When using Hibernate (JPA), if I do the following call :
MyParent parent = em.getReference("myId");
parent.getAListMappedAsOneToMany().add(record)
record.setParent(parent);
Is there any performance problem ?
My thoughts is that getReference does not load the entity and getAListeMappedAsOneToMany().add do not need to load the list as it is defined as lazy fetch.
getAListMappedAsOneToMany could return a very big list if it is really accessed (by calling get or size method).
Could you confirm that there is no performance problem with such a code ?
getReference() doesn't go to the database, and returns a proxy. But if you call a method on the proxy, it initializes the proxy and gets the entity data from the database. So since you call getAListMappedAsOneToMany() on your entity, you don't gain anything by calling getReference() instead of find().
Similarly, the list is loaded lazily. this means that it will only be loaded when you call a method on it. And you do call a method on it: add(). So the data of the elements in the list is also loaded from the database.
Turn on SQL logging in devlopment, to see and understand all the queries executed by Hibernate.
If you want to avoid loading the list, replace your code by
MyParent parent = em.getReference("myId");
record.setParent(parent);
This won't load anything from the database, and it will make the association persistent because Record.parent is the owner side of the association. But beware that this will also make your in-memory object graph inconsistent if the parent has already been loaded before.
getReference() is useful when you don't want to use any members of the object but to give the reference of the object to another object. For example, when entity A referencing entity B and you want to set your b as B of A, then getReference() is what you need.
But in your case, when you get the proxy object, you immediately try to reach one of its members. (aListMappedAsOneToMany) Thus this will result, the whole parent object will be loaded from db.
It is true that, when you getAListMappedAsOneToMany().add(record), it will not load from db yet only if you set inverse="true"
You can learn more information about performance from:
http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/performance.html#performance-collections-mostefficentinverse
I'm currently working on a project where I'm going to do a lot of comparison of similar non-database (service layer objects for this discuss) objects and objects retrieved from the database via LinqToSql. For the sake of this discussion, assume I have a service layer Product object with a string field that is represented in the database. However, in the database, there is also a primary key Id that is not represented in the service layer.
Accordingly (as I often do for unit testing etc), I overrode Equals(Object), Equals(Product), and GetHashCode and implemented IEquatable with the expectation that I would be able to write code like this:
myContext.Products.Where(p => p.Equals(passedInProduct).SingleOrDefault();
And so forth.
The Equals override is tested and works. The objects are mutable so the usual caveats apply to the GetHashCode override. However, for the purposes of this example, the objects are not modified except by LtS and could be made readonly.
Here's the simple test:
Create a test object in memory and commit to the LtS context. By committing, the test object is populated with a few auto-generated fields.
Create another identical test object in memory (separate reference)
Attempt to retrieve the first object from the database using the second object as the criteria. (see code line above).
// Setup
string productDesc = "12A";
Product testProduct1 = _CreateTestProductInDatabase(productDesc);
Product testProduct2 = _CreateTestProduct(productDesc);
// check setup
Product retrievedProduct1 = ProductRepo.Retrieve(testProduct1);
//Assert.IsNotNull(retrievedProduct1);
// execute - try to retrieve the 'equivalent' product object
Product retrievedProduct2 = ProductRepo.Retrieve(testProduct2);
A simplified version of Retrieve (cruft removed is just parameter checks etc):
using (var dbContext = new ProductDataContext()) {
Product retrievedProduct = dbContext.Products
.Where(p => p.Equals(product)).SingleOrDefault();
NB: The overridden Equals method knows not to care about the auto-generated fields from the database and only looks at the string that is represented in the service layer.
Here's what I observed:
Retrieve on testProduct1 succeeds (no surprise, equal by reference)
Retrieve on testProduct2 fails (null)
The overridden Equals method called in the Retrieve method is never hit during either Retrieve calls
However, the overridden Equals method is called multiple times by the context on SubmitChanges (called when creating the first test object in the database) (works as expected).
Statically, the compiler knows that the type of the objects being emitted and is able to resolve the type.
So my specific questions:
Am I trying to do something ill-advised? Seems like a straightforward use of Equals.
Corollary to first question: alternate suggestions to deal with linq to sql equality checking while keeping comparison details inside the objects rather than the repository
Why might I have observed the Equals method being resolved in SubmitChanges but not in the Where clause?
I'm as much interested in understanding as making my Equals calls work. But I also would love to learn how to make this 'pattern' work rather than just understand why it appears to be an 'anti-pattern' in the contest of LtS and C#.
Please don't suggest I just filter directly on the context with Where statements. Obviously, I can remove the Equals call and do that. However, some of the other objects (not presented here) are large and a bit complicated. For the sake of maintenance and clarity, I want to keep knowledge of how to compare itself to another of its own type in one place and ideally as part of the object in question.
Some other things I tried that didn't change the behavior:
Overloaded and used == instead
Casting the lambda variable to the type p => (Product)p
Getting an IQueryable object first and calling Equals in the Where clause
Some other things I tried that didn't work:
Creating a static ProductEquals(Product first, Product second) method: System.NotSupportedException:has no supported translation to SQL.
Thanks StackOverflow contributors!
Re Possible dups: I've read ~10 other questions. I'd love a pointer to an exact duplicate but most don't seem to directly address what seems to be an oddity of LinqToSql.
Am I trying to do something ill-advised?
Absolutely. Consider what LINQ to SQL does: it creates a SQL representation of your query. It doesn't know what your overridden Equals method does, so can't translate that logic into SQL.
Corollary to first question: alternate suggestions to deal with linq to sql equality checking while keeping comparison details inside the objects rather than the repository
You'd need to do something with expression trees to represent the equality that way - and then build those expression trees up into a full query. It won't be fun, but it should be possible. It will affect how you build all your queries though.
I would have expected most database representations to be ID-based though, so you should be able to just compare IDs for equality. Usually when I've seen attempts to really model data in an OO fashion but store it in a database, the leakiness of the abstraction has caused a lot of pain.
Why might I have observed the Equals method being resolved in SubmitChanges but not in the Where clause?
Presumably SubmitChanges is working against a set of in-memory objects to work out what's changed - it doesn't have to do any conversion to SQL to do that part.
I've asked the question a few different times in a few different ways and I haven't yet gotten any responses. I'm trying again because I feel like my solution is too convoluted and I must be missing something simpler to do.
Using EF 4.1, POCO, DbContext API, AutoMapper, and Razor in an MVC 3 application.
I have a many-to-many relationship between two of my entities: Proposals and CategoryTags. I can successfully map (Automapper) a Proposal to my ProposalViewModel including the collection of CategoryTags.
In my View, I use javascript to allow the user to add, update, and remove tags by dynamically creating elements, each one that stores the ID of the chosen tag.
I can successfully post my ViewModel back to my controller with it's CategoryTags collection populated (although only with the ID property for each CategoryTag).
When that ViewModel is posted back to my controller, I don't know how to get those tags from the ViewModel and add them to my Model in such a way that db.SaveChanges() updates the database properly.
The only way I've had any success is to disconnect the CategoryTags collection in mapping (by namig them differently), iterate through each tag and manually look it up in my context and then call the .add() method. This is sloppy for a number of reasons which leads me to believe I'm doing it wrong.
Can anyone offer any direction at all?
UPDATE:
For anyone who is interested, my functional code:
Dim p As New Proposal
Dim tempTag As CategoryTag
p = AutoMapper.Mapper.Map(Of ProposalViewModel, Proposal)(pvm)
db.Proposals.Attach(p)
db.Entry(p).Collection("CategoryTags").Load()
For Each ct In pvm.Tags
tempTag = db.CategoryTags.Find(ct.Id)
If tempTag Is Nothing Then
Continue For
End If
If ct.Tag = "removeMe" Then
p.CategoryTags.Remove(tempTag)
Continue For
End If
p.CategoryTags.Add(tempTag)
Next
db.Entry(p).State = EntityState.Modified
db.SaveChanges()
Return RedirectToAction("Index")
The only working way is doing this manually - you can read full description of the problem if you want. The description is related to ObjectContext API but DbContext API is just wrapper suffering same issues (actually DbContext API suffers even more issues in this scenario and because of that I will skip solution with manually setting relationships).
In short. Once you post your data back to the controller you must create new context instance and attach your Proposal and realated CategoryTags. But after that you must inform the context about changes you did. It means you must say context which tags have been added to proposal and which have been removed. Otherwise context cannot process your changes because it doesn't do any automatic merge with data in database.
The easiest way to solve this is loading current Proposal with related CategoryTags from database (= you will have attached instances) and merge incoming data into attached object graph. It means you will manually remove and add tags based on posted values.
In my collection view I need to generate an index for each item. As Items get reordered I need this index to update with its new position.
The data are Core Data entities in a managed NSArrayController.
The closest I have come to a possible solution is implementing this method on the entity class and then using representedObject.dynamicIndex to bind it to the UI.
- (NSNumber *) dynamicIndex
{
NSInteger r = [[[[self managedObjectContext] registeredObjects] allObjects] indexOfObject:self];
NSNumber *result = [NSNumber numberWithInt:r];
return result;
}
This solution is sketchy at best, and not really functional as it doesn't necessarily reflect the order in the collection view.
Anyone have a model / mechanism for generating or retrieving item indexes in an NSCollectionView?
First, make sure you understand the difference between (and properly use the terminology of) "entity" and "instance." It makes all the difference in communicating your problems/solutions with others.
Second: Don't worry about NSCollectionViewItems ... worry about each one's "represented object," which is held in some container.
Third: Did you want the display order to be a persistent attribute of your entity or do you just need to know what position the item is in at the moment, regardless of what it might be later? Important question.
Fourth: Core Data does not give you the concept of ordered collections. This is to support store types such as NSSQLiteStoreType, where you might only want to fault in a few items (or one) without loading the whole list. Therefore, you're on your own if you want a persistent sort order. To do this, just add an attribute to your entity called "sortOrder" and make it a number type.
Fifth: Because of the "no ordered collections" issue above, your attempt to find the index of a given instance of your entity from an array, built from a set, which was faulted in with a nondeterministic order is doomed to failure.
Sixth: Since you're using an array controller, you'll need to set its sort descriptors. You'll want to use your "sortOrder" key. That way, your fetched instances will always be kept sorted by their "sortOrder."
Seventh and finally: If you're trying to get the index of any objects in your array controller's set/array of objects, you'll want to ask it for its -arrangedObjects, so you're getting the index of the object in the sorted collection the array controller controls.
Hope that helps.
Update for Lion (10.7)
With regard to my sixth point: If you're targeting 10.7 and above in your application, [NSManagedObject now gives you ordered relationships.][1] Use -mutableOrderedSetValueForKey: and -mutableOrderedSetValueForKey: to set and retrieve NSOrderedSets. Yay!
Essentially, I have to get a flat file into a database. The flat files come in with the first two characters on each line indicating which type of record it is.
Do I create a class for each record type with properties matching the fields in the record? Should I just use arrays?
I want to load the data into some sort of data structure before saving it in the database so that I can use unit tests to verify that the data was loaded correctly.
Here's a sample of what I have to work with (BAI2 bank statements):
01,121000358,CLIENT,050312,0213,1,80,1,2/
02,CLIENT-STANDARD,BOFAGB22,1,050311,2359,,/
03,600812345678,GBP,fab1,111319005,,V,050314,0000/
88,fab2,113781251,,V,050315,0000,fab3,113781251,,V,050316,0000/
88,fab4,113781251,,V,050317,0000,fab5,113781251,,V,050318,0000/
88,010,0,,,015,0,,,045,0,,,100,302982205,,,400,302982205,,/
16,169,57626223,V,050311,0000,102 0101857345,/
88,LLOYDS TSB BANK PL 779300 99129797
88,TRF/REF 6008ABS12300015439
88,102 0101857345 K BANK GIRO CREDIT
88,/IVD-11 MAR
49,1778372829,90/
98,1778372839,1,91/
99,1778372839,1,92
I'd recommend creating classes (or structs, or what-ever value type your language supports), as
record.ClientReference
is so much more descriptive than
record[0]
and, if you're using the (wonderful!) FileHelpers Library, then your terms are pretty much dictated for you.
Validation logic usually has at least 2 levels, the grosser level being "well-formatted" and the finer level being "correct data".
There are a few separate problems here. One issue is that of simply verifying the data, or writing tests to make sure that your parsing is accurate. A simple way to do this is to parse into a class that accepts a given range of values, and throws the appropriate error if not,
e.g.
public void setField1(int i)
{
if (i>100) throw new InvalidDataException...
}
Creating different classes for each record type is something you might want to do if the parsing logic is significantly different for different codes, so you don't have conditional logic like
public void setField2(String s)
{
if (field1==88 && s.equals ...
else if (field2==22 && s
}
yechh.
When I have had to load this kind of data in the past, I have put it all into a work table with the first two characters in one field and the rest in another. Then I have parsed it out to the appropriate other work tables based on the first two characters. Then I have done any cleanup and validation before inserting the data from the second set of work tables into the database.
In SQL Server you can do this through a DTS (2000) or an SSIS package and using SSIS , you may be able to process the data onthe fly with storing in work tables first, but the prcess is smilar, use the first two characters to determine the data flow branch to use, then parse the rest of the record into some type of holding mechanism and then clean up and validate before inserting. I'm sure other databases also have some type of mechanism for importing data and would use a simliar process.
I agree that if your data format has any sort of complexity you should create a set of custom classes to parse and hold the data, perform validation, and do any other appropriate model tasks (for instance, return a human readable description, although some would argue this would be better to put into a separate view class). This would probably be a good situation to use inheritance, where you have a parent class (possibly abstract) define the properties and methods common to all types of records, and each child class can override these methods to provide their own parsing and validation if necessary, or add their own properties and methods.
Creating a class for each type of row would be a better solution than using Arrays.
That said, however, in the past I have used Arraylists of Hashtables to accomplish the same thing. Each item in the arraylist is a row, and each entry in the hashtable is a key/value pair representing column name and cell value.
Why not start by designing the database that will hold the data then you can use the entity framwork to generate the classes for you.
here's a wacky idea:
if you were working in Perl, you could use DBD::CSV to read data from your flat file, provided you gave it the correct values for separator and EOL characters. you'd then read rows from the flat file by means of SQL statements; DBI will make them into standard Perl data structures for you, and you can run whatever validation logic you like. once each row passes all the validation tests, you'd be able to write it into the destination database using DBD::whatever.
-steve