Ok, lets say you have two tables: Order and OrderLine and for some reason they do not have a foreign key relationship in the database (it's an example, live with it). Now, you want to join these two tables using Entity Framework and you cook up something like this:
using (var model = new Model())
{
var orders = from order in model.Order
join orderline in model.OrderLine on order.Id equals orderline.OrderId into orderlines
from ol in orderlines.DefaultIfEmpty()
select new {order = order, orderlines = orderlines};
}
Now, the above will produce orders and orderlines, left-joined and all, but it has numerous issues:
It's plain ugly
It returns an anonymous type
It returns multiple instances of the same order and I you have to do Distinct() on the client side because orders.Distinct() fails.
What I am looking for is a solution which is:
Pretty
Returns a statically well-known type instead of the anonymous type (I tried to project the query result, but I got into problems with the OrderLines)
Runs Distinct on the server side
Anyone?
Even if the database tables do not have a foreign key relationship setup, you can configure Entity Framework as if they do.
Add an OrderDetails navigation property to your Order class and then just query Orders.
Related
trying to work out how to do LINQ to Entities query for a many to many relationship which has a junction with fields table.
Below are the Domains models (I am using View models, but keeping it simple for this example).
Student Domain model
StudentID (PK)
ICollection<StudentCourse> StudentCourses
StudentCourse Domain model
StudentCourseID (PK)
StudentID (FK)
CourseID (FK)
ForAdult
ForSeniour
Description
Course Domain model
CourseID (PK)
ICollection<StudentCourse> StudentCourses
Note:
Since the junction table (i.e. StudentCourse) contains fields other than the two foreign keys, EF will create an entity for this.
Lazy Loading
I've got this working for lazy loading. The Navigation properties have been declared with the 'virtual' keyword.
The Query way - works!
var student = (from s in context.Students
where s.StudentID == id
select s).SingleOrDefault<Student>()
The Method way - works!
Student student = context.Students.Find(id);
Projection
BUT, I would prefer to do this with projection, for performance reasons, i.e. less trips to the database.
I'm really stuck on how to write up the LINQ to Entities query to return 1 student with (1 or) many StudentCourses.
I don't understand thoroughly how the Entity should be shaped, if you know what I mean.
For example, I've tried:
var myvar = from s in context.Students
from sc in s.StudentCourses
where s.StudentID == id
select s
What I require is to return an entity of Student with a collection of StudentCourses which could then be assigned to a Student and passed to the View model, then to the View.
Really would appreciate any help, as I've spent alot of time trying to solve this.
Also as a side note, I'm using the SingleOrDefault() method to cast the results of the var (IQueryable I think) to type Student. Is this the preferred way to cast?
You can get EF to eagerly load the related entities by using the Include method.
So using your LINQ example:
var student = (from s in context.Students
where s.StudentID == id
select s).Include("StudentCourses").FirstOrDefault();
And using extension methods:
var student = context.Students.Include("StudentCourses").FirstOrDefault(id);
The Student instance that is returned will have the StudentCourses collection populated with related entities. This should invoke only one SQL query that joins the tables together.
To answer your aside question: I prefer to use FirstOrDefault most of the time as above. The difference is that SingleOrDefault will expect exactly one result and throws an exception otherwise, whereas FirstOrDefault will return null if a student is not found.
Also, as the cast to Student is implicit, you don't need the <Student> type parameter.
I have a Customer Entity with a OneToMany relationship to an Invoice Entity.
In plain old sql i can do "select customer_name,customer_age,[some other fields] from customer, invoice where ... [put some filtering here]", which gets me exactly one record with the fields i need.
In JPA i use "select c from Customer c join c.invoiceCollection where ... [same filtering as above]"
This works, but i get the Customer entity with all its associated invoices.
This is nonsense, because i pull a huge amount of (invoices) data from the database, which i do not need. I need only my customer data with exactly one invoice, as specified in the where clause.
To make things worse, i have to loop over the Customer.invoiceCollection in order to find the one specific invoice needed. This costs me even more time plus it exposes my "where" clause to the middle-tier.
Question: is there a JPA select syntax which fetches exactly one record from a one-to-many relationship, as defined in the where clause?
Things tried so far:
a) lazy loading. This does not work, throws an exception whenever i try to access Customer.invoiceCollection.
Even if it worked, i'd get a Collection with some 1000 entries, which i do not need.
b) changed my jpa statement to "select c,i from Customer c join c.invoiceCollection i where ...". This returns me an array of objects, which i have to manually map to a Customer / Invoice entity.
It works, but it makes the ORM philosophy obsolete. If i perform all the mapping from relational database records/fields to java objects manually in my code, why do i need JPA?
This is one of the most infuriating things about JPA. For example, you NEED the OneToMany side if you want Customer to cascade delete Invoices. In most cases you'd like to tell Invoices to delete itself when a Customer is deleted so that Customer does not necessarily need to know about Invoice.
My suggestion for you is that you keep the OneToMany there but get the Lazy Loading working. In your code, do not access "Customer#getInvoices" directly (unless you really need all of them).
This will allow you to do queries on customers that join to invoices without loading them.
I'm guess the exception you are getting just has to do with transaction boundaries which can be easily fixed.
For a lot of these relationships I often add the OneToMany as a private instance variable but I don't create the #getter method. That way I can use it in queries, setup cascade delete, etc. but I don't provide a way to accidentally load thousands of invoices from a customer.
Oh, and for those queries where you need exactly one invoice with its associated customer you should just do the JPA query on the Invoice and then call #getCustomer on that invoice object. That will be eagerly fetched for you.
If you need exactly one invoice with the related customer, why don't you create a query simply based on Invoice?
select i from Invoice i where [same filtering..]
You should not use one-to-many relationship in this case.
One-to-many relationships are suitable for situations when objects at "many" side are logical parts of object at "one" side (e.g. relationship from Invoice to InvoiceLine).
In you case you need a unidirectional many-to-one relationship from Invoice to Customer, so that you can query it as follows:
select i from Invoice i where ...
Then you can use customer field of Invoice to access Customer or filter by its properties.
I am using JPA2 with it's Criteria API to select my entities from the database. The implementation is OpenJPA on WebSphere Application Server. All my entities are modeled with Fetchtype=Lazy.
I select an entity with some criteria from the database and want to load all nested data from sub-tables at once.
If I have a datamodel where table A is joined oneToMany to table B, I can use a Fetch-clause in my criteria query:
CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery<A> cq = cb.createQuery(A.class);
Root<A> root = cq.from(A.class);
Fetch<A,B> fetch = root.fetch(A_.elementsOfB, JoinType.LEFT);
This works fine. I get an element A and all of its elements of B are filled correctly.
Now table B has a oneToMany-relationship to table C and I want to load them too. So I add the following statement to my query:
Fetch<B,C> fetch2 = fetch.fetch(B_.elementsOfC, JoinType.LEFT);
But this wont do anything.
Does anybody know how to fetch multi level entities in one query?
It does not work with JPQL and there is no way to make it work in CriteriaQueries either. Specification limits fetched entities to the ones in that are referenced directly from the returned entity:
About fetch join with CriteriaQuery:
An association or attribute referenced by the fetch method must be
referenced from an entity or embeddable that is returned as the result
of the query.
About fetch join in JPQL:
The association referenced by the right side of the FETCH JOIN clause
must be an association or ele ment collection that is referenced from
an entity or embeddable that is returned as a result of the query.
Same limitation is also told in OpenJPA documentation.
For what is worth. I do this all the time and it works just fine.
Several points:
I'm using jpa 2.1, but I'm almost sure it used to work in jpa 2.0 as well.
I'm using the criteria api, and I know some things work diferent in jpql. So don't think it works some way or doesn't work because that's what happens in jpql. Most often they do behave in the same way, but not always.
(Also i'm using plain criteria api, no querydsl or anything. Sometimes it makes a difference)
My associations tend to be SINGULAR_ATTRIBUTE. So maybe that's the problem here. Try a test with the joins in reverse "c.fetch(b).fetch(a)" and see if that works. I know it's not the same, but just to see if it gives you any hint. I'm almost sure I have done it with onetomany left fetch joins too, though.
Yep. I just checked and found it: root.fetch("targets", LEFT).fetch("destinations", LEFT).fetch("internal", LEFT)
This has been working without problems for months, maybe more than a year.
I just run a test and it generates this query:
select -- all fields from all tables
...
from agreement a
left outer join target t on a.id = t.agreement_id
left outer join destination d on t.id = d.target_id
left outer join internal i on d.id = i.destination_id
And returns all rows with all associations with all fields.
Maybe the problem is a different thing. You just say "it wont do anyhting". I don't know if it throws an exception or what, but maybe it executes the query properly but doesn't return the rows you expect because of some conditions or something like that.
You could design a view in the DB joining tables b and c, create the entity and fetchit insted of the original entity.
Please help an EF n00b design his database.
I have several companies that produce several products, so there's a many-to-many relationship between companies and products. I have an intermediate table, Company_Product, that relates them.
Each company/product combination has a unique SKU. For example Acme widgets have SKU 123, but Omega widgets have SKU 456. I added the SKU as a field in the Company_Product intermediate table.
EF generated a model with a 1:* relationship between the company and Company_Product tables, and a 1:* relationship between the product and Company_Product tables. I really want a : relationship between company and product. But, most importantly, there's no way to access the SKU directly from the model.
Do I need to put the SKU in its own table and write a join, or is there a better way?
I just tested this in a new VS2010 project (EFv4) to be sure, and here's what I found:
When your associative table in the middle (Company_Product) has ONLY the 2 foreign keys to the other tables (CompanyID and ProductID), then adding all 3 tables to the designer ends up modeling the many to many relationship. It doesn't even generate a class for the Company_Product table. Each Company has a Products collection, and each Product has a Companies collection.
However, if your associative table (Company_Product) has other fields (such as SKU, it's own Primary Key, or other descriptive fields like dates, descriptions, etc), then the EF modeler will create a separate class, and it does what you've already seen.
Having the class in the middle with 1:* relationships out to Company and Product is not a bad thing, and you can still get the data you want with some easy queries.
// Get all products for Company with ID = 1
var q =
from compProd in context.Company_Product
where compProd.CompanyID == 1
select compProd.Product;
True, it's not as easy to just navigate the relationships of the model, when you already have your entity objects loaded, for instance, but that's what a data layer is for. Encapsulate the queries that get the data you want. If you really want to get rid of that middle Company_Product class, and have the many-to-many directly represented in the class model, then you'll have to strip down the Company_Product table to contain only the 2 foreign keys, and get rid of the SKU.
Actually, I shouldn't say you HAVE to do that...you might be able to do some edits in the designer and set it up this way anyway. I'll give it a try and report back.
UPDATE
Keeping the SKU in the Company_Product table (meaning my EF model had 3 classes, not 2; it created the Company_Payload class, with a 1:* to the other 2 tables), I tried to add an association directly between Company and Product. The steps I followed were:
Right click on the Company class in the designer
Add > Association
Set "End" on the left to be Company (it should be already)
Set "End" on the right to Product
Change both multiplicities to "* (Many)"
The navigation properties should be named "Products" and "Companies"
Hit OK.
Right Click on the association in the model > click "Table Mapping"
Under "Add a table or view" select "Company_Product"
Map Company -> ID (on left) to CompanyID (on right)
Map Product -> ID (on left) to ProductID (on right)
But, it doesn't work. It gives this error:
Error 3025: Problem in mapping fragments starting at line 175:Must specify mapping for all key properties (Company_Product.SKU) of table Company_Product.
So that particular association is invalid, because it uses Company_Product as the table, but doesn't map the SKU field to anything.
Also, while I was researching this, I came across this "Best Practice" tidbit from the book Entity Framework 4.0 Recipies (note that for an association table with extra fields, besides to 2 FKs, they refer to the extra fields as the "payload". In your case, SKU is the payload in Company_Product).
Best Practice
Unfortunately, a project
that starts out with several,
payload-free, many-to-many
relationships often ends up with
several, payload-rich, many-to-many
relationships. Refactoring a model,
especially late in the development
cycle, to accommodate payloads in the
many-to-many relationships can be
tedious. Not only are additional
entities introduced, but the queries
and navigation patterns through the
relationships change as well. Some
developers argue that every
many-to-many relationship should start
off with some payload, typically a
synthetic key, so the inevitable
addition of more payload has
significantly less impact on the
project.
So here's the best practice.
If you have a payload-free,
many-to-many relationship and you
think there is some chance that it may
change over time to include a payload,
start with an extra identity column in
the link table. When you import the
tables into your model, you will get
two one-to-many relationships, which
means the code you write and the model
you have will be ready for any number
of additional payload columns that
come along as the project matures. The
cost of an additional integer identity
column is usually a pretty small price
to pay to keep the model more
flexible.
(From Chapter 2. Entity Data Modeling Fundamentals, 2.4. Modeling a Many-to-Many Relationship with a Payload)
Sounds like good advice. Especially since you already have a payload (SKU).
I would just like to add the following to Samuel's answer:
If you want to directly query from one side of a many-to-many relationship (with payload) to the other, you can use the following code (using the same example):
Company c = context.Companies.First();
IQueryable<Product> products = c.Company_Products.Select(cp => cp.Product);
The products variable would then be all Product records associated with the Company c record. If you would like to include the SKU for each of the products, you could use an anonymous class like so:
var productsWithSKU = c.Company_Products.Select(cp => new {
ProductID = cp.Product.ID,
Name = cp.Product.Name,
Price = cp.Product.Price,
SKU = cp.SKU
});
foreach (var
You can encapsulate the first query in a read-only property for simplicity like so:
public partial class Company
{
public property IQueryable<Product> Products
{
get { return Company_Products.Select(cp => cp.Product); }
}
}
You can't do that with the query that includes the SKU because you can't return anonymous types. You would have to have a definite class, which would typically be done by either adding a non-mapped property to the Product class or creating another class that inherits from Product that would add an SKU property. If you use an inherited class though, you will not be able to make changes to it and have it managed by EF - it would only be useful for display purposes.
Cheers. :)
Greetings,
Considering the Northwind sample tables Customers, Orders, and OrderDetails I would like to eager load the related entities corresponding to the tables mentioned above and yet I need ot order the child entities on the database before fetching entities.
Basic case:
var someQueryable = from customer in northwindContext.Customers.Include("Orders.OrderDetails")
select customer;
but I also need to sort Orders and OrderDetails on the database side (before fetching those entities into memory) with respect to some random column on those tables. Is it possible without some projection, like it is in T-SQL? It doesn't matter whether the solution uses e-SQL or LINQ to Entities. I searched the web but I wasn't satisfied with the answers I found since they mainly involve projecting data to some anonymous type and then re-query that anonymous type to get the child entities in the order you like. Also using CreateSourceQuery() doesn't seem to be an option for me since I need to get the data as it is on the database side, with eager loading but just by ordering child entities. That is I want to do the "ORDER BY" before executing any query and then fetch the entities in the order I'd like. Thanks in advance for any guidance. As a personal note, please excuse the direct language since I am kinda pissed at Microsoft for releasing the EF in such an immature shape even compared to Linq to SQL (which they seem to be getting away slowly). I hope this EF thingie will get much better and without significant bugs in the release version of .NET FX 4.0.
Actually I have Tip that addresses exactly this issue.
Sorting of related entities is not 'supported', but using the projection approach Craig shows AND relying on something called 'Relationship Fixup' you can get something very similar working:
If you do this:
var projection = from c in ctx.Customers
select new {
Customer = c,
Orders = c.Orders.OrderByDescending(
o => o.OrderDate
)
};
foreach(var anon in projection )
{
anon.Orders //is sorted (because of the projection)
anon.Customer.Orders // is sorted too! because of relationship fixup
}
Which means if you do this:
var customers = projection.AsEnumerable().Select(x => x.Customer);
you will have customers that have sorted orders!
See the tip for more info.
Hope this helps
Alex
You are confusing two different problems. The first is how to materialize entities in the database, the second is how to retrieve an ordered list. The EntityCollection type is not an ordered list. In your example, customer.Orders is an EntityCollection.
On the other hand, if you want to get a list in a particular order, you can certainly do that; it just can't be in a property of type EntityCollection. For example:
from c in northwindContext.Customers
orderby c.SomeField
select new {
Name = c.Name,
Orders = from o in c.Orders
orderby c.SomeField
select new {
SomeField = c.SomeField
}
}
Note that there is no call to Include. Because I am projecting, it is unnecessary.
The Entity Framework may not work in the way you expect, coming from a LINQ to SQL background, but it does work. Be careful about condemning it before you understand it; deciding that it doesn't work will prevent you from learning how it does work.
Thank you both. I understand that I can use projection to achieve what I wanted but I thought there might be an easy way to do it since in T-SQL world it's perfectly possible with a few nested queries (or joins) and order bys. On the other hand seperation of concerns sounds reasonable and we are in the entity domain now so I will use the way you two both recommended though I have to admit this is easier and cleaner to achieve in LINQ to SQL by using AssociateWith.
Kind regards.