I'm trying to use LINQ to query the following Entity Data Model
based on this db model
I'd like to be able to pull a list of products based on ProductFacets.FacetTypeId.
Normally, I'd use joins and this wouldn't be a problem but I don't quite understand how to query many-to-many tables under the Entity DataModel.
This is an example sql query:
select p.Name, pf.FacetTypeId from Products p
inner join ProductFacets pf on p.ProductId = pf.ProductId
where pf.FacetTypeId in(8, 12)
Presuming EF 4:
var facetIds = new [] { 8, 12 };
var q = from p in Context.Products
where p.FacetTypes.Any(f => facetIds.Contains(f.FacetTypeId))
select p;
In EF (assuming the mapping is done correctly), joins are hardly ever used; navigation properties are used instead.
Your original SQL returns a tuple with repeated Name entries. With LINQ, it's often easier
to "shape" the queries into non-tuple results.
The following should be the same as the SQL, only instead of returning (Name, FacetTypeId) pairs with repeated Names, it will return a type that has a Name and a sequence of FacetTypeIds:
var facetIds = new [] { 8, 12 };
var result = from p in db.Products
select new
{
p.Name,
FacetTypeIds = from pf in p.FacetTypes
where pf.FacetTypeId == 8 || pf.FacetTypeId == 12
select pf.FacetTypeId,
};
Related
How do I left outer join two tables on two fields in linq?
I have a sql:
select a.*, b.* from courselist as a
left outer join Summary as b
on a.subject = b.Subject and a.catalog =
b.Catalogno
where a.degree_id = 1
order by a.sequenceNo
Below is my linq query, but there is error underline "join", failed in the call to "Groupjoin". I don't know how to correct that.
var searchResults = (from a in db.courselist
join b in db.Summary on
new { a.subject,a.catalog } equals
new { b.Subject, b.Catalogno } into ab
where a.degree_id == 1
orderby a.degree_sequenceNo
from b in ab.DefaultIfEmpty()
select new
{
Courselist = a,
Summary = b
}
).ToList();
Thanks.
I've checked your code again,
I found it's fault
you just need to specify join parameters name like this:
new { suject = a.subject, catalog = a.catalog } equals
new { suject = b.subject, catalog = b.Catalogno } into ab
It seems you are missing the reference, the query doesn't have an error
try to use this:
using System.Linq;
The main issue when people start using LINQ is that they keep thinking in the SQL way, they design the SQL query first and then translate it to LINQ. You need to learn how to think in the LINQ way and your LINQ query will become neater and simpler. For instance, in your LINQ you don't need joins. You should use Associations/Navigation Properties instead. Check this post for more details.
There should be a relationship between courselist and Summary, in which case, you can access Summary through courselist like this:
var searchResults = (from a in db.courselist
where a.degree_id == 1
orderby a.degree_sequenceNo
select new {
Courselist = a,
Summary = a.Summary
}).ToList();
If there is no relationship between the two, then you should reconsider your design.
I am having some trouble with a linq query I am trying to write.
I have List A of products that have been modified so I am trying to get the list of products from the db to allow me to apply the changes to them.
I have tried 2 different queries
var query = from p in db.Products
where products.Select(z => z.id).Contains(p.Id)
select p;
var query2 = from p in db.Products where (from o in products
select o.id)
.Contains(p.Id)
select p;
Both attempts are returning an error
base {System.SystemException} = {"Unable to create a constant value of type 'ProjectABC.Models.ProductModel'. Only primitive types ('such as Int32, String, and Guid') are supported in this context."}
What am I doing wrong?
I had the same problem the other day, seems EF doesn't support Select().Contains() without giving that error. After testing around for a bit, I ended up splitting it up in what in your case would correspond to;
var IDs = products.Select(z=>z.id);
var query = from p in db.Products
where IDs.Contains(p.Id)
select p;
which worked well in my case when the "products" collection was in memory anyway (ie a ToList()'ed result from the database)
I have this sql that i want to have written in linq extension method returning an entity from my edm:
SELECT p.[Id],p.[Firstname],p.[Lastname],prt.[AddressId],prt.[Street],prt.[City]
FROM [Person] p
CROSS APPLY (
SELECT TOP(1) pa.[AddressId],a.[ValidFrom],a.[Street],a.[City]
FROM [Person_Addresses] pa
LEFT OUTER JOIN [Addresses] AS a
ON a.[Id] = pa.[AddressId]
WHERE p.[Id] = pa.[PersonId]
ORDER BY a.[ValidFrom] DESC ) prt
Also could this be re-written in linq extension method using 3 joins?
Assuming you have set the Person_Addresses table up as a pure relation table (i.e., with no data besides the foreign keys) this should do the trick:
var persons = model.People
.Select(p => new { p = p, a = p.Addresses.OrderByDescending(a=>a.ValidFrom).First() })
.Select(p => new { p.p.Id, p.p.Firstname, p.p.LastName, AddressId = p.a.Id, p.a.Street, p.a.City });
The first Select() orders the addresses and picks the latest one, and the second one returns an anonymous type with the properties specified in your query.
If you have more data in your relation table you're gonna have to use joins but this way you're free from them. In my opinion, this is more easy to read.
NOTE: You might get an exception if any entry in Persons have no addresses connected to them, although I haven't tried it out.
Suppose I have three tables:
Person(pid, ...)
PersonAddress(pid, aid,...)
Address(aid, ...)
Then I want to get the person address like sql:
select a.* from address a join PersonAddress pa on a.addressID=pa.addressID
where pa.personID = myPersonID
Use Entity Framework to create Entity model, then want to write a linq equivalent as above sql.
I tried it in following way:
var addresses = this.GetAddress();
var personaddresses = this.GetPersonAddress();
var query = from ad in addresses
from pa in personaddresses
where ((ad.AddressID == pa.AddressID)&&(pa.PersonID==person.personID))
select ad;
but I got error. Or I try to start from:
var result = this.Context.Address;
var result = result.Join .... //how to write linq in this way?
How to write the linq?
This is untested but if you have all of your relationships setup and you create the model (I have used Model as the name for this) from this you should be able to use the following:
var values = this.Model.Address.Select(a => a.PersonAddress.Where(pa => pa.Id == myPersonID));
You almost never use join in LINQ to Entities.
Try:
var q = from p in Context.People
where p.PersonId == personId
from a in p.Addresses // presumes p.Addresses is 1..*
select a;
Assuming you have three entities: Person, PersonAddress and Address, here is a query that should meet your needs (this example assumes an Entity Framework context named context):
var values = context.PersonAddress.Where(pa => pa.Person.PersonId == myPersonId).Select(pa => pa.Address);
However, if the PersonAddress table exists as a pure many-to-many relationship table (i.e. contains only keys), you'd be better off setting up your Entity Framework model in such a way that the intermediate table isn't necessary, which would leave you with the much simpler:
var values = context.Person.Where(p => p.PersonId == myPersonId).Addresses;
Based on the additional feedback
Because you need to include the country table, you should originate your query from the Address table. In that case:
var values = context.Address.Where(a => a.PersonAddress.Where(pa => pa.Product.Id == myProductId).Count() > 0)
To include the Country table in the result:
var values = context.Address.Include("Country").Where(a => a.PersonAddress.Where(pa => pa.Product.Id == myProductId).Count() > 0)
Suppose I have 3 tables: A(aid, ...), B(bid, ...) Relationship(aid, bid, ...)
Then I have a aid as parameter, I want to get the result from b. If use SQL, will be like
select b.* from B join Relationship on b.bid = Relationship.bid
where relationship.aid = aid_param
how to write linq with same result as above SQL. (the foreign key was not created correctly in this case)?
Assuming that you've used the designer to add your tables to the LINQ data context and you have either foreign key relationships defined in the database or have hand-coded the associations in the designer, you should simply be able to reference the B table and it's EntitySet of Relationship properties, filtered by your parameter, and check if there are any matches.
var relationships = db.B.Any( b => b.Relationships.Where( r => r.aid == aid_param ) );
var bQuery = from r in MyContext.Relationship
where r.aid == aid_param
select r.B;
If you don't have a relationship set in the model, then u can:
var bQuery = from b in myContext.B
where myContext.Relationship
.Any( r => r.aid == aid_param && b.bid == r.bid)
select b;
var output =
from b
from Relationship
where (b.bid = Relationship.bid,
relationship.aid = aid_param)