I am using Google App Engine and using Google's datastore interface for a database .
My question is this , I have the following code : I have a network object that I want to either update if it exists on db , or to create if it's the first time. . For this I have to catch an exception and repeat the same code twice - it seems ugly and redundant and makes me think I'm doing something wrong .
The second thing that strikes me as odd is that there is no method I can think of that copies an object to an entity or vice versa . Am I expected to implement this myself ? It is very uncomfrotable to use the setProperty or getProperty for each property and well ...I am just wondering why there is no objectToEntity method or something of the sort.
This is how my code currently looks ...
try {
Entity network=datastore.get(KeyFactory.stringToKey(networks.get(i)._ipDigits));
//If I get here no exception was thrown - entity already exists on db.
Network contextNet= //fetch the network object from servlet context ...
network.setProperty("ip", contextNet._ip); //update the fields using setProperty - no better way??
network.setProperty("offlineUsers",contextNet._offlineUsers);
datastore.put(network);
}
//Entity doesn't exist , create a new entity and save it (while repeating the same code)...
catch (EntityNotFoundException e) {
Entity network=new Entity("network",Long.parseLong(networks.get(i)._ipDigits));
Network contextNet= // ...fetch the network object from servlet context
network.setProperty("ip", contextNet._ip);
network.setProperty("offlineUsers",contextNet._offlineUsers);
datastore.put(network);
}
You don't have to get and put the entity in order to update it. If you know the ID of the entity you can just put it. If it exists it will be updated, if not it will be created.
Use objectify to automatically map your classes to entities.
Related
I am using Breeze with Angular and EF 6 in my project and I have a form where I perform CRUD operations.
I have this entity called: Car2Sale which is a many-to-many table (Id, CarId, SaleId). Id is the PK for this table while CardId and SaleId are FKs to their associated tables and they are grouped together in a unique index. (CarId, SaleId).
When I want to add a new Car2Sale entity with a CarId and SaleId that already exist in the database I get this error on server side: Violation of unique constraint... at this method:
[HttpPost]
public SaveResult SaveChanges(JObject saveBundle) {
return _repository.SaveChanges(saveBundle);
}
which is correct because I wanted to prevent the user from introducing similar keys in the same table.
On the client side I receive the error in the
entity.entityAspect.getValidationErrors();
and I display it using Toastr.js.
I was wondering what is the best practice to do exception handling in the SaveChanges() method in this case on the server side.
I was thinking about using a try catch on the server side and return a SaveResult of my own, like below:
[HttpPost]
public SaveResult SaveChanges(JObject saveBundle) {
SaveResult myResult = null;
try {
myResult = _repository.SaveChanges(saveBundle);
} catch(Exception ex) {
Logger.Log(ex.toString());
}
return myResult;
}
Many thanks
My main objection to your particular approach is that you're swallowing the error and returning what appears to be a "happy" SaveResult to the client. The BreezeJS client will assume that the save succeeded and will update your cached entities accordingly. That's not a good idea!
You can certainly catch it on the server-side and might want to do so in order to reshape the error response. I think that I would do so INSIDE the repository as I'm disinclined to have persistence logic in my controllers. Encapsulating that stuff is the raison d'etre of the repository/Unit-of-Work pattern.
It's not clear to me how this helps you on the client side. You still need to send the error to the client and have the client do something reasonable with it.
You might want to look at the Breeze Labs SaveErrorExtensions for ideas about interpreting errors on the client. To my mind, the hard thing is communicating actionable intelligence to the user. That's a business/design decision that we can't solve for you. We can give you the information; you have to make sense with it.
HTH.
I have a similar code below that I'm trying to figure out transaction locking:
DAOT.repeatInTransaction(new Transactable() {
#Override
public void run(DAOT daot)
{
Points points = daot.ofy().find(Points.class, POINTS_ID);
// do something with points
takes_a_very_long_time_delay(); // perhaps 10 secs
daot.ofy().put(points);
}
});
The code above is executed from within a Java servlet. The operation is expected to work for 10 seconds for example. In between that time, I have a test that will invoke another servlet that will delete a Points entity, I was expecting that the delete operation would fail or at least delete the entity after the transaction above has finished.
However the entity was deleted during the period that the above code is executing. In my real application, I added exception handling to throw exception when trying to access or edit a entity that does not exist.
From there, the application is throwing "Entity not found" exception just after I executed the servlet that will delete the Entity in the code above.
Although I am using GAE Transactions already, however I think I am still missing something that's why my test fails.
Code for the delete Transaction from withing the Delete servlet:
DAOT.repeatInTransaction(new Transactable() {
#Override
public void run(DAOT daot)
{
Points points = daot.ofy().find(Points.class, POINTS_ID);
daot.ofy().delete(points);
}
});
How can I ensure that a new operation like a delete for a entity will wait until the current operation is happening on a entity during a transaction?
App Engine uses optimistic concurrency, not locking. That is, a transaction on a group of entities will not prevent other processes from modifying those entities while the transaction runs. Instead, when the transaction attempts to commit, it will check if any modifications were made while the transaction was executing, and if it has, discard any changes and run your function again from the beginning.
I assume you use objectify to work with datastore.
First you need to make sure daot.ofy() returns objectify instance with explicit transaction set (ObjectifyFactory.beginTransaction()) instead of ObjectifyFactory.begin(). Then make sure you use the same objectify instance for both find() and delete() calls (as well as for find()/put pairs).
We noticed a slight oddity in the Sitecore API code. The code is below for your reference. The code is trying to get a database by doing new Database(database). But randomly it was failing.
This code worked for a while with Database db = new Database(database); but started failing randomly yesterday. When we changed the code to Database db = Database.GetDatabase(database);, the code started working again. What is the difference between the two approaches and what is recommended by Sitecore?
I've seen this happen twice now - multiple times in production and a couple of times in my development environment.
public static void DeleteItem(string id, stringdatabase)
{
//get the database
Database db = new Database(database);
//get the item
item = db.GetItem(new ID(id));
if (item != null)
{
using(new Sitecore.SecurityModel.SecurityDisabler())|
{
//delete the item
item.Delete();
}
}
}
A common way you will see people get a specific database is:
Sitecore.Data.Database master = Sitecore.Configuration.Factory.GetDatabase("master");
This is equivalent to Sitecore.Data.Database.GetDatabase("master").
When you call either of these methods it will first check the cache for the database. If not found it will build up the database with all of the configuration values within the config file via reflection. Once the database is created it will be placed in the cache for future use.
When you use the constructor on the database it is simply creating a rather empty database object. I am rather suprised to hear it was working at all when you used this method.
The proper approach to get a specific database would be to use:
Sitecore.Configuration.Factory.GetDatabase("master");
// or
Sitecore.Data.Database.GetDatabase("master");
If you are looking to get the database used with the current request (aka context database) you can use Sitecore.Context.Database. You can also use Sitecore.Context.ContentDatabase.
I am learning ASP.NET MVC, and confused as to how can I ensure unique values for columns (username & email) for a table.
Can anybody help me with a sample code or a link to the tutorial which shows & explains this?
EDIT:
I know that I can apply an unique key constraint on my table columns and achieve it. However, I want to know how can I achieve it via ASP.NET MVC code?
UPDATE:
I wish to do a check in my application such that no duplicated values are passed to DAL, i.e. achieve a check before inserting a new row.
Mliya, this is something you are controlling at the database level, not at the application level.
If you are designing a database with a users table in which you would like to constraint the username and email columns to be UNIQUE you should create a UNIQUE INDEX on such columns.
without knowing your backend database (mySQL, SQL Server, MS Access, Oracle...) it's not the case to show you pictures or tell much more, just create the table with the designer and add these unique constraints to those columns and by design you will be sure no duplicated values will ever be inserted for username and email.
I also suggest you to create an ID column which would be set as PK (primary key, which means it will be automatically set as NON NULL and UNIQUE).
From your ASP.NET MVC application you should of course make sure that no duplicated values are then passed to the DAL for username and email. You could do this in different ways, the easiest is probably to check before inserting a new row if any user already exists with that username and/or email and if so you can show a notification message telling the user to please select another pair of values.
In an ASP.NET MVC architecture, you should try to do most of your validation in the Model, but with low-level validation rules like these, it's sometimes impossible. What you should look to for answers then is Domain-driven Design (DDD) where Application Services can solve such low-level needs.
Application Services will have access to the database (either directly, or better yet; indirectly through repositories) and can perform low-level validation and throw ValidationException or something similar (with detailed information the Controller can act upon and respond to the user) when a prerequisite or business rule isn't met.
S#arp Architecture implementes all of this in a best-practice framework that you can use as a basis for your ASP.NET MVC applications. It is highly opinionated towards DDD principles and NHibernate, and it will sometimes force your hand on how you do stuff, which is kind of the point. The most important part about it is that it learns you how to deal with these kinds of problems.
To answer your question more concretely and in the spirit of DDD, this is how I would solve it:
public class UserController
{
private readonly IUserService userService;
public UserController(IUserService userService)
{
// The IUserService will be injected into the controller with
// an "Inversion of Control" container like NInject, Castle Windsor
// or StructureMap:
this.userService = userService;
}
public ActionResult Save(UserFormModel userFormModel)
{
if (userFormModel.IsValid)
{
try
{
// Mapping can be performed by AutoMapper or some similar library
UserDto userDto = Mapper.Map<UserDto>(userFormModel);
this.userService.Save(userDto);
}
catch (ValidationException ve)
{
ViewBag.Error = ve.Detail;
}
}
// Show validation errors or redirect to a "user saved" page.
}
}
public class UserService : IUserService
{
private readonly IUserRepository userRepository;
public UserService(IUserRepository userRepository)
{
// The IUserRepository will be injected into the service with
// an "Inversion of Control" container like NInject, Castle Windsor
// or StructureMap:
this.userRepository = userReposityr;
}
public UserDto Save(UserDto userDto)
{
using (this.userRepository.BeginTransaction())
{
if (!this.userRepository.IsUnique(userDto.UserName))
{
// The UserNameNotUniqueValidationException will inherit from ValidationException
// and build a Detail object that contains information that can be presented to
// a user.
throw new UserNameNotUniqueValidationException(userDto.UserName);
}
userDto = this.userRepository.Save(userDto);
this.userRepository.CommitTransaction();
return userDto;
}
}
}
I'm developing my first windows phone 7 app, and I've hit a snag. basically it's just reading a json string of events and binding that to a list (using the list app starting point)
public void Load()
{
// form the URI
UriBuilder uri = new UriBuilder("http://mysite.com/events.json");
WebClient proxy = new WebClient();
proxy.OpenReadCompleted += new OpenReadCompletedEventHandler(OnReadCompleted);
proxy.OpenReadAsync(uri.Uri);
}
void OnReadCompleted(object sender, OpenReadCompletedEventArgs e)
{
if (e.Error == null)
{
var serializer = new DataContractJsonSerializer(typeof(EventList));
var events = (EventList)serializer.ReadObject(e.Result);
foreach (var ev in events)
{
Items.Add(ev);
}
}
}
public ObservableCollection<EventDetails> Items { get; private set; }
EventDetails is my class that wraps the json string. this class has to be correct because it is an exact copy of the class used by that website internally from which the json is generated...
I get the json string correctly from the webclient call (I read the memorystream and the json is indeed there) but as soon as I attempt to deserialize the string, the application exits and the debugger stops.
I get no error message or any indication that anything happen, it just stops. This happens if I type the deserialize method into the watch window as well...
I have already tried using JSON.net in fact I thought maybe it was a problem with JSON.net so I converted it to use the native deserializer in the .net framework but the error is the same either way.
why would the application just quit? shouldn't it give me SOME kind of error message?
what could I be doing wrong?
many thanks!
Firstly, the fact that you have some string there that looks like JSON does not mean that you have a valid JSON. Try converting a simple one.
If your JSON is valid, it might be that your JSON implementation does not know how to convert a list to EventList. Give it a try with ArrayList instead and let me know.
The application closes because an unhandled exception happens. If check the App.xaml.cs file you will find the code that closes your app. What you need to do is try catch your deserialization process and handle it locally. So most likely you have some JSON the DataContractJsonSerializer does not like. I have been having issue with it deserializing WCF JSON and have had to go other routes.
You may want to check to ensure your JSON is valid, just because your website likes it does not mean it is actually valid, the code on your site may be helping to correct the issue. Drop a copy of your JSON object (the string) in http://jsonlint.com/ to see if it is valid or not. Crokford (the guy who created JSON) wrote this site to validate JSON, so I would rely on it more than your site ;) This little site has really helped me out of some issues over the past year.
I ran into this same kind of problem when trying to migrate some existing WM code to run on WP7. I believe that the WP7 app crashes whenever it loads an assembly (or class?) that references something that's not available in WP7. In my case, I think it was Assembly.Load or something in the System.IO namespace, related to file access via paths.
While your case might be something completely different, the symptoms were exactly the same.
The only thing I can recommend is to go through the JSON library and see if it's referencing base classes that are not allowed in WP7. Note that it doesn't even have to hit the line of code that's causing the issue - it'll crash as soon as it tries to hit the class that contains the bad reference.
If you can step into the JSON library, you can get a better idea of which class is causing the problem, because as soon as the code references it, the whole app will crash and the debugger will stop.