I've got a custom OData feed that for books. Each book can have multiple authors and an author can be involved in multiple books so I implemented this using a join table (Book - BookAuthorJoin - Author). My proxy object has Book.BookAuthorJoins BookAuthorJoin.Books & BookAuthorJoin.Authors.
What I want todo is have a single query where I get all the books for an author in a single LINQ query, but having trouble applying the filter. Seems I want two Expand() methods, but that isn't working. The following query doesn't work, but shows what I'm trying to do:
var query = from book in ODataContext.Books.Expand("BookAuthorJoins").Expand("Authors")
where book.BookAuthorJoins.Author.AuthorID = authorID
select book;
On the server side, the 1-to-many or many-to-many relationship is usually exposed as just a navigation property, exposing the join table in the middle will make your life much harder. If you use EF you should be able to hide the table and just expose the relationship as a navigation property.
In any case, to get all books for a certain author the query should look like:
/Authors(123)/Books
The result of this query is just a feed of books.
If you do keep the join table exposed then something like this migth work:
/Authors(123)/BookAuthorJoins?$expand=Book
But this time you get all the BookAuthorJoins with the Book for each as well.
Related
Hi I have a question that is braking my mind for some days.
I have my SQL server Database and my C# application.
In the DB I have differemt tables, let me show you a simple ex
Tables:
Person
Relationship
City
Business Rules:
The person are from a City, so the person has IdCity
A person has a relationship with other person, and about that relationship you need to save the starting date.
In other projects I already did something like that, but in this proyect this is not working for me.
When I retrieved with LinQ the information about the person, the city is not coming, and an error appears when I try "person.city.description", for ex.
I try using Include("City") in the linq query, but it didn't work. Besides that, I don't know how to manage the circular reference to the person to person relationship.
One important thing, that I think that can be the problem, is that I rename all the tables from the DataModel, for example, the table in database is called Prd_City, so I change the Name and the Entity Set Name for City in c# project. So in the included I have to use the real table name, in other case the query fail, but if I use the real name nothing happens.
using (var context = new MyContext())
{
List<Person> oPeople = (from p in context.Person.Include("Prd_City")
select p).ToList();
return oPeople ;
}
Any help will be welcome.
Thanks!
"It didn't work" is never a good description of your problem. But from the rest of your question I can infer that Person has a navigation property named "Prd_City", while you expected it to be "City". The thing is: you renamed the entities, but not the navigation properties in the entities.
My advice (for what it's worth): it seems that your work database-first. If you can, change to code-first and manually map the POCO classes to their table names, and properties to their database columns. It may be a considerable amount of work (depending on the size of your data model), but after that you will never run the risk of EF "un-renaming" your entities. Besides, the DbContext API is easier to use than ObjectContext. Currently, it's the preferred EF API.
This sounds rather simple (and maybe I'm missing the obvious here) but I can't find a solution. I know I can query an entity and return one, or many direct child entities doing this:
var query = from c in Service.Clients.Expand("Addresses,Comments,PhoneNumbers")..
What I would like to be able to do is do the same with 3 levels (Children of child), lets say "Country->Province->City" or "Brand->Family->Model"
I tried to expand all entities, but it fails
var query = from c in Service.Brands.Expand("Families,Models").. //fails,
//which even makes some sense, since Models is a Child of Family, not Brand
var query = from c in Service.Brands.Expand("Families").. //this works,
//but Family.Models is empty
Is there a way to do this in one query, or do I have to split this in two separate queries?
In ASP.NET OData v4, the URL convention seems to have changed. It is now:
~/Brands?$expand=Families($expand=Models)
You can also select the stuff you want in sub-entities.
For example if you want just the brand family identifiers:
~/Brands?$expand=Families($select=Id)
Furthermore, if you want only the identifiers of the brand family models, you would do that:
~/Brands?$expand=Families($expand=Models($select=Id))
...and so on. Hope this helps !
The following statement should return what you are looking for:
var query = from c in Service.Brands.Expand("Families/Models")
This will run the following odata query:
.../OData.svc/Brands?$expand=Families/Models
Here is a link to some further odata documentation:
The syntax of a $expand query option is a comma-separated list of Navigation Properties. Additionally each Navigation Property can be followed by a forward slash and another Navigation Property to enable identifying a multi-level relationship.
Consider the database schema in the picture.
I need a linq-to-entities statement that will retrieve authors and their respective books grouped by author. For example, I want to display something as follows:
authorName8
bookTitle27
bookTitle35
bookTitle62
authorName37
bookTitle9
bookTitle51
An additional constraint is the oject of grouped items has to be returnable by a method.
Can anybody formulate join and group-by statement (a) ordered by author name, (b) ordered by author name and book title?
You didn't mention which version of EF you are using, or whether you need to filter the authors by some condition, or filter the books by some condition. A general query that will fetch everything will look like this:
List<Author> authorsWithBooks =
context.
Authors.
Include(a => a.Books). // For EF 4.1
/*Include("Books"). instead for previous versions of EF */
/*Where(a => !a.IsDeleted). if you want to filter the authors */
ToList();
If you want to filter the book collections, you will have to write a more elaborate query though.
var query = (from customer in MyDataContext.Aurthor select author)
.Including(author=> author.Books);
This will get you all the authors and eager load the books for best performance.
Let's say I have three tables:
Office
ID
SalespeopleOffice
ID
OfficeID
PersonID
People
ID
ManagerID
In LINQ to SQL, how can I start from the SalespeopleOffices table and "walk" from that table to the People table or the Office table via the relationships between the tables? Specifically without knowing what those relationships are; pull the data about the relationships instead of interacting with the objects directly. I'm looking for a way to programatically analyze table relationships. The database I'm working with has many more tables than this, so in reality it's a lot more complex than this.
I'd ideally like to create a LinqPad script to do this.
You can use reflection to examine the properties on each type in the context. (In LinqPad, the context is this).
Value and string properties will be scalar fields on the table,
EntitySet properties will represent [something]-to-many relationships, and
other types will be [something]-to-one relationships.
If you connect the two sides of the relationships you can figure out what the [something] is in each case. Does that make sense?
Edit
I was just poking around a little, and there's a better approach. The model information is available via the Mapping property. Try this:
var tableData = from t in this.Mapping.GetTables()
select new
{
t.TableName,
Associations =
from a in t.RowType.Associations
select new
{
a.ThisMember.Name,
TypeName = a.ThisMember.Type.Name
}
};
tableData.Dump();
Assuming you've activated Auto Completion, it should be a piece of cake to find the exact data you're interested in by exploring the properties on this meta data.
If you're using LINQ to SQL, your SalespeopleOffices instances shoud have People and Office properties.
Something like:
var salesPO = dataContext.SalespeopleOffices.First()
var office = salesPO.Office
var man = salesPO.People
I am reading this asp.net article on building your first asp.net mvc 2 website and I came across a Linq query that uses the Include method in the query. I have used some linq, but I have never used the Include method and would like a better explanation. Does it translate to an join in linq? What is the benefit? Here is the query from the article:
var genreModel = storeDB.Genres
.Include("Albums")
.Single(g => g.Name == genre);
The article states that:
Entity Framework feature that allows us to specify other related entities we want loaded as well, called Query Result Shaping. We want to load the Albums for the matching Genre, so we'll query from Genres.Include("Albums") to indicate that we want related albums as well. This is more efficient, since it will retrieve both our Genre and Album data in a single database request.
I sort of understand what the author is saying above, but feel I would need a better example or explanation, especially since I have never used the Include method before.
If you inspect the generated sql, you'll notice that the Albums table is joined in. It should look something like:
SELECT [t0].*, [t1].*
FROM Genres [t0]
LEFT JOIN Albums [t1] ON [t0].GenreId = [t1].GenreId
WHERE [t0].Name == #p0
When the results get back to the client side, the ObjectContext will turn the row-column shape into instances of Genres and Albums. These instances will be related hierarchically - the single Genre with all of its Albums.
Suppose this genre has 5 albums. The query will return 5 rows. The object context will create one instance of Genre (each of the 5 rows has the same Genre primary key value).