This is my code:
string queryString = "Marco".ToLower();
utenti = db.User.Where(p =>
queryString.Contains(p.Nickname.ToLower()) ||
queryString.Contains(p.Nome.ToLower()) ||
queryString.Contains(p.Cognome.ToLower())).ToList();
but I get:
Only arguments that can be evaluated on the client are supported for the String.Contains method.
Why? Can't I use .Contains()?
Try .IndexOf. It is not LINQ that can't do Contains, it's LINQ to Entities and LINQ to SQL that can't.
string queryString = "Marco";
utenti = db.User.Where(p =>
queryString.IndexOf(p.Nickname, StringComparison.OrdinalIgnoreCase) >= 0 ||
queryString.IndexOf(p.Nome, StringComparison.OrdinalIgnoreCase) >= 0 ||
queryString.IndexOf(p.Cognom, StringComparison.OrdinalIgnoreCasee) >= 0)
.ToList();
Why?
LINQ uses deferred execution. This means it waits until you want to iterate over your query results before it does anything. There are 3 main types of LINQ:
LINQ to Objects - when your IEnumerable is already on the heap.
LINQ to Entities - when you want to query a database using Entity Framework.
LINQ to SQL - when you want to query a database using LINQ to SQL.
Deferred execution in the context of the second 2 means that your query is not executed on the database until you enumerate the results in a foreach block, or invoke an enumeration method like .ToList, .ToArray, etc. Until then, your query is just stored as expression trees in memory.
Your query would work just peachy if db.User was a collection in memory. However when the data is in a database, LINQ to Entities (or LINQ to SQL) must translate your expression trees to what it calls a "store expression" -- which is just fancy talk for "convert my LINQ expressions to SQL".
Now imagine you had a custom C# algorithm you wanted to use for your query, and you did something like this:
var result = db.User.Where(x => MyCustomMethod(x));
There is no way today that LINQ to Entities can convert your C# code into a SQL query (store expression). It is the same with a lot of other C# methods you rely on daily. It also does not support .ToLower, .ToUpper, .StartsWith, .EndsWith, etc. There is a limited number of C# methods that can be converted to store expressions, and .IndexOf just happens to be one of them.
However keep in mind that it is only the string object's Contains method that we are talking about here that is not supported for store expressions. LINQ to Entities does support .Contains on IEnumerables. The following is valid and will work with LINQ to Entities (not sure about LINQ to SQL):
var idsIWantToFind = new[] { 1, 2, 3 };
var users = db.Where(x => idsIWantToFind.Contains(x.UserId));
The above is the equivalent of doing a SQL WHERE UserId IN (1, 2, 3) predicate.
Related
I've searched through numerous examples on how to filter collection based on another collection and I have found an easy way, like so:
var oShipnotes = await DbContext.ShipNotes.Where(s => oManageRouteDto.ShipNotes.Any(mr => mr.Id == s.Id)).ToListAsync();
however it throws an exception that says it cannot be translated to SQL query.
Could anyone point me the right direction how to solve this?
Thanks!
Replace nested LINQ query to materialized list of identifiers:
// 1) get the list of target ship note identifiers
var ids = oManageRouteDto.ShipNotes.Select(mr => mr.Id).ToList();
// 2) pass this list into Where using Contains
var oShipnotes = await DbContext.ShipNotes.Where(s => ids.Contains(s.Id)).ToListAsync();
EF is aware of this pattern and translates IList<T>.Contains into SQL's IN condition.
Since EF deals with IQueryables, each LINQ query must be translated into valid SQL expression. As a result, EF and underlying provider cannot translate every valid LINQ query (from C# perspective) just because SQL is not C#.
I have this code and it gives me this following error "LINQ to Entities does not recognize the method"
var AuxiliarValue = _context.company.LastOrDefault(x => x.StartValue.HasValue && (x.StartValue.Value < InicialValue));
InicialValue is Double
But when I put a ToList(), it works
var AuxiliarValue = _context.company.ToList().LastOrDefault(x => x.StartValue.HasValue && (x.StartValue.Value < InicialValue));
Can anyone explain to me why it works with ToList()?
LastOrDefault is not supported with LINQ to Entities. You can use OrderByDescending and then use FirstOrDefault
var AuxiliarValue = _context.company
.OrderByDescending(r=> yourFieldtoOrder)
.FirstOrDefault(x => x.StartValue.HasValue && (x.StartValue.Value < InicialValue));
The reason it works with ToList is that ToList will iterate all the results and bring them in memory, so the LastOrDefault is executed on an in-memory collection, rather than at the database end.
It works when you add ToList because the query is no longer being translated into SQL and executed by the database. Instead, the entire table of data is returned from the database to your application, a List is built to hold that data, and then the operation is performed using LINQ to Objects.
You probably don't want to do that; you probably want to adjust the way that you query the data such that it can be translated into SQL and run against the database.
Here is the code:
public ActionResult Index()
{
var crView = db.CRCases.ToList();
return View(crView);
}
Would like to filter like we have in sql statement using WHERE and ORDER BY.
Is there any good reference on how to apply it?
linq is your friend here.
you can do
crView = crView.Where(x => x.yourPredicate).OrderBy( y => y.OrderClause);
CRCases should be a DBSet<T>. You can use LinQ like this :
db.CRCases.Where(x => x.TheProperty =xxx).ToList()
You can use LINQ = Language Integrated Query.
This have two different notations:
Functional: you can change several function calls beginning on an IEnumerable.
db.CrCases.Where(predicate).OrderBy(sortExpression)
"SQL like": it really looks like an "unordered SQL".
from crCase in db.CrCases where condition orderby sortExpression select crCase
The condition and sort expression in the first notation require lambda expressions (you can see them in action here). In the second case the condition is expressed.
Most of the queries can be written either way.
You'll usually use var to store the query, and materialize it later.
var query = from crCase in db.CrCases
where condition orderby sortExpression
select crCase
CrCase firstCase = query.Firts();
List<CrCase> cases = query.ToList();
There are several flavors of LINQ. They all look equal on the surface, but are translated into another thing. For example:
LINQ to objects: it's the more general, an allows to make queries on collections. Tis is what you'll use for this particular case.
LINQ to SQL: this allows to write queries in C# which are transalated to SQL queries in the underlying database
LINQ to Entities: similar to the previous, but you make your queries to an Entity Framework model, and the queries are finally transalated to the DB
There are some others flavors of LINQ. And, in fact, you can create your own, but that's not easy.
Finally, you have to take into account that a LINQ query isn't executed until you materialize it. I.e. it's not executed until you try to get data from it (converting it ToList(), ToArray() or whatever, enumerating it, or accesing any of it's data First(), Sum(), Count()). Depending on the flavor, the executing can be running C# code (LINQ to objects) or run a SQL Query in a DB (LINQ to SQL) or whatever.
Here you have a great place to learn how to use LINQ:
LINQ 101
I built a Linq query on a MongoDB collection, which retrieves properly my docs:
var parts = mongoParts.AsQueryable()
.Where(mongo => mongo.Prop1 == bmbMatch.Prop1 && mongo.Prop2 == bmbMatch.Prop2)
.ToList();
This works fine, but to make this less code I wrote a Generic function taking a
Func<T, T, bool> selector
Which take the exact same lamdba expression as in Where clause above, which makes it:
var mongo = mongoQuery.AsQueryable()
.Where(tmongo => selector(tmongo, localMatch))
.ToList();
with selector as :
(mongo, local) => mongo.Prop1== local.Prop1 && mongo.Prop2 == local.Prop2
This gives me a "Unsupported where clause: "
Any ideas?
From the internet
Not all LINQ queries can be turned into equivalent MongoDB queries,
because C# allows you to write expressions that don't have any
equivalent in the MongoDB query language.
That being said, LINQ to MongoDB probably can't translate delegates.
You can find all supported LINQ queries on this address.
Conclusion: If you need to query very large data or make queries as fast as possible, you will have to stick with the former solution. Otherwise, pull all data from your db and query them in-memory.
I want to order by my results with the matches count in my string line.
So here is code
.ThenByDescending(p => p.Title.ToLower()
.Split(' ')
.Count(w => words.Any(w.Contains)));
But it bring me error and says that LINQ can't parse Split into SQL.
LINQ to Entities does not recognize the method 'System.String[]
Split(Char[])' method, and this method cannot be translated into a
store expression.
How can I implement Split via LINQ?
For example, for this array it must order in this way
words = { "a", "ab" }
ab a ggaaag gh //3 matches
ba ab ggt //2 matches
dd //0 matches
it means that Linq to entities failed to find translation of split method that can be written as sql query. if you want to perform split functions you have to bring the record in memory by calling ToList(), AsEnumerable() etc.
var result = (from t in db.Table
select t).AsEnumerable().OrderBy(x=>x.Column).ThenByDescending(p=>p.Title.ToLower.Split(' ')....);
You will need to perform the sorting in LINQ to Objects because LINQ to Entities cannot translate the C# code into SQL (or the language of whatever DB you're using).
You can do it like this.
var results =
objectContext
.Where(a => a == b) //Whatever
.AsEnumerable()
.ThenByDescending(p=>p.Title.ToLower().Split(' ').Count(w=>words.Any(w.Contains)));
AsEnumerable() (along with ToArray() or ToList()) turn the LINQ to Entities back into LINQ to Objects.
One can't expect LINQ to Entities to be able to convert that to SQL.
The best solution is to change the schema such that each word in a post's title is stored as a separate row in a separate table (with the appropriate associations). The query can then use explicit (group) join operations or the FK association property.
If you can't do this and yet want the query to run on the database, you'll have to look into writing a user-defined function to work with delimited strings. Read this question for more info. This will be a lot of work though.
The easiest solution (if you can afford to do it) of course is to pull everything back to the client just use LINQ to Objects for that part of the query, as mentioned by #Muhammad Adeel Zahid.