startManagedCursor (c) versus c.close()? - android-sqlite

Wondering how startManagedCursor() acts on / versus db.close() and / or cursor.close() Did read docs, but I'm still fuzzy on it %-) ... and, understood that startManagedCursor() is deprecated api >= 11. I think I have to use it for < android 2.3.x.
Two questions are in the code comments. Thanks !
SOR ! ... SO Rocks :)
Cursor c = null;
try {
dbHelper.open() ;
c = dbHelper.getMyRecords() ;
startManagingCursor(c) ;
if ( c.moveToFirst() ) { uberCool (stuff, here) ; }
dbHelper.close() ;// <** Question 1 : Is cursor also closed here ?
} catch (Exception e) {
Log.d ("OOPS", Caught exception: " + e.toString() ) ;
} finally {
// ** Question 2 : Is the close() just below redundant
// - will the managed cursor just close when function
// goes out of scope ?
if (c != null) c.close() ;
}

My understanding is that a managed cursor is owned by the activity. I have used one with a ListView inside a ListActivity. The primary reason for this is that the views in the ListView are automatically updated when the data changes in the data source (in my case a SQLite database) underlying the Cursor. I believe that the Cursor is closed when the Activity which manages it is killed. Calling close() in the finally block might be redundant. It depends first that my assumption is correct and also when this code is executed.

Related

Should 'require' go inside or outside of the Future?

How do I replace my first conditional with the require function in the context of a Future? Should I wrap the entire inRange method in a Future, and if I do that, how do I handle the last Future so that it doesn't return a Future[Future[List[UserId]], or is there a better way?
I have a block of code that looks something like this:
class RetrieveHomeownersDefault(depA: DependencyA, depB: DependencyB) extends RetrieveHomeowners {
def inRange(range: GpsRange): Future[List[UserId]] = {
// I would like to replace this conditional with `require(count >= 0, "The offset…`
if (count < 0) {
Future.failed(new IllegalArgumentException("The offset must be a positive integer.")
} else {
val retrieveUsers: Future[List[UserId]] = depA.inRange(range)
for (
userIds <- retrieveUsers
homes <- depB.homesForUsers(userIds)
) yield FilterUsers.withoutHomes(userIds, homes)
}
}
}
I started using the require function in other areas of my code, but when I tried to use it in the context of Futures I ran into some hiccups.
class RetrieveHomeownersDefault(depA: DependencyA, depB: DependencyB) extends RetrieveHomeowners {
// Wrapped the entire method with Future, but is that the correct approach?
def inRange(range: GpsRange): Future[List[UserId]] = Future {
require(count >= 0, "The offset must be a positive integer.")
val retrieveUsers: Future[List[UserId]] = depA.inRange(range)
// Now I get Future[Future[List[UserId]]] error in the compiler.
for (
userIds <- retrieveUsers
homes <- depB.homesForUsers(userIds)
) yield FilterUsers.withoutHomes(userIds, homes)
}
}
Any tips, feedback, or suggestions would be greatly appreciated. I'm just getting started with Futures and still having a tough time wrapping my head around many concepts.
Thanks a bunch!
Just remove the outer Future {...} wrapper. It's not necessary. There's no good reason for the require call to go inside the Future. It's actually better outside since then it will report immediately (in the same thread) to the caller that the argument is invalid.
By the way, the original code is wrong too. The Future.failed(...) is created but not returned. So essentially it didn't do anything.

How to indiciate a failure for a function with a void result

I have a function in scala which has no return-value (so unit). This function can sometimes fail (if the user provided parameters are not valid). If I were on java, I would simply throw an exception. But on scala (although the same thing is possible), it is suggested to not use exceptions.
I perfectly know how to use Option or Try, but they all only make sense if you have something valid to return.
For example, think of a (imaginary) addPrintJob(printJob: printJob): Unit command which adds a print job to a printer. The job definition could now be invalid and the user should be notified of this.
I see the following two alternatives:
Use exceptions anyway
Return something from the method (like a "print job identifier") and then return a Option/Either/Try of that type. But this means adding a return value just for the sake of error handling.
What are the best practices here?
You are too deep into FP :-)
You want to know whether the method is successful or not - return a Boolean!
According to this Throwing exceptions in Scala, what is the "official rule" Throwing exceptions in scala is not advised as because it breaks the control flow. In my opinion you should throw an exception in scala only when something significant has gone wrong and normal flow should not be continued.
For all other cases it generally better to return the status/result of the operation that was performed. scala Option and Either serve this purpose. imho A function which does not return any value is a bad practice.
For the given example of the addPrintJob I would return an job identifier (as suggested by #marstran in comments), if this is not possible the status of addPrintJob.
The problem is that usually when you have to model things for a specific method it is not about having success or failure ( true or false ) or ( 0 or 1 - Unit exit codes wise ) or ( 0 or 1 - true or false interpolation wise ) , but about returning status info and a msg , thus the most simplest technique I use ( whenever code review naysayers/dickheads/besserwissers are not around ) is that
val msg = "unknown error has occurred during ..."
val ret = 1 // defined in the beginning of the method, means "unknown error"
.... // action
ret = 0 // when you finally succeeded to implement FULLY what THIS method was supposed to to
msg = "" // you could say something like ok , but usually end-users are not interested in your ok msgs , they want the stuff to work ...
at the end always return a tuple
return ( ret , msg )
or if you have a data as well ( lets say a spark data frame )
return ( ret , msg , Some(df))
Using return is more obvious, although not required ( for the purists ) ...
Now because ret is just a stupid int, you could quickly turn more complex status codes into more complex Enums , objects or whatnot , but the point is that you should not introduce more complexity than it is needed into your code in the beginning , let it grow organically ...
and of course the caller would call like
( ret , msg , mayBeDf ) = myFancyFunc(someparam, etc)
Thus exceptions would mean truly error situations and you will avoid messy try catch jungles ...
I know this answer WILL GET down-voted , because well there are too much guys from universities with however bright resumes writing whatever brilliant algos and stuff ending-up into the spagetti code we all are sick of and not something as simple as possible but not simpler and of course something that WORKS.
BUT, if you need only ok/nok control flow and chaining, here is bit more elaborated ok,nok example, which does really throw exception, which of course you would have to trap on an upper level , which works for spark:
/**
* a not so fancy way of failing asap, on first failing link in the control chain
* #return true if valid, false if not
*/
def isValid(): Boolean = {
val lst = List(
isValidForEmptyDF() _,
isValidForFoo() _,
isValidForBar() _
)
!lst.exists(!_()) // and fail asap ...
}
def isValidForEmptyDF()(): Boolean = {
val specsAreMatched: Boolean = true
try {
if (df.rdd.isEmpty) {
msg = "the file: " + uri + " is empty"
!specsAreMatched
} else {
specsAreMatched
}
} catch {
case jle: java.lang.UnsupportedOperationException => {
msg = msg + jle.getMessage
return false
}
case e: Exception => {
msg = msg + e.getMessage()
return false
}
}
}
Disclaimer: my colleague helped me with the fancy functions syntax ...

Ormlite exception with single quote in text using LIKE query

I've got what seems to be a bug. I can add an entry to the database which has a single quote in the text.
However, when I search using QueryBuilder, for any text LIKE xyz, if xyz has a single quote in it, I get MySQL complaining about malformed SQL.
Other than parsing all strings myself, is there some method in Ormlite I can call to "santize" my strings?
Sample code is below:
public boolean isDuplicate () {
QueryBuilder<Company, Long> qb = getDao().queryBuilder() ;
Where<Company, Long> where = qb.where() ;
try {
if (Strings.isValid(name))
where.like("name", name) ;
if (Strings.isValid(regNo)) {
if (Strings.isValid(name))
where.or() ;
where.eq("regNo", regNo) ;
List<Company> res = where.query() ;
if (res != null && res.size() > 0)
return true ;
else
return false ;
}
} catch (SQLException e) {
GlobalConfig.log(e, true);
}
return false ;
}
This creates a SQL error if the company name has a single quote in it:
Creating default entries for Well Don't Delete Me 2 Please Pte Ltd.
[12-07-2013 13:45:42] You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 't Delete Me 2 Please Pte Ltd.' OR regNo = 'delete' )' at line 1
Any suggestions welcome.
Ok, I figured this out- I need to use the SelectArg method of querying the database.
So it now looks like this:
...
if (Strings.isValid(name))
{
SelectArg arg = new SelectArg () ; // Added
arg.setValue (name) ; / Added
where.like("name", arg) ; // Changed
}
if (Strings.isValid(regNo))
{
if (Strings.isValid(name))
where.or() ;
SelectArg arg = new SelectArg () ; // Added
arg.setValue (regNo) ; // Added
where.eq("regNo", arg) ; // Changed
List<Company> res = where.query() ;
if (res != null && res.size() > 0)
return true ;
else
return false ;
...
What I've learned is this: you must use one SelectArg PER item.
Now my question to Gray is why not make this a default behaviour? When I insert or update it seems to happen automatically, and to get the problem I found solved I have to add more
lines of code that could easily be part of the internal query handling.
I understand his concerns in this post but I agree with Dale. Maybe a halfway house is to have a flag to say which way Ormlite should treat parameters to the query methods.
I admire the flexibility and simplicity of the "programmable" SQL in Ormlite and in almost every case, Ormlite related code is concise, easy to follow and logical. This is one rare case where I feel it is more verbose than necessary, for no net benefit. Just my opinion.

How to continue loop in Map( /Reduce ) function?

I have a Map function in MongoDB which I'm later using Reduce on. I use a collection which has a bunch of users in it and users own some channels. However, there are users that do not have any channels and the Map/Reduce function raises an error in my script.
map = Code("function () {"
" if(!this.channels) continue;"
" this.channels.forEach(function(z) {"
" emit(z, 1);"
" });"
"}")
When I use return instead of continue to quit the function it works flawlessly except that I don't want to end the loop. Is there any smart way around this?
Thanks for your advice and better widsom.
If you return from map, it returns only from map for this document. Maps for other documents will be executed regardless of that.
I suggest rewriting your map to this form
function () {
if(this.channels) {
this.channels.forEach(function(z) {
emit(z, 1);
});
}
}
I think, this form is more clear. It will emit something for users that have channels, and skip those that don't have any.

Best way to check if object exists in Entity Framework? [closed]

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What is the best way to check if an object exists in the database from a performance point of view? I'm using Entity Framework 1.0 (ASP.NET 3.5 SP1).
If you don't want to execute SQL directly, the best way is to use Any(). This is because Any() will return as soon as it finds a match. Another option is Count(), but this might need to check every row before returning.
Here's an example of how to use it:
if (context.MyEntity.Any(o => o.Id == idToMatch))
{
// Match!
}
And in vb.net
If context.MyEntity.Any(function(o) o.Id = idToMatch) Then
' Match!
End If
From a performance point of view, I guess that a direct SQL query using the EXISTS command would be appropriate. See here for how to execute SQL directly in Entity Framework: http://blogs.microsoft.co.il/blogs/gilf/archive/2009/11/25/execute-t-sql-statements-in-entity-framework-4.aspx
I had to manage a scenario where the percentage of duplicates being provided in the new data records was very high, and so many thousands of database calls were being made to check for duplicates (so the CPU sent a lot of time at 100%). In the end I decided to keep the last 100,000 records cached in memory. This way I could check for duplicates against the cached records which was extremely fast when compared to a LINQ query against the SQL database, and then write any genuinely new records to the database (as well as add them to the data cache, which I also sorted and trimmed to keep its length manageable).
Note that the raw data was a CSV file that contained many individual records that had to be parsed. The records in each consecutive file (which came at a rate of about 1 every 5 minutes) overlapped considerably, hence the high percentage of duplicates.
In short, if you have timestamped raw data coming in, pretty much in order, then using a memory cache might help with the record duplication check.
I know this is a very old thread but just incase someone like myself needs this solution but in VB.NET here's what I used base on the answers above.
Private Function ValidateUniquePayroll(PropertyToCheck As String) As Boolean
// Return true if Username is Unique
Dim rtnValue = False
Dim context = New CPMModel.CPMEntities
If (context.Employees.Any()) Then ' Check if there are "any" records in the Employee table
Dim employee = From c In context.Employees Select c.PayrollNumber ' Select just the PayrollNumber column to work with
For Each item As Object In employee ' Loop through each employee in the Employees entity
If (item = PropertyToCheck) Then ' Check if PayrollNumber in current row matches PropertyToCheck
// Found a match, throw exception and return False
rtnValue = False
Exit For
Else
// No matches, return True (Unique)
rtnValue = True
End If
Next
Else
// The is currently no employees in the person entity so return True (Unqiue)
rtnValue = True
End If
Return rtnValue
End Function
I had some trouble with this - my EntityKey consists of three properties (PK with 3 columns) and I didn't want to check each of the columns because that would be ugly.
I thought about a solution that works all time with all entities.
Another reason for this is I don't like to catch UpdateExceptions every time.
A little bit of Reflection is needed to get the values of the key properties.
The code is implemented as an extension to simplify the usage as:
context.EntityExists<MyEntityType>(item);
Have a look:
public static bool EntityExists<T>(this ObjectContext context, T entity)
where T : EntityObject
{
object value;
var entityKeyValues = new List<KeyValuePair<string, object>>();
var objectSet = context.CreateObjectSet<T>().EntitySet;
foreach (var member in objectSet.ElementType.KeyMembers)
{
var info = entity.GetType().GetProperty(member.Name);
var tempValue = info.GetValue(entity, null);
var pair = new KeyValuePair<string, object>(member.Name, tempValue);
entityKeyValues.Add(pair);
}
var key = new EntityKey(objectSet.EntityContainer.Name + "." + objectSet.Name, entityKeyValues);
if (context.TryGetObjectByKey(key, out value))
{
return value != null;
}
return false;
}
I just check if object is null , it works 100% for me
try
{
var ID = Convert.ToInt32(Request.Params["ID"]);
var Cert = (from cert in db.TblCompCertUploads where cert.CertID == ID select cert).FirstOrDefault();
if (Cert != null)
{
db.TblCompCertUploads.DeleteObject(Cert);
db.SaveChanges();
ViewBag.Msg = "Deleted Successfully";
}
else
{
ViewBag.Msg = "Not Found !!";
}
}
catch
{
ViewBag.Msg = "Something Went wrong";
}
Why not do it?
var result= ctx.table.Where(x => x.UserName == "Value").FirstOrDefault();
if(result?.field == value)
{
// Match!
}
Best way to do it
Regardless of what your object is and for what table in the database the only thing you need to have is the primary key in the object.
C# Code
var dbValue = EntityObject.Entry(obj).GetDatabaseValues();
if (dbValue == null)
{
Don't exist
}
VB.NET Code
Dim dbValue = EntityObject.Entry(obj).GetDatabaseValues()
If dbValue Is Nothing Then
Don't exist
End If