Using NHibernate I usually query for single records using the Get() or Load() methods (depending on if I need a proxy or not):
SomeEntity obj = session.Get<SomeEntity>(new PrimaryKeyId(1));
Now, if I execute this statement twice, like the example below, I only see one query being executed in my unittests:
SomeEntity obj1 = session.Get<SomeEntity>(new PrimaryKeyId(1));
SomeEntity obj2 = session.Get<SomeEntity>(new PrimaryKeyId(1));
So far, so good. But I noticed some strange behaviour when getting the same object using a ICriteria query. Check out my code below: I get the first object instance. I then change the value of a property to 10 (the value in the database is 8), get another instance and finally check the values of the second object instance.
//get the first object instance.
SomeEntity obj1 = session.CreateCriteria(typeof(SomeEntity))
.Add(Restrictions.Eq("Id", new PrimaryKeyId(1)))
.UniqueResult<SomeEntity>();
//the value in the database and the property is 8 at this point. Let's set it to 10.
obj1.SomeValue = 10;
//get the second object instance.
SomeEntity obj2 = session.CreateCriteria(typeof(SomeEntity))
.Add(Restrictions.Eq("Id", new PrimaryKeyId(1)))
.UniqueResult<SomeEntity>();
//check if the values match.
Assert.AreEqual(8, obj2.SomeValue);
Now, for some reason the assert fails, because the value is 10 of obj2 even though I asked for the object with a new query. the funny thing is, there are 2 exactly the same select queries being executed according to my unit test output window. My question: why are there 2 queries being executed if the second object is fetched from the first level cache?
Am I missing something or is this a bug?
Regards, Ted
edit #1: using NHibernate v2.1.2GA
edit #2: I added some extra explanation about the 2 queries being executed to the last paragraph.
Well, having learned a lot more about NHibernate I can now answer this question myself:
The ICriteria query returns a list of objects fetched by NHibernate. NHibernate does not know which objects are returned until they are matched one by one with the object in the first level cache. If the item is already in the first level cache map the item read from the database is discarded. if it is not in the identity map, the item is put into the first level cache.
Another "a-ha!" moment: suppose you run the query for the first time while there are 5 rows in the database all rows are fetched and put into first level cache. now over time 5 more records are added to the table and you rerun the query. Now all 10 records are fetched, but NHibernate sees 5 of them are already in the cache and will only add the 5 latter records. So basically you fetched 5 records for nothing (just to match the identifiers with the object identifiers in the identity map).
Get/Load use the 1st level cache, this is why you don't see the 2nd call out the db. Queries do not use the 1st level cache. However, you can set up queries to use the 2nd level cache. See details here
UPDATE What's likely happening is the query is doing a 2 phase load. So it's getting the result set, but also checking the 1st level cache to see if any entities exist there. If they do, then it returns the cached object. See NHibernate.Loader.Loader.GetRow method.
Here is the relevant line:
//If the object is already loaded, return the loaded one
obj = session.GetEntityUsingInterceptor(key);
AFAIK, only 'Get' (and maybe Load) use the 1st level cache.
Using the Criteria API always results in a query hitting the DB, unless the 2nd level cache is enabled.
Edit: more information can be found here
I am not sure why a second query is ran, but the expected behavior of NHibernate is if you ask for the same object by ID from the same session, you get the first level cache.
In my understanding, when using a Criteria, you are basically saying to NHibernate: "I want to filter rows based on expressions".
When seen that way, NHibernate has no way of knowing if the query will always return the same filtered row(s) from the database, so it has to query it again.
Also, you can use query caching only with second-level caching, as per the documentation:
So the query cache should always be used in conjunction with the second-level cache.
From here
NHibernate is probably issuing an update between the first and second queries to protect you from a concurrency problem. As Frederik pointed out, you should always use Get to retrieve an object by its key.
I'm curious, what is the PrimaryKeyId wrapper adding?
EDIT:
However it's working (my money's still on an update before select), this behavior is by design. If you want to discard your in-memory object and load a new instance of it from the session, then Evict the original from the session first. There is also a Refresh method you could try.
Related
I have been reading about MediatR and CQRS latelly and I saw many people saying that commands shouldn't return domain objects. They can return values but they're limited to returning erros values, failure/success information and the Id of the newly created entities.
My question is how to return this new objetct to the client if the command can return only the Id of the new entity.
1) Should I query the database again with this new Id? If so, isn't that bad that I making a new trip to the database to get an object that was in the memory a few seconds ago?
2) What's the correct way of returning the entities created by the commands?
I think the more important question is why you shouldn't return domain objects from commands. If the reason for that seems like a valid reason for you, you should look into alternatives such as executing a query right after the command to fetch the domain object.
If, however, returning the domain object from the command fits your needs and does not impose any direct problems, then why not just do it and keep things simple and straightforward?
We are going to implement gemfire for our project. We are currently syncing gemfire cache with our DB2 database. So, we are facing issue while putting DB data into cache.
To put DB data into region. I have implement com.gemstone.gemfire.cache.CacheLoader and override load method of it. As written in java doc load method will return only one Object. But for our requirement we will have to return multiple VO from load method
public List<CmDvceInvtrGemfireBean> load(LoaderHelper<CmDvceInvtrGemfireBean, CmDvceInvtrGemfireBean> helper)
throws CacheLoaderException
While returining multiple VO in form of List<CmDvceInvtrGemfireBean> gemfire region consider it's as single value.
So, when i invoke,
System.out.println("return COUNT" + cmDvceInvtrRecord.query("SELECT COUNT(*) FROM /cmDvceInvtrRecord"));
It return count of one. But i can see total 7 number of data into it.
So, I want to implement the kind of mechanism that will put all the 7 values as a separate VO in Region
Is there any way to do this using Gemfire CacheLoader?
A CacheLoader was meant to load a value only for a single entry in the GemFire Region on a cache miss. As the Javadoc states...
..creates the value for the desired key..
While a key can map to a multi-valued (e.g. an array/Collection) value, the CacheLoader can only populate a single entry.
You will have to resort to other means of populating the cache with multiple "entries" in a single operation.
Out of curiosity, why do you need (requirement?) to load multiple entries (from the DB) at once? Are you trying to minimize the number of round trips to the DB?
Also, what logic are you using to decide what VO from the DB will be loaded based on the information (i.e. key) provided in the CacheLoader?
For instance, are you somehow trying to predictably select values from the DB based on the CacheLoader key that would subsequently minimize cache misses on future Region.get(key) calls?
Sorry, I don't have a better answer for you right now, but answers to some of these questions may help me give you some ideas for alternatives.
Cheers,
John
i need to get a large amount of data from a remote database. the idea is do a sort of pagination, like this
1 Select a first block of datas
SELECT * FROM TABLE LIMIT 1,10000
2 Process that block
while(mysql_fetch_array()...){
//do something
}
3 Get next block
and so on.
Assuming 10000 is an allowable dimension for my system, let us suppose i have 30000 records to get: i perform 3 call to remote system.
But my question is: when executing a select, the resultset is transmitted and than stored in some local part with the result that fetch is local, or result set is stored in remote system and records coming one by one at any fetch? Because if the real scenario is the second i don't perform 3 call, but 30000 call, and is not what i want.
I hope I explained, thanks for help
bye
First, it's highly recommended to utilize MySQLi or PDO instead of the deprecated mysql_* functions
http://php.net/manual/en/mysqlinfo.api.choosing.php
By default with the mysql and mysqli extensions, the entire result set is loaded into PHP's memory when executing the query, but this can be changed to load results on demand as rows are retrieved if needed or desired.
mysql
mysql_query() buffers the entire result set in PHP's memory
mysql_unbuffered_query() only retrieves data from the database as rows are requested
mysqli
mysqli::query()
The $resultmode parameter determines behaviour.
The default value of MYSQLI_STORE_RESULT causes the entire result set to be transfered to PHP's memory, but using MYSQLI_USE_RESULT will cause the rows to be retrieved as requested.
PDO by default will load data as needed when using PDO::query() or PDO::prepare() to execute the query and retrieving results with PDO::fetch().
To retrieve all data from the result set into a PHP array, you can use PDO::fetchAll()
Prepared statements can also use the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY constant, though PDO::fetchALL() is recommended.
It's probably best to stick with the default behaviour and benchmark any changes to determine if they actually have any positive results; the overhead of transferring results individually may be minor, and other factors may be more important in determining the optimal method.
You would be performing 3 calls, not 30.000. That's for sure.
Each 10.000 results batch is rendered on the server (by performing each of the 3 queries). Your while iterates through a set of data that has already been returned by MySQL (that's why you don't have 30.000 queries).
That is assuming you would have something like this:
$res = mysql_query(...);
while ($row = mysql_fetch_array($res)) {
//do something with $row
}
Anything you do inside the while loop by making use of $row has to do with already-fetched data from your initial query.
Hope this answers your question.
according to the documentation here all the data is fetched to the server, then you go through it.
from the page:
Returns an array of strings that corresponds to the fetched row, or FALSE if there are no more rows.
In addition it seams this is deprecated so you might want to use something else that is suggested there.
What state is saved between rerunning queries in Linqpad? I presumed none, so if you run a script twice it will have the same results both time.
However run the C# Program below twice in the same Linqpad tab. You'll find the first it prints an empty list, the second time a list with the message 'hey'. What's going on?
System.ComponentModel.TypeDescriptor.GetAttributes(typeof(String)).OfType<ObsoleteAttribute>().Dump();
System.ComponentModel.TypeDescriptor.AddAttributes(typeof(String),new ObsoleteAttribute("hey"));
LINQPad caches the application domain between queries, unless you request otherwise in Edit | Preferences (or press Ctrl+Shift+F5 to clear the app domain). This means that anything stored in static variables will be preserved between queries, assuming the types are numerically identical. This is why you're seeing the additional type description attribute in your code, and also explains why you often see a performance advantage on subsequent query runs (since many things are cached one way or another in static variables).
You can take advantage of this explicitly with LINQPad's Cache extension method:
var query = <someLongRunningQuery>.Cache();
query.Select (x => x.Name).Dump();
Cache() is a transparent extension method that returns exactly what it was fed if the input was not already seen in a previous query. Otherwise, it returns the enumerated result from the previous query.
Hence if you change the second line and re-execute the query, the query will execute quickly since will be supplied from a cache instead of having to re-execute.
Out of the blue, i am getting this error when doing a number of updates using nhibernate.
Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [MyDomainObject]
there is no additional information in the error. Is there some recommended way to help identify the root issue or can someone give me a better explanation on what this error indicated or is a sympton around.
Some additional info
I looked at the object and all of the data looks fine, it has an ID, etc . .
Note this is running in a single call stack from an asp.net-mvc website so i wouldn't expect there to be any threading issues to worry about in terms of concurrency.
NHibernate has an object, let's call it theObject. theObject.Id has a value of 42. NHibernate notices that the object is dirty. The object's Id is different than the unsaved-value, which is zero (0) for integer primary keys. So NHibernate issues an update statement, but no rows are updated, which means that there is no row in the database for that type of object with an Id of 42. So the object has been deleted without NHibernate knowing about it. This could happen inside another transaction (e.g. you have threading issues) or if someone (or another application) deleted/altered the row using SQL directly against the database.
The other possibility is that your unsaved-value is wrong. e.g. You are using -1 to indicate an unsaved-entity, but your mapping has a unsaved-value of zero. This is unlikely as your application is generally working from the sounds of it. If the unsaved-value was wrong, you wouldn't have been able to save any entities to the database as NHibernate would have been issuing UPDATE statements when it should have been issuing INSERT.
It means that you have multiple transactions accessing the same data, thus producing concurrency issues. You should improve on your data access handling, you probably are updating data from multiple threads, syndicate the changed data into a queue first which handles all the access to the db.
An old post, but hopefully my info will help someone. I was getting a similar error but only when persisting associations, after I had added in a new object. The error was of the form:
NHibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) [My.Entity#0]
Note the zero on the end, which is my identifier property. It should not be trying to save with key zero as I was using identity specification in SQL Server (generator class=native). I had not changed my unsaved-value in my xml so I had no idea what the problem was; for some reason NHibernate was trying to do an update using key value as 0 instead of a save (and getting the next key identity) for my new object.
In the end I found the cause was that I was initialising Version number to 1 for the new object in my constructor! Even though my identifier property was zero, for some reason NHibernate was also looking for a version property of zero as well, to identify it as an unsaved transient instance. The book "NHibernate in Action" does actually mention this on page 120, but for some reason my objects were fine when persisting with version number of 1 normally, and only failing if saving a new object through an association.
So make sure you do not set your Version value (leave as zero or null).
You say that your data is ok, but check if for example you are mapping the ID as self generate. I had the exact same problem, but I was sending an object with an ID different from 0.
Hope it helps!
My problem was this:
[Bind(Include="Name")] EventType eventType
Should have been:
[Bind(Include="EventTypeId,Name")] EventType eventType
Just as other answers suggest nhibernate was using zero as the id for my entity.
If you have a trigger on the table, it can be the reason. In this case, add inside it
SET ROWCOUNT 0;
SET NOCOUNT ON;
This error happened to me in the following way:
List < Device > allDevices = new List < Device > ();
//Add Devices to the list
allDevices.Add(aDevice);
//Add allDevices to database //Will work fine
// allDevices.Clear(); //Should be used here
//Later we add more devices
allDevices.Add(anotherDevice);
//Add allDevices to Database -> We get the error
//Solution to this
allDevices.Clear(); //Before adding new transaction with the oldData,