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.
Related
I have a simple data model: a Product can have many Tag objects.
The Tag model has a tag field and a value field, both strings (value is not important here).
The following DQL query gets me all Products and their tags:
SELECT p, t FROM Product p LEFT JOIN p.tags t
However, how do I only select Products that have a certain tag (e.g. "blue")? It is important that I get all the tags for the returned product, so I cannot simply do a WHERE t.tag = 'blue'.
As a side question; do you have any thoughts on whether it would be better to implement this using a separate Tag table, and then having a ProductTag table knitting them together (proper many-to-many relation)?
You can use a DQL query like this:
"SELECT p, t FROM Product p LEFT JOIN p.tags t WHERE p.id IN (
SELECT sp.id FROM Product sp INNER JOIN sp.tags st WHERE st.tag = 'blue'
)"
This will return all product object where a Tag.tag = 'blue' is found in their collection of tags, but also other tags are joined.
I tried this out in a Symfony2 project and it worked like this. Because I got all the information used for this from the Doctrine Documentation I think it will work too in the standalone version.
If it doesn't work tell me the error and I will take a close look at this problem.
I'm relatively new to the Entity Framework and I'd like to do a query that includes a link table. Any suggestions on how to do a basic join query using LINQ?
Entity Structure
News
NewsID
CommunityNews
CommunityID
NewsID
Community
CommunityID
If you're generating the context from a database using the EDMX editor, and you have the appropriate foreign key constraints set up, you should be able to just add those three tables to the context, and it will create a many-to-many mapping between News and Community.
var newsForCommunity = context.News.Where(
n => n.Communities.Any(
c => c.CommunityId == communityId);
Here is another way you can write the query:
var newsForCommunity =
(from c in context.Communities
from n in c.News
where c.CommunityID == communityID
select n.NewsID
).ToList();
How would I translate this into LINQ?
Say I have A parent table (Say, customers), and child (addresses).
I want to return all of the Parents who have addresses in California, and just the california address. (but I want to do it in LINQ and get an object graph of Entity objects)
Here's the old fashioned way:
SELECT c.blah, a.blah
FROM Customer c
INNER JOIN Address a on c.CustomerId = a.CustomerId
where a.State = 'CA'
The problem I'm having with LINQ is that i need an object graph of concrete Entity types (and it can't be lazy loaded.
Here's what I've tried so far:
Edit: added context instantiation as requested
// this one doesn't filter the addresses -- I get the right customers, but I get all of their addresses, and not just the CA address object.
var ctx = new CustomersContext() // dbContext -- using EF 4.1
from c in ctx.Customer.Include(c => c.Addresses)
where c.Addresses.Any(a => a.State == "CA")
select c
// this one seems to work, but the Addresses collection on Customers is always null
var ctx = new CustomersContext() // dbContext -- using EF 4.1
from c in ctx.Customer.Include(c => c.Addresses)
from a in c.Addresses
where a.State == "CA"
select c;
Any ideas?
Based on the code above, it appears that you already have a rehydrated collection of Customer objects in the Customer variable.
You will need to call the Include function when you fill your Customer collection above. This way, the framework will rehydrate the included objects at the time of retrieval from your data context.
The actual query code appears good though.
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();
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