I want to do the following...
FROM o IN orders
SELECT new OrderContainer { Contact = (PostalContact) o.Contact }
So hopefully you can see that the order's 'Contact' will be of a derived type. Unfortunately however it doesn't seem to do a polymorphic fetch! Is there anyway of achieving this?
Cheers, Ian.
Try using the extention method .OfType()
from o in orders
select new OrderContainer { Contact = o.Contact.OfType<PostalContact>().FirstOrDefault() }
Edit:
a way to get the full object data, but i doubt that this is good enough for your needs.
from c in contacts.OfType<PostalContact>()
where c.Orders.Any(o=>o.Contact.Id == c.id)
select new OrderContainer { Contact = c }
on the other hand, if you set the base class (entity) to abstract, you may find that entity will load the full objects. but this is not recomended due to the queries that are generated. if you are looking into this you may want to look at (TPH) Table per Hierarchy for your contacts
Related
There is a table, it is a poco entity generated by entity framework.
class Log
{
int DoneByEmpId;
string DoneByEmpName
}
I am retrieving a list from the data base. I want distinct values based on donebyempid and order by those values empname.
I have tried lot of ways to do it but it is not working
var lstLogUsers = (context.Logs.GroupBy(logList => logList.DoneByEmpId).Select(item => item.First())).ToList(); // it gives error
this one get all the user.
var lstLogUsers = context.Logs.ToList().OrderBy(logList => logList.DoneByEmpName).Distinct();
Can any one suggest how to achieve this.
Can I just point out that you probably have a problem with your data model here? I would imagine you should just have DoneByEmpId here, and a separate table Employee which has EmpId and Name.
I think this is why you are needing to use Distinct/GroupBy (which doesn't really work for this scenario, as you are finding).
I'm not near a compiler, so i can't test it, but...
Use the other version of Distinct(), the one that takes an IEqualityComparer<TSource> argument, and then use OrderBy().
See here for example.
Let's say we have 3 domain classes: 2 classes related with each other through a 3rd class.
Ok, some code:
class A {
String subject
String description
static hasMany = [cs: C]
static transients = ['numberOfCs']
Long getNumberOfCs() {
return cs.size()
}
}
class B {
String title
}
class C {
A objectA
B objectB
static belongsTo = [a: A]
}
Pretty clear? I hope so. This work perfectly with my domain.
You can see the transient property numberOfCs, which is used to calculate the number of C instances related to my A object. And it works just fine.
The problem: listing all my A objects, I want to sort them by the number of relationships with C objects, but the transient property numberOfCs cannot be used for the scope.
How can I handle the situation? How can I tell GORM to sort the As list by numberOfCs as it would be a regular (non transient) field?
Thanks in advance.
I'm not sure that Grails' criteria do support this, as you need both to select the A object itself and aggregate by a child objects (C). That means grouping by all the A's fields, which is not done automatically.
If you only need some fields from A, you can group by them:
def instances = A.withCriteria {
projections {
groupProperty('subject')
count('cs', 'cCount')
}
order 'cCount'
}
otherwise you'll need to retrieve only ids and make a second query, like in this question.
Another way is to use derived properties like described here (not sure it will work though):
class A {
static mapping = {
numberOfCs formula: 'select count(*) from C where c.b_id = id'
}
}
I wouldn't consider your Problem GORM related but rather general Hibernate or even SQL related.
Take a look at the HQL Docu they are a lot of examples.
check the following HQL this pretty close what you are asking.
select mySortedAs from A mySortedAs left join mySortedAs.cs myCsOfA order by
count(myCsOfA)
I think I saw somewhere that you also can do something like this myCsOfA.length or myCsOfA.size
I have an Abstract type called Product, and five "Types" that inherit from Product in a table per type hierarchy fashion as below:
I want to get all of the information for all of the Products, including a smattering of properties from the different objects that inherit from products to project them into a new class for use in an MVC web page. My linq query is below:
//Return the required products
var model = from p in Product.Products
where p.archive == false && ((Prod_ID == 0) || (p.ID == Prod_ID))
select new SearchViewModel
{
ID = p.ID,
lend_name = p.Lender.lend_name,
pDes_rate = p.pDes_rate,
pDes_details = p.pDes_details,
pDes_totTerm = p.pDes_totTerm,
pDes_APR = p.pDes_APR,
pDes_revDesc = p.pDes_revDesc,
pMax_desc = p.pMax_desc,
dDipNeeded = p.dDipNeeded,
dAppNeeded = p.dAppNeeded,
CalcFields = new DAL.SearchCalcFields
{
pDes_type = p.pDes_type,
pDes_rate = p.pDes_rate,
pTFi_fixedRate = p.pTFi_fixedRate
}
}
The problem I have is accessing the p.pTFi_fixedRate, this is not returned with the Products collection of entities as it is in the super type of Fixed. How do I return the "super" type of Products (Fixed) properties using Linq and the Entity Framework. I actually need to return some fields from all the different supertypes (Disc, Track, etc) for use in calculations. Should I return these as separate Linq queries checking the type of "Product" that is returned?
This is a really good question. I've had a look in the Julie Lerman book and scouted around the internet and I can't see an elegant answer.
If it were me I would create a data transfer object will all the properties of the types and then have a separate query for each type and then union them all up. I would insert blanks into the DTO properies where the properties aren't relevant to that type. Then I would hope that the EF engine makes a reasonable stab at creating decent SQL.
Example
var results = (from p in context.Products.OfType<Disc>
select new ProductDTO {basefield1 = p.val1, discField=p.val2, fixedField=""})
.Union(
from p in context.Products.OfType<Fixed>
select new ProductDTO {basefield1 = p.val1, discField="", fixedField=p.val2});
But that can't be the best answer can it. Is there any others?
So Fixed is inherited from Product? If so, you should probably be querying for Fixed instead, and the Product properties will be pulled into it.
If you are just doing calculations and getting some totals or something, you might want to look at using a stored procedure. It will amount to fewer database calls and allow for much faster execution.
Well it depends on your model, but usually you need to do something like:
var model = from p in Product.Products.Include("SomeNavProperty")
.... (rest of query)
Where SomeNavProperty is the entity type that loads pTFi_fixedRate.
I'm a total LINQ noob so I guess you'll probably have a good laugh reading this question. I'm learning LINQ to create queries in LightSwitch and what I don't seem to understand is to select an entity based on a value in a lookup table. Say I want to select all employees in a table that have a job title that is picked from a related lookup table. I want the descriptive value in the lookup table for the user to pick from a list to use as a parameter in a query, not the non-descriptive id's.
Can someone point me to an article or tutorial that quickly explains this, or give me a quick answer? I AM reading books and have a Pluralsight account but since this is probably the most extensive knowledge I will need for now a simple tutorial would help me more that watching hours of videos and read thousands of pages of books.
Thanks in advance!
Edit: this is the code. As far as I know this should but won't work (red squigly line under EmployeeTitle, error says that EmployeeContract does not contain a definition for EmployeeTitle even though there is a relationship between the two).
partial void ActiveEngineers_PreprocessQuery(ref IQueryable<Employee> query)
{
query = from Employee e in query
where e.EmployeeContract.EmployeeTitle.Description == "Engineer"
select e;
}
Edit 2: This works! But why this one and not the other?
partial void ActiveContracts_PreprocessQuery(ref IQueryable<EmployeeContract> query)
{
query = from EmployeeContract e in query
where e.EmployeeTitle.Description == "Engineer"
select e;
}
The red squiggly line you've described is likely because each Employee can have 1-to-many EmployeeContracts. Therefore, Employee.EmployeeContracts is actually an IEnumerable<EmployeeContract>, which in turn does not have a "EmployeeTitle" property.
I think what you're looking for might be:
partial void ActiveEngineers_PreprocessQuery(ref IQueryable<Employee> query)
{
query = from Employee e in query
where e.EmployeeContract.Any(x => x.EmployeeTitle.Description == "Engineer")
select e;
}
What this is saying is that at least one of the Employee's EmployeeContracts must have an EmployeeTitle.Description == "Engineer"
Try something like this:
partial void RetrieveCustomer_Execute()
{
Order order = this.DataWorkspace.NorthwindData.Orders_Single
(Orders.SelectedItem.OrderID);
Customer cust = order.Customer;
//Perform some task on the customer entity.
}
(http://msdn.microsoft.com/en-us/library/ff851990.aspx#ReadingData)
Assuming you have navigation properties in place for the foreign key over to the lookup table, it should be something like:
var allMonkies = from employee in context.Employees
where employee.EmployeeTitle.FullTitle == "Code Monkey"
select employee;
If you don't have a navigation property, you can still get the same via 'manual' join:
var allMonkies = from employee in context.Employees
join title in context.EmployeeTitles
on employee.EmployeeTitleID equals title.ID
where title.FullTitle == "Code Monkey"
select employee;
I want to create an Entity Object from a LinQ statement, but I don't want to load all its columns.
My ORDERS object has a lot of columns, but I just want to retrieve the REFERENCE and OPERATION columns so the SQL statement and result will be smaller.
This LinQ statement works properly and loads all my object attributes:
var orders = (from order in context.ORDERS
select order);
However the following statement fails to load only two properties of my object
var orders = (from order in context.ORDERS
select new ORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The error thrown is:
The entity or complex type
'ModelContextName.ORDERS' cannot be
constructed in a LINQ to Entities
query.
What is the problem? Isn't it possible to partially load an object this way?
Thank you in advance for your answers.
ANSWER
Ok I should thank you both Yakimych and Dean because I use both of your answers, and now I have:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
})
.AsEnumerable()
.Select(o =>
(ORDERS)new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
}
).ToList().AsQueryable();
And I get exactly what I want, the SQL Statement is not perfect but it returns only the 2 columns I need (and another column which contains for every row "1" but I don't know why for the moment) –
I also tried to construct sub objects with this method and it works well.
No, you can't project onto a mapped object. You can use an anonymous type instead:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The problem with the above solution is that from the moment you call AsEnumerable(), the query will get executed on the database. In most of the cases, it will be fine. But if you work with some large database, fetching the whole table(or view) is probably not what you want. So, if we remove the AsEnumerable, we are back to square 1 with the following error:
The entity or complex type 'ModelContextName.ORDERS' cannot be constructed in a LINQ to Entities query.
I have been struggling with this problem for a whole day and here is what I found. I created an empty class inheriting from my entity class and performed the projection using this class.
public sealed class ProjectedORDERS : ORDERS {}
The projected query (using covariance feature):
IQueryable<ORDERS> orders = (from order in context.ORDERS
select new ProjectedORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
});
Voilà! You now have a projected query that will map to an entity and that will get executed only when you want to.
I think the issue is creating new entities within the query itself, so how about trying this:
context.ORDERS.ToList().Select(o => new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
});