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)
Related
This question already has answers here:
EntityFramework - contains query of composite key
(12 answers)
Closed 2 years ago.
I have a table as follows:
PersonalDetails
Columns are:
Name
BankName
BranchName
AccountNo
Address
I have another list that contains 'Name' and 'AccountNo'.
I have to find all the records from table that whose respective 'Name' and 'AccountNo' are present in given list.
Any suggestion will be helpful.
I have done following but not of much use:
var duplicationhecklist = dataAccessdup.MST_FarmerProfile
.Join(lstFarmerProfiles,
t => new { t.Name,t.AccountNo},
t1 => new { t1.Name, t1.AccountNo},
(t, t1) => new { t, t1 })
.Select(x => new {
x.t1.Name,
x.t1.BankName,
x.t1.BranchName,
x.t1.AccountNo
}).ToList();
where lstFarmerProfiles is a list.
You probably found out that you can't join an Entity Framework LINQ query with a local list of entity objects, because it can't be translated into SQL. I would preselect the database data on the account numbers only and then join in memory.
var accountNumbers = lstFarmerProfiles.Select(x => x.AccountNo).ToArray();
var duplicationChecklist =
from profile in dataAccessdup.MST_FarmerProfile
.Where(p => accountNumbers
.Contains(p.AccountNo))
.AsEnumerable() // Continue in memory
join param in lstFarmerProfiles on
new { profile.Name, profile.AccountNo} equals
new { param.Name, param.AccountNo}
select profile
So you will never pull the bulk data into memory but the smallest selection you can probably get to proceed with.
If accountNumbers contains thousands of items, you may consider using a better scalable chunky Contains method.
Since you have the lists in .net of values you want to find, try to use the Contains method, for sample:
List<string> names = /* list of names */;
List<string> accounts = /* list of account */;
var result = db.PersonalDetails.Where(x => names.Contains(x.Name) && accounts.Contains(x.AccountNo))
.ToList();
If MST_FarmerProfile is not super large I think you best option is to bring it into memory using AsEnumerable() and do the joining there.
var duplicationhecklist =
(from x in dataAccessdup.MST_FarmerProfile
.Select(z => new {
z.Name,
z.BankName,
z.BranchName,
z.AccountNo
}).AsEnumerable()
join y in lstFarmerProfiles
on new { x.Name, x.AccountNo} equals new { y.Name, y.AccountNo}
select x).ToList();
Since data is usually located on different machines or in separate processes at least: DB - is one and your in-memory list is your app, there is just 2 ways to do it.
Download as small data part from DB to local as possible and join locally (usually using AsEnumerable() or basically ToList()). You got many good thoughts on this in other answers.
Another one is different - upload your local data to server somehow and perform query on DB side. Uploading can be done differently: using some temp tables OR using VALUES. Fortunately there is a small extension for EF now (for both EF6 and EF Core) which you could try. It is EntityFrameworkCore.MemoryJoin (name might be confusing, but it supports both EF6 and EF Core). As stated in author's article it modifies SQL query passed to server and injects VALUES construction with data from your local list. And query is executed on DB server.
If accountNo identifies the record then you could use:
var duplicationCheck = from farmerProfile in dataAccessdup.MST_FarmerProfile
join farmerFromList in lstFarmerProfiles
on farmerProfile.AccountNo equals farmerFromList.AccountNo
select new {
farmerProfile.Name,
farmerProfile.BankName,
farmerProfile.BranchName,
farmerProfile.AccountNo
};
If you need to join on name and account then this should work:
var duplicationCheck = from farmerProfile in dataAccessdup.MST_FarmerProfile
join farmerFromList in lstFarmerProfiles
on new
{
accountNo = farmerProfile.AccountNo,
name = farmerProfile.Name
}
equals new
{
accountNo = farmerFromList.AccountNo,
name = farmerFromList.Name
}
select new
{
farmerProfile.Name,
farmerProfile.BankName,
farmerProfile.BranchName,
farmerProfile.AccountNo
};
If you are only going to go through duplicateChecklist once then leaving .ToList() out will be better for performance.
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'm trying to get Entity Framework to select an object and filter its collection at the same time. I have a JobSeries object which has a collection of jobs, what I need to do is select a jobseries by ID and filter all the jobs by SendDate but I can't believe how difficult this simple query is!
This is the basic query which works:
var q = from c in KnowledgeStoreEntities.JobSeries
.Include("Jobs.Company")
.Include("Jobs.Status")
.Include("Category")
.Include("Category1")
where c.Id == jobSeriesId
select c;
Any help would be appreciated, I've been trying to find something in google and what I want to do is here:http://blogs.msdn.com/bethmassi/archive/2009/07/16/filtering-entity-framework-collections-in-master-detail-forms.aspx
It's in VB.NET though and I couldn't convert it to C#.
EDIT: I've tried this now and it doesn't work!:
var q = from c in KnowledgeStoreEntities.JobSeries
.Include("Jobs")
.Include("Jobs.Company")
.Include("Jobs.Status")
.Include("Category")
.Include("Category1")
where (c.Id == jobSeriesId & c.Jobs.Any(J => J.ArtworkId == "13"))
select c;
Thanks
Dan
Include can introduce performance problems. Lazy loading is guaranteed to introduce performance problems. Projection is cheap and easy:
var q = from c in KnowledgeStoreEntities.JobSeries
where c.Id == jobSeriesId
select new
{
SeriesName = c.Name,
Jobs = from j in c.Jobs
where j.SendDate == sendDate
select new
{
Name = j.Name
}
CategoryName = c.Category.Name
};
Obviously, I'm guessing at the names. But note:
Filtering works.
SQL is much simpler.
No untyped strings anywhere.
You always get the data you need, without having to specify it in two places (Include and elsewhere).
No bandwith penalties for retrieving columns you don't need.
Free performance boost in EF 4.
The key is to think in LINQ, rather than in SQL or in materializing entire entities for no good reason as you would with older ORMs.
I've long given up on .Include() and implemented Lazy loading for Entity Framework
In another posting: Does Linq-To-Sql support composable queries there was discussion on how to compose/concat where clauses dynamically. This appears to be done with an "AND" (i.e. the first where clause and the second where clause are joined by an AND). What I am wondering is if there is a way to compose Linq queries with an OR.
Example:
var people = from p in Person
where p.age < 18
select p
var otherPeople = from p in people
where p.firstName equals "Daniel"
select p
This gives people with a first name of "Daniel" and that are under 18. I'm looking for the syntax to join these to find people who have a first name of "Daniel" or are under 18.
Note: I am using ADO.net Data Services so I do not have .Contains() available to me.
EDIT: The Union Suggestion (by Garry Shutler) is exactly what I am looking for functionality-wise. I did run into two possible issues with it:
It looks like it would make multiple database hits if I was to do a third condition (union seems to take an IEnumerable as its parameter) - I was hoping to build up multiple AND and OR statements in code and then execute one request.
Union is not supported by ADO.Net Data Services (very disappointing)
Is what you want as simple as:
var people = from p in Person
where p.age < 18 || p.firstName == "Daniel"
select p;
or have you just given a simple example?
In which case you can use:
var under18 = from p in Person
where p.age < 18
select p;
var daniels = from p in Person
where p.firstName == "Daniel"
select p;
var combined = under18.Union(daniels);
LinqToSql may be intelligent enough to convert that to an OR but I'm not so sure.
What about using PredicateBuilder by Joe Albahari?
var predicate = PredicateBuilder.False<Person>();
predicate = predicate.Or(p => p.age < 18);
predicate = predicate.Or(p => p.firstName == "Daniel");
var query = Person.Where(predicate);
The predicate option is the way to go. The Union option DOES NOT build good sql. Reference http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/925b245d-5529-4a64-8cd4-4bc83ee6fe7a/
I wrote about how to achieve queries which search for a key value within a set on my blog .
Here are the relevant links.
Contains Operations in ADO.NET Data Services Part I
Contains Operations in ADO.NET Data Services Part II
Using this , you can write queries which look like this
//The set in which we have to search for a match
List<string> citiesIWillVisit = new List<string>() {"London","Berlin","Prague"};
var customersAround = nwContext.Customers
.IsIn<Customers>(citiesIWillVisit, c=> c.City);
foreach (Customers localCustomer in customersAround) {
System.Console.WriteLine(localCustomer.ContactName);
}
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