I have two entities with Many-to-Many relationship: Author and Book.
I want to get a non-deleted author with all his non-deleted books (these entities have field deleted). And I try this #Query:
select a from Author a left join fetch a.books b
where a.id = :id and a.deleted = false and b.deleted = false
But it returns nothing, for example, if an author has only deleted books.
Of course, without b.deleted = false author is returned. But I want a query to return author without any deleted books.
How can I do that?
Maybe, with a native query?
Related
I have the following model: a "classical" many-to-many book/author association but with 2 peculiarities.
Author does not have an association to Books.
The Author personal data is stored on other associated entity, called Person.
The model:
public class Book {
#ManyToMany
List<Author> authors;
}
public class Author {
#OneToOne
Person person;
}
public class Person {
String name;
}
I would like to write a service which, given a book, will return all the authors associated with it. I would also like to retrieve the personal data avoiding the N+1 problem.
Inside my Book repo I wrote a custom query like that:
#Query("select b.authors from Book b join b.authors a join fetch a.person where b = :book")
List<Author> listAuthors(#Param("book") Book book);
But I have the folloiwing error:
query specified join fetching, but the owner of the fetched
association was not present in the select list (...)
I could manage to solve it by adding the association to Books on the Author side and fetching Person from it. Unfortunately , in my case, it is really desirable that Authors do not know about Books.
Is there any way to fetch Person data from a Book?
This is actually a Hibernate Bug HHH-14116 - I recently stumbled across a related bug and in the discussion in the Hibernate Chat this bug was filed too. There is already a fix, but AFAIK it has not been released yet.
One way to circumvent this would be to have the relationship be bi-directional and query from the Author side.
Otherwise you could do it with a subquery, but I'm not sure whether that works with Spring Data JPA:
select a from Author a join fetch a.person where a.id in
(select a.id from Book b join b.authors a where b = :book)
Have you tried selecting Person with your #Query as well?
#Query("select b.authors, a.person from Book b join b.authors a join fetch a.person where b = :book")
List<Author> listAuthors(#Param("book") Book book);
I solved it using an EXISTS clause:
#Query("select a from Author a join fetch a.person " +
"where exists (select 1 from Book b where b = :book and a member of b.authors)")
List<Author> listAuthors(#Param("book") Book book);
I perform a simple query like this, but loose all the objects that do not have an association with episodes:
$query = $this->getEntityManager()
->createQuery('
SELECT p,e
FROM AcmeDemoBundle:Place p
JOIN p.episodes e
WHERE p.id = :id'
)
->setParameter('id',$id);
This is a simple asso:
/**
* #ORM\OneToMany(targetEntity="Episode", mappedBy="place")
*/
protected $episodes;
My query automatically discards the objects that have an empty $episodes Collection. Does anyone know why? I am sure it makes sense but I can't figure this out.
Is there a way to fetch the object anyway whether there is or there is not the asso episodes?
Many thanks.
I think you want a LEFT JOIN instead of just a JOIN.
JOIN gives only records from the left that also have a record on the right.
LEFT JOIN gives records from the left regardless of if they also have a record on the right.
My data structure is set up this way
A user takes a number of modules
A module contains a number of courses
Here's how the relationship looks like:
How do I get a list of courses the user takes?
The query I have now is:
var courses = (from ClassEnrollment enrolment in entities.ClassEnrollment
where enrolment.UserID == UserID
join Module module in entities.Module
on enrolment.ModuleID equals module.ID
select module.Course
).ToList();
However, this doesn't result in a list of courses, but rather a list of list of courses.
How can I flatten this query to a list of distinct courses?
According to your data structure screenshot, you have a one-to-many relationship between the ClassEnrollment and Module, as well as navigational property called Module. You also have a many-to-many relationship between Module and Course, but the navigational property should be called Courses. Given your code, you want something like this:
var courses = entities.
ClassEnrollment.
Where(e => e.UserID == UserID).
SelectMany(e => e.Module.Courses).
ToList();
Your question, however, mentions a user: A user takes a number of modules, How do I get a list of courses the user takes?. I don't see any User entity anywhere else, though, so it would be nice if you could clarify. Are you using LINQ-to-SQL, btw?
Something like this:
var courses = from ClassEnrollment enrolment in entities.ClassEnrollment
from module in entities.Module
where enrolment.ModuleID equals module.ID && enrolment.UserID equals UserID
select module.Course
Use SelectMany.
You can use
courses.SelectMany(c => c);
In your query you don't need explicitly specify the type for the range variables
Or you can join course to the query
var query = from enrolment in entities.ClassEnrollment
join module in entities.Module on enrolment.ModuleID equals module.ID
join course in entities.Course on module.CourseID equals course.ID
where enrolment.UserID == UserID
select course;
var course = query.ToList();
Here is the expression
x => x.stf_Category.CategoryID == categoryId
x refers to an Product Entity that contains a Category. I am trying to load all Products that match given categoryId.
In the db the Product table contains a Foreign Key reference to Category (via CategoryId).
Question: I think I am doing it wrong. Is there something else one has to do in EF4 to create a LINQ expression of this type?
Are there any good examples of EF4 Linq expressions out there? Specifically something that queries on the basis of related entities such as my problem ?
Thanks !
You're looking for the Include method.
var query = db.Products.Include("Categories");
This is commonly referred to as eager loading.
Entity Framework will 'infer' the JOIN constraint based on the mapping you have specified.
The "magic string" needs to match the Entity Set name on your EDMX.
Check out this post for more info.
EDIT
I'm a little confused as to whether you want the Products and Categories, or just the Products which have a specific Category ID.
If the latter, this is the way to go:
var query = from p in db.products
join c in db.categories
on p.CategoryId equals c.CategoryId
where c.CategoryId == someCategoryId
select p;
Keep in mind though, the above query is exactly the same result as your original query.
If p is a product, then p.Categories will look at the Navigational Property of your Product entity on the EDMX, in which case it will be your Category FK.
As long as you setup your Navigational properties right, p.Categories is fine.
If you are using EF4 and the association between Category and Product classes has been picked up and defined in your Model, then all products with a specific categoryID can be selected as simple as:
x => x.CategoryID == categoryID
You don't need to join nor an eager loading for that.
Let's say I have an Order table which has a FirstSalesPersonId field and a SecondSalesPersonId field. Both of these are foreign keys that reference the SalesPerson table. For any given order, either one or two salespersons may be credited with the order. In other words, FirstSalesPersonId can never be NULL, but SecondSalesPersonId can be NULL.
When I drop my Order and SalesPerson tables onto the "Linq to SQL Classes" design surface, the class builder spots the two FK relationships from the Order table to the SalesPerson table, and so the generated Order class has a SalesPerson field and a SalesPerson1 field (which I can rename to SalesPerson1 and SalesPerson2 to avoid confusion).
Because I always want to have the salesperson data available whenever I process an order, I am using DataLoadOptions.LoadWith to specify that the two salesperson fields are populated when the order instance is populated, as follows:
dataLoadOptions.LoadWith<Order>(o => o.SalesPerson1);
dataLoadOptions.LoadWith<Order>(o => o.SalesPerson2);
The problem I'm having is that Linq to SQL is using something like the following SQL to load an order:
SELECT ...
FROM Order O
INNER JOIN SalesPerson SP1 ON SP1.salesPersonId = O.firstSalesPersonId
INNER JOIN SalesPerson SP2 ON SP2.salesPersonId = O.secondSalesPersonId
This would make sense if there were always two salesperson records, but because there is sometimes no second salesperson (secondSalesPersonId is NULL), the INNER JOIN causes the query to return no records in that case.
What I effectively want here is to change the second INNER JOIN into a LEFT OUTER JOIN. Is there a way to do that through the UI for the class generator? If not, how else can I achieve this?
(Note that because I'm using the generated classes almost exclusively, I'd rather not have something tacked on the side for this one case if I can avoid it).
Edit: per my comment reply, the SecondSalesPersonId field is nullable (in the DB, and in the generated classes).
The default behaviour actually is a LEFT JOIN, assuming you've set up the model correctly.
Here's a slightly anonymized example that I just tested on one of my own databases:
class Program
{
static void Main(string[] args)
{
using (TestDataContext context = new TestDataContext())
{
DataLoadOptions dlo = new DataLoadOptions();
dlo.LoadWith<Place>(p => p.Address);
context.LoadOptions = dlo;
var places = context.Places.Where(p => p.ID >= 100 && p.ID <= 200);
foreach (var place in places)
{
Console.WriteLine(p.ID, p.AddressID);
}
}
}
}
This is just a simple test that prints out a list of places and their address IDs. Here is the query text that appears in the profiler:
SELECT [t0].[ID], [t0].[Name], [t0].[AddressID], ...
FROM [dbo].[Places] AS [t0]
LEFT OUTER JOIN (
SELECT 1 AS [test], [t1].[AddressID],
[t1].[StreetLine1], [t1].[StreetLine2],
[t1].[City], [t1].[Region], [t1].[Country], [t1].[PostalCode]
FROM [dbo].[Addresses] AS [t1]
) AS [t2] ON [t2].[AddressID] = [t0].[AddressID]
WHERE ([t0].[PlaceID] >= #p0) AND ([t0].[PlaceID] <= #p1)
This isn't exactly a very pretty query (your guess is as good as mine as to what that 1 as [test] is all about), but it's definitively a LEFT JOIN and doesn't exhibit the problem you seem to be having. And this is just using the generated classes, I haven't made any changes.
Note that I also tested this on a dual relationship (i.e. a single Place having two Address references, one nullable, one not), and I get the exact same results. The first (non-nullable) gets turned into an INNER JOIN, and the second gets turned into a LEFT JOIN.
It has to be something in your model, like changing the nullability of the second reference. I know you say it's configured as nullable, but maybe you need to double-check? If it's definitely nullable then I suggest you post your full schema and DBML so somebody can try to reproduce the behaviour that you're seeing.
If you make the secondSalesPersonId field in the database table nullable, LINQ-to-SQL should properly construct the Association object so that the resulting SQL statement will do the LEFT OUTER JOIN.
UPDATE:
Since the field is nullable, your problem may be in explicitly declaring dataLoadOptions.LoadWith<>(). I'm running a similar situation in my current project where I have an Order, but the order goes through multiple stages. Each stage corresponds to a separate table with data related to that stage. I simply retrieve the Order, and the appropriate data follows along, if it exists. I don't use the dataLoadOptions at all, and it does what I need it to do. For example, if the Order has a purchase order record, but no invoice record, Order.PurchaseOrder will contain the purchase order data and Order.Invoice will be null. My query looks something like this:
DC.Orders.Where(a => a.Order_ID == id).SingleOrDefault();
I try not to micromanage LINQ-to-SQL...it does 95% of what I need straight out of the box.
UPDATE 2:
I found this post that discusses the use of DefaultIfEmpty() in order to populated child entities with null if they don't exist. I tried it out with LINQPad on my database and converted that example to lambda syntax (since that's what I use):
ParentTable.GroupJoin
(
ChildTable,
p => p.ParentTable_ID,
c => c.ChildTable_ID,
(p, aggregate) => new { p = p, aggregate = aggregate }
)
.SelectMany (a => a.aggregate.DefaultIfEmpty (),
(a, c) => new
{
ParentTableEntity = a.p,
ChildTableEntity = c
}
)
From what I can figure out from this statement, the GroupJoin expression relates the parent and child tables, while the SelectMany expression aggregates the related child records. The key appears to be the use of the DefaultIfEmpty, which forces the inclusion of the parent entity record even if there are no related child records. (Thanks for compelling me to dig into this further...I think I may have found some useful stuff to help with a pretty huge report I've got on my pipeline...)
UPDATE 3:
If the goal is to keep it simple, then it looks like you're going to have to reference those salesperson fields directly in your Select() expression. The reason you're having to use LoadWith<>() in the first place is because the tables are not being referenced anywhere in your query statement, so the LINQ engine won't automatically pull that information in.
As an example, given this structure:
MailingList ListCompany
=========== ===========
List_ID (PK) ListCompany_ID (PK)
ListCompany_ID (FK) FullName (string)
I want to get the name of the company associated with a particular mailing list:
MailingLists.Where(a => a.List_ID == 2).Select(a => a.ListCompany.FullName)
If that association has NOT been made, meaning that the ListCompany_ID field in the MailingList table for that record is equal to null, this is the resulting SQL generated by the LINQ engine:
SELECT [t1].[FullName]
FROM [MailingLists] AS [t0]
LEFT OUTER JOIN [ListCompanies] AS [t1] ON [t1].[ListCompany_ID] = [t0].[ListCompany_ID]
WHERE [t0].[List_ID] = #p0