three:

  1. A file system path. The file does not have to exist on disk. If it does not exist, it is created for you.
  2. An empty string (@""). An empty database is created at a temporary location. This database is deleted with the FMDatabase connection is closed.
  3. NULL. An in-memory database is created. This database will be destroyed with the FMDatabase connection is closed.

    FMDatabase *db = [FMDatabase databaseWithPath:@"/tmp/tmp.db"];

Opening

Before you can interact with the database, it must be opened. Opening fails if there are insufficient resources or permissions to open and/or create the database.

if (![db open]) {
    [db release];
    return;
}

Executing Updates

Any sort of SQL statement which is not a SELECT statement qualifies as an update. This includes CREATE, PRAGMA, UPDATE, INSERT, ALTER, COMMIT, BEGIN, DETACH, DELETE, DROP, END, EXPLAIN, VACUUM, and REPLACE statements (plus many more). Basically, if your SQL statement does not begin with SELECT, it is an update statement.

Executing updates returns a single value, a BOOL. A return value of YES means the update was successfully executed, and a return value of NO means that some error was encountered. If you use the -[FMDatabase executeUpdate:error:withArgumentsInArray:orVAList:] method to execute an update, you may supply an NSError ** that will be filled in if execution fails. Otherwise you may invoke the -lastErrorMessage and -lastErrorCode methods to retrieve more information.

Executing Queries

A SELECT statement is a query and is executed via one of the -executeQuery... methods.

Executing queries returns an FMResultSet object if successful, and nil upon failure. Like executing updates, there is a variant that accepts an NSError ** parameter. Otherwise you should use the -lastErrorMessage and -lastErrorCode methods to determine why a query failed.

In order to iterate through the results of your query, you use a while() loop. You also need to "step" from one record to the other. With FMDB, the easiest way to do that is like this:

FMResultSet *s = [db executeQuery:@"SELECT * FROM myTable"];
while ([s next]) {
    //retrieve values for each record
}

You must always invoke -[FMResultSet next] before attempting to access the values returned in a query, even if you're only expecting one:

FMResultSet *s = [db executeQuery:@"SELECT COUNT(*) FROM myTable"];
if ([s next]) {
    int totalCount = [s intForColumnIndex:0];
}

FMResultSet has many methods to retrieve data in an appropriate format:

  • intForColumn:
  • longForColumn:
  • longLongIntForColumn:
  • boolForColumn:
  • doubleForColumn:
  • stringForColumn:
  • dateForColumn:
  • dataForColumn:
  • dataNoCopyForColumn:
  • UTF8StringForColumnIndex:
  • objectForColumn:

Each of these methods also has a {type}ForColumnIndex: variant that is used to retrieve the data based on the position of the column in the results, as opposed to the column's name.

Typically, there's no need to -close an FMResultSet yourself, since that happens when either the result set is deallocated, or the parent database is closed.

Closing

When you have finished executing queries and updates on the database, you should -close the FMDatabase connection so that SQLite will relinquish any resources it has acquired during the course of its operation.

[db close];

Transactions

FMDatabase can begin and commit a transaction by invoking one of the appropriate methods or executing a begin/end transaction statement.

Data Sanitization

When providing a SQL statement to FMDB, you should not attempt to "sanitize" any values before insertion. Instead, you should use the standard SQLite binding syntax:

INSERT INTO myTable VALUES (?, ?, ?)

The ? character is recognized by SQLite as a placeholder for a value to be inserted. The execution methods all accept a variable number of arguments (or a representation of those arguments, such as an NSArray, NSDictionary, or a va_list), which are properly escaped for you.

Alternatively, you may use named parameters syntax:

INSERT INTO myTable VALUES (:id, :name, :value)

The parameters must start with a colon. SQLite itself supports other characters, but internally the Dictionary keys are prefixed with a colon, do not include the colon in your dictionary keys.

NSDictionary *argsDict = [NSDictionary dictionaryWithObjectsAndKeys:@"My Name", @"name", nil];
[db executeUpdate:@"INSERT INTO myTable (name) VALUES (:name)" withArgumentsInDictionary:argsDict];

Thus, you SHOULD NOT do this (or anything like this):

[db executeUpdate:[NSString stringWithFormat:@"INSERT INTO myTable VALUES (%@)", @"this has \" lots of ' bizarre \" quotes '"]];

Instead, you SHOULD do:

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", @"this has \" lots of ' bizarre \" quotes '"];

All arguments provided to the -executeUpdate: method (or any of the variants that accept a va_list as a parameter) must be objects. The following will not work (and will result in a crash):

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", 42];

The proper way to insert a number is to box it in an NSNumber object:

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:42]];

Alternatively, you can use the -execute*WithFormat: variant to use NSString-style substitution:

[db executeUpdateWithFormat:@"INSERT INTO myTable VALUES (%d)", 42];

Internally, the -execute*WithFormat: methods are properly boxing things for you. The following percent modifiers are recognized: %@, %c, %s, %d, %D, %i, %u, %U, %hi, %hu, %qi, %qu, %f, %g, %ld, %lu, %lld, and %llu. Using a modifier other than those will have unpredictable results. If, for some reason, you need the % character to appear in your SQL statement, you should use %%.

Using FMDatabaseQueue and Thread Safety.

Using a single instance of FMDatabase from multiple threads at once is a bad idea. It has always been OK to make a FMDatabase object per thread. Just don't share a single instance across threads, and definitely not across multiple threads at the same time. Bad things will eventually happen and you'll eventually get something to crash, or maybe get an exception, or maybe meteorites will fall out of the sky and hit your Mac Pro. This would suck.

So don't instantiate a single FMDatabase object and use it across multiple threads.

Instead, use FMDatabaseQueue. It's your friend and it's here to help. Here's how to use it:

First, make your queue.

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:aPath];

Then use it like so:

[queue inDatabase:^(FMDatabase *db) {
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:1]];
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:2]];
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:3]];

    FMResultSet *rs = [db executeQuery:@"select * from foo"];
    while ([rs next]) {
        …
    }
}];

An easy way to wrap things up in a transaction can be done like this:

[queue inTransaction:^(FMDatabase *db, BOOL *rollback) {
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:1]];
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:2]];
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:3]];

    if (whoopsSomethingWrongHappened) {
        *rollback = YES;
        return;
    }
    // etc…
    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:4]];
}];

FMDatabaseQueue will make a serialized GCD queue in the background and execute the blocks you pass to the GCD queue. This means if you call your FMDatabaseQueue's methods from multiple threads at the same time GDC will execute them in the order they are received. This means queries and updates won't step on each other's toes, and every one is happy.

History

The history and changes are availbe on its GitHub page and are summarized in the "CHANGES_AND_TODO_LIST.txt" file.

Contributors

The contributors to FMDB are contained in the "Contributors.txt" file.

License

The license for FMDB is contained in the "License.txt" file.

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