Here is my scenario. I have a Document class. The document is associated with a DocumentClasses table through a many to many relationship, so a document can have one or more classes. When running a search, the user can choose to filter documents by class. So, I need to be able to append a where clause to my query if the user chooses to select any classes. The logic is that if the document is assigned to any of the classes in the classes the user selected, the document should be returned. So the basic query needed in pseudo code. So basically, if any number in list A belongs to list B, then return the record.
I have tried this (RestrictByClasses is just a List(Of Integer)):
query = query.where(Function(resultItem) RestrictByClasses.Contains(resultItem.DocumentClassIds.Any())
But I get the following exception:
The nested query is not supported. Operation1='Case' Operation2='Collect'
Is there any way to get linq to filter records out like this?
Thanks!
UPDATE:
After doing a little more debugging, I think that it is more to do with how I am projecting onto the object in order to load it with values that can be used to filter. Here is how I am doing the projection:
Dim query = From document In dbContext.Documents
Select New FeeAndReceptionReportIntermediateItem With
{
.BookTypeId = If(restrictByBookTypes AndAlso document.DocumentInstruments.Any(), document.DocumentInstruments.FirstOrDefault().Instrument.BookTypeId, Nothing),
.CustomerId = document.CustomerId,
.DocumentClassIds = If(restrictByDocumentClasses, document.DocumentClasses.Select(Function(group) group.ClassId), Nothing),
.DocumentId = document.DocumentId,
.DocumentNumber = document.DocumentNumber,
.DepartmentId = document.DepartmentId,
.InstrumentGroupIds = If(restrictByInstrumentGroup, document.DocumentInstruments.FirstOrDefault().Instrument.InstrumentGroups.Select(Function(group) group.InstrumentGroupId), Nothing),
.RecordDateTime = document.RecordDateTime,
.RestrictedInstrument = (includeRestrictedDocuments AndAlso document.DocumentInstruments.Any() AndAlso document.DocumentInstruments.FirstOrDefault().Instrument.Restricted)
}
I think it is complaining about how the .DocumentClassIds and .InstrumentGroupId's are being loaded into the POCO object (FeeAndReceptionReportIntermediateItem). I would really like to load these up in the initial query, before a .ToList() has been called and I would really like to not even do the join if the user did not pass in the restrictions that require me to create the join, that's why I am using the navigation properties and an if statement when loading these collecctions, because I am assuming if "restrictByDocumentClasses" is false, the navigation property won't be accessed and the join won't be included.
This works as a general pattern. The first line gets an IQueryable<> from the DbSet<>. The select does this for us, so that we can continue reusing query to hold our query as we build it up. Then just keep adding on If...Then...query=query.Where(...)...Endif to continue whittling down the resultset.
var query=db.MyTable.Select(x=>x);
if (RestrictByClasses.Any())
query=query.Where(r =>
r.DocumentClasses.Select(x=>x.ClassId)
.Intersect(RestrictByClasses)
.Any());
if (RestrictBySomethingElse)
query=query.Where(x=>SomethingElse)
I think this is the equivalent in VB.NET:
Dim query = db.MyTable.[Select](Function(x) x)
If RestrictByClasses.Any() Then
query = query.Where(Function(r) r.DocumentClasses.Select(Function(group) group.ClassId).Intersect(RestrictByClasses).Any())
End If
'Repeat as necessary
If RestrictBySomethingElse Then
query = query.Where(Function(x) SomethingElse)
End If
'End repeat
' Rest here is pseudo code
' Sort
SELECT/SWITCH sortonfield
CASE 'name': query=query.OrderBy(Function(x) x.name)
CASE 'dob': query=query.OrderBy(Function(x) x.dob)
DEFAULT: query=query.OrderBy(Function(x) x.id)
END CASE
'Paginate
query=query.Skip((pagenumber-1)*pagesize).Take(pagesize)
'Project
Dim finalresult=query.Select(Function(x) new something {
name=x.Name,
id=x.id,
things=x.things
});
Once all your filters have been put in place (and optionally a sort, and pagination), then project your resultset into whatever you need.
Related
Ok I have seen many questions that based on their text could be something like this but not quite. Say I have something like this
(from r in reports
join u in SECSqlClient.DataContext.GetTable<UserEntity>()
on r.StateUpdateReportUserID equals u.lngUserID
select r).
If reports have a bunch of say reportDTO class and I want to select from a list of that DTO but at the same time set one property to a property in userEntity how would I do that? Basically I want all other fields on the report maintained but set a user name from the user table. (There is a reason this is not done in one big query that gets a list of reports)
What I am looking for is something like Select r).Something(SOME LAMBDA TO SET ONE FIELD TO userEntity property).
There is a dirty way to do this, which is
var repQuery = from r in reports ... select new { r, u };
var reps = repQuery.Select(x => { x.r.Property1 = x.u.Property1; return x.r; };
However, When it comes to functional programming (which Linq is, arguably) I like to adhere to its principles, one of which to prevent side effects in functions. A side effect is a change in state outside the function body, in this case the property value.
On the other hand, this is a valid requirement, so I would either use the ForEach method after converting the query to list (ToList()). Foreach is expected to incur side effects. Or I would write a clearly named extension method on IEnumerable<T> (e.g. DoForAll) that does the same, but in a deferred way. See Why there is no ForEach extension method on IEnumerable?.
I'm using NHibernate 3.2 and I have a repository method that looks like:
public IEnumerable<MyModel> GetActiveMyModel()
{
return from m in Session.Query<MyModel>()
where m.Active == true
select m;
}
Which works as expected. However, sometimes when I use this method I want to filter it further:
var models = MyRepository.GetActiveMyModel();
var filtered = from m in models
where m.ID < 100
select new { m.Name };
Which produces the same SQL as the first one and the second filter and select must be done after the fact. I thought the whole point in LINQ is that it formed an expression tree that was unravelled when it's needed and therefore the correct SQL for the job could be created, saving my database requests.
If not, it means all of my repository methods have to return exactly what is needed and I can't make use of LINQ further down the chain without taking a penalty.
Have I got this wrong?
Updated
In response to the comment below: I omitted the line where I iterate over the results, which causes the initial SQL to be run (WHERE Active = 1) and the second filter (ID < 100) is obviously done in .NET.
Also, If I replace the second chunk of code with
var models = MyRepository.GetActiveMyModel();
var filtered = from m in models
where m.Items.Count > 0
select new { m.Name };
It generates the initial SQL to retrieve the active records and then runs a separate SQL statement for each record to find out how many Items it has, rather than writing something like I'd expect:
SELECT Name
FROM MyModel m
WHERE Active = 1
AND (SELECT COUNT(*) FROM Items WHERE MyModelID = m.ID) > 0
You are returning IEnumerable<MyModel> from the method, which will cause in-memory evaluation from that point on, even if the underlying sequence is IQueryable<MyModel>.
If you want to allow code after GetActiveMyModel to add to the SQL query, return IQueryable<MyModel> instead.
You're running IEnumerable's extension method "Where" instead of IQueryable's. It will still evaluate lazily and give the same output, however it evaluates the IQueryable on entry and you're filtering the collection in memory instead of against the database.
When you later add an extra condition on another table (the count), it has to lazily fetch each and every one of the Items collections from the database since it has already evaluated the IQueryable before it knew about the condition.
(Yes, I would also like to be the extensive extension methods on IEnumerable to instead be virtual members, but, alas, they're not)
I am trying to bind distinct records to a dropdownlist. After I added distinct function of the linq query, it said "DataBinding: 'System.String' does not contain a property with the name 'Source'. " I can guarantee that that column name is 'Source'. Is that name lost when doing distinct search?
My backend code:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
var query = (from p in dc.Promotions
select p.Source).Distinct();
return query;
}
Frontend code:
PromotionDAL dal = new PromotionDAL();
ddl_Source.DataSource = dal.GetAllSource();
ddl_Source.DataTextField = "Source";
ddl_Source.DataValueField = "Source";
ddl_Source.DataBind();
Any one has a solution? Thank you in advance.
You're already selecting Source in the LINQ query, which is how the result is an IQueryable<string>. You're then also specifying Source as the property to find in each string in the databinding. Just take out the statements changing the DataTextField and DataValueField properties in databinding.
Alterantively you could remove the projection to p.Source from your query and return an IQueryable<Promotion> - but then you would get distinct promotions rather than distinct sources.
One other quick note - using query syntax isn't really helping you in your GetAllSources query. I'd just write this as:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
return dc.Promotions
.Select(p => p.Source)
.Distinct();
}
Query expressions are great for complicated queries, but when you've just got a single select or a where clause and a trivial projection, using the dot notation is simpler IMO.
You're trying to bind strings, not Promotion objects... and strings do not have Source property/field
Your method returns a set of strings, not a set of objects with properties.
If you really want to bind to a property name, you need a set of objects with properties (eg, by writing select new { Source = Source })
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
});
Is there a way to ensure a particular conditional clause is added to the expression tree on each select from a particular table?
For example, a table with a field with the date a record was deleted should never come back or be included in any kind of statement.
Rather than include a where clause each time, is there a way, without creating a view, to add a conditional to each select?
--- Edit for clarity below ---
I'm looking for a function I can partial, much like the Insert/Update/Delete functions, but for Selecting. I want to apply a blanket filter to all queries against a table.
Furthermore, if I get a collection of items from a parent, I want that set to be filtered as well.
Something like:
Private Function BaseItems() As IQueryable(Of Item)
Return (From mi In dataContext.Items Where mi.DeletedAt Is Nothing Select mi)
End Function
Public Function GetItems() as list(of Item)
Return (From mi in BaseItems() select mi).ToList()
End Function
works for functions I write and call. ITEMS, can be a child of MASTER, for example.
'assume TheMaster is a LinqToSQL data class which has a one to many child of Items
TheMaster.Items.Count '<-- will bring back all Items.
How do I always filter what populates by data classes?
You should be able to do this my putting the items into a list and then use lambda expressions to filter the list?
MyListObject.Where(x => x == x.Date);
I'm not sure I understand the context of your question. However you can add more where conditionals on a linq query. If you return an IQueryable, you'll only return a runnable query and you can chain on it other LINQ queries. Afaik it won't execute until you start making it to an IEnumerable or List to iterate through.
Example on LINQ-to-SQL:
MyDataContext context = new MyDataContext();
public IQueryable<MyTable> GetTable() {
return from record in context.Records
where record.Date > DateTime.Now
select record;
}
public IEnumerable<MyTable> GetTableWithinWeek() {
return from record in GetTable()
where record.Date < DateTime.Now.AddDays(7);
select record;
}
I hope my answer makes sense.
AssociateWith in the DataLoadOptions of the DataContext seems to do what I'm needing.