sqlite3 prepared statement fails if fk is null. It should not - ios

Question

I use Sqlite3 in my iOS app.
If I try to run the command from the console:
INSERT INTO documents (name,fk_1) VALUES ('pluto','')
everything is fine.
But when doing it via prepared statement, the same fails:
INSERT INTO documents (name,fk_1) VALUES (?,?)
saying "foreign key constraint failed".The value of fk_1 = ''.
The table documents is
CREATE TABLE "documents" ("id" integer PRIMARY KEY AUTOINCREMENT NOT NULL ,"name" varchar,"attivita_id" integer NULL REFERENCES attivita(id) ON DELETE CASCADE)
I was supposing that I can use NULL value as FK, and I can, indeed, because from the command line (first query) it works. The same is not possible with prepared statements?
The code to execute a write is:
-(NSInteger)writeData:(NSDictionary* )data table:(NSString* )table
{
sqlite3_stmt *sqlStatement;
NSMutableArray *columns = [[NSMutableArray alloc] init];
NSMutableArray *values = [[NSMutableArray alloc] init];
NSMutableDictionary *temp = [[NSMutableDictionary alloc] initWithDictionary:data];
#try {
assert([data count] != 0);
if ([[data allKeys] count] == 0) return 1;
// Names/Values for INSERT
// id is AUTOINCREMENT
[temp removeObjectForKey:#"id"];
[columns addObjectsFromArray:[temp allKeys]];
NSString *cols = [columns componentsJoinedByString:#","];
NSMutableString *colNames = [[NSMutableString alloc] initWithString:
[NSString stringWithFormat:#"INSERT INTO %s (",[table UTF8String]]];
[colNames appendString:cols];
[colNames appendString:#")"];
// Values for INSERT
[values addObjectsFromArray:[temp allValues] ];
// Prepare the ? marks for bindings
NSMutableArray * marks = [NSMutableArray new];
for (int i=0 ; i < [values count]; i++)
[marks addObject:#"?"];
NSMutableString *s = [[NSMutableString alloc] init];
NSMutableString *valNames = [[NSMutableString alloc] initWithString:#" VALUES ("];
[valNames appendString:[marks componentsJoinedByString:#","]];
[valNames appendString:#")"];
[colNames appendString:valNames];
const char *sql = [colNames UTF8String];
#ifdef DEBUG
// NSLog(#"avvDB writeDATA insert string %#",colNames);
#endif
if(sqlite3_prepare_v2(db, sql, -1, &sqlStatement, NULL) != SQLITE_OK)
{
NSLog(#"DB:Problem with prepare statement write %s, data: %#",sqlite3_errmsg(db),data);
NSLog(#"DB:Query %#",colNames);
return 0;
}
// binding
for(int i = 0; i < [values count]; i++)
{
// According to the doc, if the 3rd value is a null pointer, the effect is similar to
// sqlite3_bind_null
if ([[values objectAtIndex:i] length] == 0 || [values objectAtIndex:i] == nil)
sqlite3_bind_text(sqlStatement, i+1, nil,-1, SQLITE_TRANSIENT);
else
{
[s setString:[NSString stringWithFormat:#"%#",[values objectAtIndex:i]]];
const char* currentValue = [s UTF8String];
sqlite3_bind_text(sqlStatement, i+1, currentValue,-1, SQLITE_TRANSIENT);
}
}
if (sqlite3_step(sqlStatement) == SQLITE_DONE) {
// NSLog(#"Location inserted on database");
}
else {
NSLog(#"DB:Error on write: %i %s %#",sqlite3_errcode(db),sqlite3_errmsg(db),data);
NSLog(#"DB:Query %#",colNames);
}
// if (sqlite3_exec(db, sql, NULL, NULL, NULL) == SQLITE_OK)
// {
// NSLog(#"Last id %llu %s",sqlite3_last_insert_rowid(db),sqlite3_errmsg(db));
// }
// NSLog(#"Ecexecuted write %#",colNames);
valNames = nil;
colNames = nil;
marks = nil;
s = nil;
}// end try
#catch(NSException* e)
{
NSLog(#"Eccezione in write %#",[e reason]);
}
#finally {
sqlite3_finalize(sqlStatement);
sqlStatement = nil;
return sqlite3_last_insert_rowid(db);
}
}

Answer 1

An empty string is not the same as NULL.
To set a parameter to NULL, use sqlite3_bind_null.
Alternatively, just don't mention empty parameters in the INSERT statement.