I have two entities which are Student and Class entities.
Student and Class are many to one relationship. So student contains class attribute.
Now i want to save or create a student associate with existing class(means i know primary key ID already).
Solution 1:
Student student = new Student();
Class class = session.load(classId);
student.setClass(class);
session.save(student);
Solution 2:
Student student = new Student();
Class class = new Class();
class.setClassId(classId);
student.setClass(class);
session.save(student);
My question here is in solution 1 it will issue two SQL, one is to get Class another is to insert student. But in solution 2 only need to have one SQL. If I have more class attribute,
i will load and issue more select sql before insert. It seems not that efficient. Is there any side-effect in solution 2?
which way to do save/insert is better? By the way, i do not set up cascade.
Thank you
Yee Chen
Solution 1 won't issue an SQL query to load Class. Unlike get(), load() returns a proxy object with the specified identifier and doesn't perform a database query immediately. Thus, load() method is a natural choice for this scenario (when you actually need to load an object, use get()).
Possible side effect of solution 2 depends on cascading configuration of relationship and so on. Even if it works fine in your current case, it makes your code more fragile, since seemingly unrelated changes in the code may break it.
So, I recommend you to use Solution 1 and don't worry about performance.
Is there any side-effect in solution 2?
First of all, you haven't associated your student with any class there.
Secondly, where do you get that class id from in the general case? At some earlier point in time, you had to either fetch an existing class instance from the DB, or create a new instance and persist it, so that you get its id. Of course, reusing an entity you already have is fine, but juggling with ids like you do above is IMHO not.
Thirdly, it is not a good idea to prematurely optimize your app. Get it to work properly first, then measure performance, and optimize only if and where needed.
Related
This question aims to get the most clean and "best" way to handle this kind of problem.
I've read many questions about how to handle inheritance in SQL and like the Table Per Type model most and would like to use it. The problem with this is that you have to know what type you are going to query to do the proper join.
Let's say we have three tables:Son, Daughter and Child.
This works very well if you for example want to query all daughters. You can simply join the child and get all the information.
What I'm trying to do is to query a Child by ID and get the associated sub class information. What I could do is to add a column Type to the child and select the associated data with a second select, but that does not seem pretty nice. Another way to do it would be to join all sub tables, but that doesn't seem to be that nice either.
Is there an inheritance model to solve this kind of problem in a clean, nice and performant way?
I'm using MySQL btw
Given your detailed definition in the comment with the use case
The Server gets the http request domain.com/randomID.
it becomes apparent, that you have a single ID at hand for which you want to retrieve the attributes of derived entities. For your case, I would recommend to use the LEFT JOIN approach:
SELECT age,
son.id is not null as isSon,
randomColumn,
daughter is not null as isDaughter,
whatEver
FROM child
LEFT JOIN son on child.id = son.id
LETT JOIN daughter on child.id = daughter.id
WHERE
child.id = #yourRandomId
This approach, BTW, stays very close to your current database design and thus you would not have to change much. Yet, you are able to benefit from the storage savings that the improved data model provides.
Besides that, I do not see many chances to do it differently:
You have different columns with different datatypes (esp. if looking at your use case), so it is not possible to reduce the number of columns by combining some of them.
Introducing a type attribute is already rejected in your question; sending single SELECT statements as well.
In the comment you are stating that you are looking for something like Map<ID, Child> in MySQL. Please note that this java'ish expression is a compile-time expression which gets instantiated during runtime with the corresponding type of the instance. SQL does not know the difference between runtime and compile-time. Thus, there is also no need for such a generic expression. Finally, also please note that in case of your Java program, you also need to analyse (by introspection or usage of instanceof) which type your value instance has -- and that is also a "single-record" activity which you need to perform.
So I have the following model structure in my Django App:-
class SuperModel(models.Model):
f1 = models.CharField()
f2 = models.CharField()
class Model(SuperModel):
f3 = models.CharField()
class OverrideModel(models.Model):
fpk = models.OneToOneField(Model, primary_key=True)
f1 = models.CharField()
f2 = models.CharField()
Basically, in my application, the fields f1 and f2 in the Model table contain user information that I have entered. The user has the ability to override this information and any changes he/she makes in the data is stored in the OverrideModel table (because I do not want to lose the information that I had entered first). Think of it as me creating user profiles earlier while now I want the user to be able to edit his/her own profile without losing the information that I had entered about them.
Now, since the rest of my application (views/templates etal) work with the field names in the Model class, what I want is to create a view of the data that fetches the field f1 from the override table if it exists, otherwise it should pickup f1 from the table it used to earlier without resorting to a raw queryset.
I will describe everything I have considered so far so that some of the other constraints I am working with become clear:-
Model.objects.annotate(f1=Case(When(overridemodel__f1__isnull=True, then=F('f1')), default=F('overridemodel__f1'))).
This throws the error that the annotate alias conflicts with a field already in the table.
Model.objects.defer('f1').extra(select={'f1': 'CASE WHEN ... END'}, tables=..., where=...).
This approach cannot be applied because I could not figure out a way to apply an outer join using extra. The override model may not have a row corresponding to each model row. Specifying the override table in the tables clause performs a cross product operation which combined with where can be used to perform an inner join, not an outer join (although I'd be happy to be proved wrong).
EDIT: I have realized that select_related might be able to solve the above problem but if I filter the queryset generated by Model.objects.select_related('overridemodel').defer('f1').extra(select={'f1': 'CASE WHEN ... END'}, tables=..., where=...) on the field f1, say qs.filter(f1='Random stuff') the where clause for the filter query uses the Model.f1 field rather than the f1 field generated in extra. So this approach is also futile.
Using Model.objects.raw() to get a raw queryset.
This is a non-starter because the Django ORM becomes useless after using raw and I need to be able to filter / sort the model objects as part of the application.
Defining methods/properties on the Model class.
Again, I will not be able to use the same field names here which involves hunting through code for all usages and making changes.
Creating a view in the database that gives me what I want and creating an unmanaged model that reads the data from that view.
This is probably the best solution for my problem but having never used an unmanaged model before, I'm not sure how to go about it or what pitfalls I might encounter. One problem that I can think of off the top of my head is that my view always has to be kept in sync with the models but that seems a small price to pay compared to hunting through the codebase and making changes and then testing to see if anything broke.
So, there you have it. As always, any help / pointers will be greatly appreciated. I have tried to provide as minimal an example as possible; so if any more information is required I'll be happy to provide it.
Also, I am using Django 1.8 with MySQL.
I realized that there is no easy canonical way to solve my problem. Even with using option 5 (creating a view that is ORM manipulated using an unmanaged Model), I would lose the related query names on the original model that are being used in my filtering / sorting.
So, for anyone else with a similar problem I would recommend the approach I finally went with which is not keeping an OverrideModel but an OverriddenModel which keeps the values that are overridden whenever the user makes changes and updating the original Model with the override values so that the model always contains the values on which filtering / querying is going to occur
I have a table Users and a table Items
In the Items table, I have fields such as
ModifiedBy
CreatedBy
AssignedTo
which all have a userId integer. The database is set up to have these as foreign keys back to the Users table.
When using LINQToSQL, the relationships which are automatically built from the dbml end up giving me names like User, User1 and User2
e.g. myItem.User1.Name or myItem.User2.Name
Obviously this isn't very readable and I'd like it be along the lines of
myItem.CreatedByUser.Name or myItem.ModifiedByUser.Name etc
I could change the names of the relationships but that means I have to redo that every time I change the db schema and refresh the dbml.
Is there any way round this?
The simple answer: No.
Someone has suggested the idea of creating partial Association classes where the property names get defined, but that won't work either: Renaming LINQ 2 SQL Entity Properties Through Partial Classes.
Your choice is to either spend a little time learning more about LINQ-to-SQL "behind-the-scenes" so that you can manually make the necessary modifications or to just change the property names through the Properties window. Personally, I just delete/redrag/rename, because not setting a property correctly is a pain to debug because the exceptions that get thrown give you little to no clue as to what caused it. I even went so far as to create a unit test library that takes each MetaTable object in the model and verifies the field count, the ServerDataType contents of each field, the association count, the names of each association, and the names of each end of the association. Every few changes, I run the unit tests to make sure that the model is intact.
Firstly, no... the names are created based on the second table in the relationship.
But what you should know is that you don't have to "refresh" (meaning, delete the table in the DBML then re-drag-and-drop it).
For the project I'm working on, we have over 200 tables... about 50 of which we have manually tweaked after dragging them from the database. We never delete and re-drag tables as there have been so many changes post-auto-generation.
I just add a small partial class to extend the object with suitably named properties, example below:
namespace Database.TableModels {
partial class WTSR_Induction {
public EmailTemplate ConfirmationEmailTemplate {
get { return EmailTemplate1; }
}
public EmailTemplate InviteEmailTemplate {
get { return EmailTemplate; }
}
}
}
In this example, the WTSR_Inductions table has two links to the EmailTemplates table, hence the EmailTemplate and EmailTemplate1 properties.
A bit late but you can do this by selecting the relationship on the linq model and go to properties and update the parent property name.
You could use linq to sql without the dbml it may be extra work upfront but from the perspective of a change to a table column name it may be easier than changes to the dbml as you have described.
I suggest creating extension methods mapping the names you want to the names you get from the autogenerated code. That way after each auto-generation you don't have to change the autogenerated code, but only your own extension methods. That, plus the unit tests to do sanity checks as suggested elsewhere on this page should work fine.
I have just faced this problem myself and I'm off to try to implement my own suggestion.
EDIT: This seems relevant:
SQLMetal Multiple Foreign Keys Pointing to One Table Issue
I have a question related to this one. I don't want to do a calculation (aggregation), but I need to get display values from an association. In my C# code, I can directly reference the value, because the foreign key constraint made Linq generate all the necessary wiring.
When I specify the IQueryable as the Gridview datasource property, and reference something that is not a column of the primary entity in the result set, I get an error that the column does not exist.
As a newbie to Linq, I am guessing the assignment implicitely converts the IQueryable to a list, and the associations are lost.
My question is, what is a good way to do this?
I assume that I can work around this by writing a parallel query returning an anonymous type that contains all the columns that I need for the gridview. It seems that by doing that I would hold data in memory redundantly that I already have. Can I query the in-memory data structures on the fly when assigning the data source? Or is there a more direct solution?
The gridview is supposed to display the physician's medical group associations, and the name of the association is in a lookup table.
IQueryable<Physician> ph =
from phys in db.Physicians
//from name in phys.PhysicianNames.DefaultIfEmpty()
//from lic in phys.PhysicianLicenseNums.DefaultIfEmpty()
//from addr in phys.PhysicianAddresses.DefaultIfEmpty()
//from npi in phys.PhysicianNPIs.DefaultIfEmpty()
//from assoc in phys.PhysicianMedGroups.DefaultIfEmpty()
where phys.BQID == bqid
select phys;
(source: heeroz.com)
So, based on Denis' answer, I removed all the unneeded stuff from my query. I figured that I may not be asking the right question to begin with.
Anyways, the page shows a physician's data. I want to display all medical group affiliations in a grid (and let the user insert, edit, and update affiliations). I now realize that I don't need to explicitly join in these other tables - Linq does that for me. I can access the license number, which is in a separate table, by referencing it through the chain of child associations.
I cannot reference the medical group name in the gridview, which brings me back to my question:
AffiliationGrid.DataSource = ph.First().PhysicianMedGroups;
This does not work, because med_group_print_name is not accessible for the GridView:
A field or property with the name 'med_group_print_name' was not found on the
selected data source.
Again, bear with me, if it is all too obvious that I don't understand Linq ... because I don't.
Your query seems strange. You should try to simply display
ph = from phys in db.Physicians
where phys.BQID == bqid
select phys;
in your grid. That should work.
Also, why the calls to Load()? If the DataContext is not disposed when the grid is binding, you should not need it.
If you still have issues, can you please post the error message you get, that would help...
Part 2
The problem is that you have the name is effectively not in the PhysMedGroup. You need to navigate one level down to the MedGroupLookup to access the name, since it is a property of that class.
Depending on the technology you are using (it seems to be either WinForms or Web Forms), you will need to configure your data-binding to access MedGroupLookup.med_group_print_name.
I am trying to inherit from my generated datacontext in LinqToSQL - something like this
public class myContext : dbDataContext {
public System.Data.Linq.Table<User>() Users {
return (from x in base.Users() where x.DeletedOn.HasValue == false select x);
}
}
But my Linq statement returns IQueryable which cannot cast to Table - does anyone know a way to limit the contents of a Linq.Table - I am trying to be certain that anywhere my Users table is accessed, it doesn't return those marked deleted. Perhaps I am going about this all wrong - any suggestions would be greatly appreciated.
Hal
Another approach would to be use views..
CREATE VIEW ActiveUsers as SELECT * FROM Users WHERE IsDeleted = 0
As far as linq to sql is concerned, that is just the same as a table. For any table that you needed the DeletedOn filtering, just create a view that uses the filter and use that in place of the table in your data context.
You could use discriminator column inheritance on the table, ie. a DeletedUsers table and ActiveUsers table where the discriminator column says which goes to which. Then in your code, just reference the Users.OfType ActiveUsers, which will never include anything deleted.
As a side note, how the heck do you do this with markdown?
Users.OfType<ActiveUsers>
I can get it in code, but not inline
Encapsulate your DataContext so that developers don't use Table in their queries. I have an 'All' property on my repositories that does a similar filtering to what you need. So then queries are like:
from item in All
where ...
select item
and all might be:
public IQueryable<T> All
{
get { return MyDataContext.GetTable<T>.Where(entity => !entity.DeletedOn.HasValue); }
}
You can use a stored procedure that returns all the mapped columns in the table for all the records that are not marked deleted, then map the LINQ to SQL class to the stored procedure's results. I think you just drag-drop the stored proc in Server Explorer on to the class in the LINQ to SQL designer.
What I did in this circumstance is I created a repository class that passes back IQueryable but basically is just
from t in _db.Table
select t;
this is usually referenced by tableRepository.GetAllXXX(); but you could have a tableRepository.GetAllNonDeletedXXX(); that puts in that preliminary where clause to take out the deleted rows. This would allow you to get back the deleted ones, the undeleted ones and all rows using different methods.
Perhaps my comment to Keven sheffield's response may shed some light on what I am trying to accomplish:
I have a similar repository for most
of my data access, but I am trying to
be able to traverse my relationships
and maintain the DeletedOn logic,
without actually calling any
additional methods. The objects are
interrogated (spelling fixed) by a StringTemplate
processor which can't call methods
(just props/fields).
I will ultimately need this DeletedOn filtering for all of the tables in my application. The inherited class solution from Scott Nichols should work (although I will need to derive a class and relationships for around 30 tables - ouch), although I need to figure out how to check for a null value in my Derived Class Discriminator Value property.
I may just end up extended all my classes specifically for the StringTemplate processing, explicitly adding properties for the relationships I need, I would just love to be able to throw StringTemplate a [user] and have it walk through everything.
There are a couple of views we use in associations and they still appear just like any other relationship. We did need to add the associations manually. The only thing I can think to suggest is to take a look at the properties and decorated attributes generated for those classes and associations.
Add a couple tables that have the same relationship and compare those to the view that isn't showing up.
Also, sometimes the refresh on the server explorer connection doesn't seem to work correctly and the entities aren't created correctly initially, unless we remove them from the designer, close the project, then reopen the project and add them again from the server explorer. This is assuming you are using Visual Studio 2008 with the linq to sql .dbml designer.
I found the problem that I had with the relationships/associations not showing in the views. It seems that you have to go through each class in the dbml and set a primary key for views as it is unable to extract that information from the schema. I am in the process of setting the primary keys now and am planning to go the view route to isolate only non-deleted items.
Thanks and I will update more later.