I'm sure I must be missing something really simple here..
OK I have a list of AreaIds. I want to compare that list to the MapArea Table and return any IDs that exist in the table but NOT in the supplied list.
This is my list of supplied areas that I want to check:
var currentAreas = (from c in _entities.mapAreaLink
where c.listingId == id
select new
{
c.MapArea.areaId
}
).ToList();
This is the getting the exhaustive list of mapAreas..
var availableAreas = (from m in _entities.MapAreas
select new
{
m.areaId
}
).ToList();
This compares the two lists and gets items that exist in the maparea table but not in the maparealink (constrained by an id of the item I am looking at).
var unusedAreas = availableAreas.Except(currentAreas).ToList();
I seem to get the list back ok, but what I need to do is now return a list of maparea objects based on the results of the Except.tolist above.
I thought I could do this:
var mapareas = (from e in _entities.MapAreas
where unusedAreas.Contains(e.areaId)
select e).ToList();
I am getting an ambiguous invocation on the where & "Cannot resolve method Contains(int)" on the e.areaId.
Ive tried using:
var unusedAreas = availableAreas.Except(currentAreas).ToArray();
No Joy.. Can anyone help me out here - I am guessing I must be missing a fundamental basic here.
many thanks
You create anonymous types with just one int property. That's not necessary and it causes the later problem. If you create lists of int you'll be OK:
var currentAreas = (from c in _entities.mapAreaLink
where c.listingId == id
select c.MapArea.areaId).ToList();
var availableAreas = (from m in _entities.MapAreas
select m.areaId).ToList();
Related
I have 2 queries that work, I was hoping to combine them to reduce the database calls.
var locations = from l in db.Locations
where l.LocationID.Equals(TagID)
select l;
I do the above because I need l.Name, but is there a way to take the above results and put them into the query below?
articles = from a in db.Articles
where
(
from l in a.Locations
where l.LocationID.Equals(TagID)
select l
).Any()
select a;
Will I actually be reducing any database calls here?
This seems a bit complicated because Locations appears to be a multi-value property of Articles and you want to only load the correct one. According to this answer to a similar question you need to use a select to return them separately in one go so e.g.
var articles = from a in db.Articles
select new {
Article = a,
Location = a.Locations.Where(l => l.LocationId == TagId)
};
First failed attempt using join:
var articlesAndLocations = from a in db.Articles
join l in a.Locations
on l.LocationID equals TagID
select new { Article = a, Location = l };
(I usually use the other LINQ syntax though so apologies if I've done something stupid there.)
Could you not use the Include() method here to pull in the locations which are associated with each article, then select both the article and location object? or the properties you need from each.
The include method will ensure that you don't need to dip into the db twice, but will allow you to access properties on related entities.
You would need to use a contains method on an IEnumerable I believe, something like this:
var tagIdList = new List() { TagID };
var articles = from a in db.Articles.Include("Locations")
where tagIdList.Contains(from l in a.Locations select l.LocationID)
select new { a, a.Locations.Name };
(Untested)
Can anybody tell me how I would get the records in the first statement that are not in the second statement (see below)?
from or in TblOrganisations
where or.OrgType == 2
select or.PkOrgID
Second query:
from o in TblOrganisations
join m in LuMetricSites
on o.PkOrgID equals m.FkSiteID
orderby m.SiteOrder
select o.PkOrgID
If you only need the IDs then Except should do the trick:
var inFirstButNotInSecond = first.Except(second);
Note that Except treats the two sequences as sets. This means that any duplicate elements in first won't be included in the results. I suspect that this won't be a problem since the name PkOrgID suggests a unique ID of some kind.
(See the documentation for Enumerable.Except and Queryable.Except for more info.)
Do you need the whole records, or just the IDs? The IDs are easy...
var ids = firstQuery.Except(secondQuery);
EDIT: Okay, if you can't do that, you'll need something like:
var secondQuery = ...; // As you've already got it
var query = from or in TblOrganisations
where or.OrgType == 2
where !secondQuery.Contains(or.PkOrgID)
select ...;
Check the SQL it produces, but I think it should do the right thing. Note that there's no point in performing any ordering in the second query - or even the join against TblOrganisations. In other words, you could use:
var query = from or in TblOrganisations
where or.OrgType == 2
where !LuMetricSites.Select(m => m.FkSiteID).Contains(or.PkOrgID)
select ...;
Use Except:
var filtered = first.Except(second);
I have an entity object (Company) which has 1 or more subobjects (CompanyRevision) represented as a non-null FK relationship in the database.
Using LINQ, I want to get all the Companies from the database, but I also only want the latest CompanyRevision for each company.
This is how I do it today, but I have a feeling this could be done using one query.
IEnumerable<Company> companyList = from p in ctx.Company.Include("CompanyRevisions")
select p;
foreach(Company c in companyList)
{
CompanyRevision cr = (from p in c.CompanyRevisions
orderby p.Timestamp descending
select p).First();
// Do something with c and cr...
}
As you can see, I would like to add this second LINQ query (the one that gets the latest CompanyRevision) into the first one, so that companyList[i].CompanyRevisions is basicly a list with just one entry (the latest one). I can't for the life of my figure out how to do this. Please help!
Thanks in advance
how about this: mixing the linq language and extension methods:
var results = from p in ctx.Company.Include("CompanyRevisions")
select new {Company = p,
Revision = p.CompanyRevisions.OrderByDescending(cr => cr.Timestamp).First()
}
Each result now has a Company and Revision member.
It's possible that you could also do this -
var results = from p in ctx.Company.Include("CompanyRevisions")
select new {Company = p,
Revision = (from pcr in p.CompanyRevisions
orderby pcr.Timestamp descending
select pcr).First()
}
To give the same results.
Although that's a guess - I haven't labbed that one out; but it's how I would try it first.
I have a strange problem when deleteting records using linq, my suspicion is that it has something to do with the range variable (named source). After deleting a record all targets for a customer are retrieved using the following statement:
var q = from source in unitOfWork.GetRepository<db_Target>().Find()
where source.db_TargetBase.db_Person.fk_Customer == customerID
select source.FromLinq();
where FromLinq is in extention method on db_target:
public static Target FromLinq(this db_Target source)
{
return new Target
{
id = source.id,
LastModified = source.db_TargetBase.LastModified,
...
}
}
When a record is deleted both db_Target and db_TargetBase are deleted. When, for example, two users are deleting records, linq tries to retrieve a record for user2 which is deleted by user1, causing a crash on the LastModified = source.db_TargetBase.LastModified line because db_TargetBase is null.
When using the following code the problem does not occure and only the non-deleted records are retrieved:
var q = from source in unitOfWork.GetRepository<db_Target>().Find()
where source.db_TargetBase.db_Person.fk_Customer == customerID
select new Target
{
id = source.id,
LastModified = source.db_TargetBase.LastModified,
...
};
This spawns two questions:
What is happening here? Am I making a copy of the range variable source because I'm using it in a extention method?
How can I "wrap" the return new Target code? I am using this in multiple places and do not want to copy it every time. Making my code harder to maintain.
TIA,
JJ
In the first set of code - since the initializer lives an a non-translatable method (extension or otherwise), it cannot be translated - so it is run locally.
In the second set of code - the initializer is represented by an elementinit expression, which is translated (examine/compare the select clause of the generated sql for proof).
if you want to wrap this, you need to have an Expression<Func<db_Target, Target>> that anyone can grab and use in thier query. Fortunately, that's easy to do:
public Expression<Func<db_Target, Target>> GetFromLinqExpressionForTarget()
{
return
source => new Target
{
id = source.id,
LastModified = source.db_TargetBase.LastModified,
...
}
}
Which may be used like so:
var FromLinq = GetFromLinqExpressionForTarget();
var q =
(
from source in ...
...
...
select source
).Select(FromLinq);
Now ... I'm really running on a guess here and am only about 60% confident that my answer is correct. So if someone wants to confirm this, that'll make my day. :)
I am new to LINQ. I am trying to find the rows that does not exists in the second data table.
report_list and benchmark both type are : DataTable. Both these datatables are being populated using OleDbCommand,OleDbDataAdapter. I am getting an error "Specified cast is not valid." in foreach ... loop. I would appreciate your help.
var result = from a in report_list.AsEnumerable()
where !(from b in benchmark.AsEnumerable()
select b.Field<int>("bench_id")
)
.Contains(a.Field<int>("BenchmarkID"))
select a;
foreach (var c in result)
{
Console.WriteLine(c.Field<string>("Name"));
}
I don't know if I understood your question. Are you trying to get the items that exists in the first table but not in the second?
var first = new string[] { "b", "c" };
var second = new string[] { "a", "c" };
//find the itens that exist in "first" but not in "second"
var q = from f in first
where !second.Contains(f)
select f;
foreach (var s in q) {
Console.WriteLine(s);
}
//Prints:
//b
I suggest you to make the inner query first, once it does not depend on the outer record.
From a in report_list
Group Join b in benchmark On a.bench_id Equals b.bench_id Into g = Group
Where g.Count = 0
Select a
Note that this is VB syntax.
My suspicion is that one of the fields you are comparing is not an integer in the database. I believe that the invalid cast exception is being thrown by one of the Field<int>() calls since that is one of the three different exceptions that this method can throw. See docs here.
Perhaps use the .Except() extension to get the set difference of the two sets?
(from b in benchmark.AsEnumerable()
select new { id = b.Field<int>("bench_id")}).Except(
from a in report_list.AsEnumerable()
select new {id = a.Field<int>("BenchmarkID")})
Not actually sure of the precise syntax, but that should work by taking the ids in benchmark, and then removing all equivalent ids in report_list, leaving only the ids that don't match. (I hope this is the order you were after...)
Note: This is also assuming that the above issue mentioned by tvanfosson isn't also a problem