I stumbled over some strange LINQ to SQL behaviour - can anybody shed some light on this?
I want to define a lambda expression and use it in my LINQ statement. The following code works fine:
[...]
Func<Table1, bool> lambda = x => x.Id > 1000;
var result = dataContext.Table1s.Where(lambda);
[...]
But when I try to use my lambda expression in a statement on an associated table
[...]
Func<Table1, bool> lambda = x => x.Id > 1000;
var result = dataContext.Table2s.Where(x => x.Table1s.Any(lambda));
[...]
I get an exception:
Unsupported overload used for query operator 'Any'.
But, and this I don't get: It works fine when I put my lambda directly into the query:
[...]
var result = dataContext.Table2s.Where(x => x.Table1s.Any(y => y.Id > 1000));
[...]
WHY?!
Thanks.
Okay, here's the deal: dataContext.Table1s is of type IQueryable<T>. IQueryable<T> defines Where and Any methods that take a predicate of type Expression<Func<T, bool>>. The Expression<> wrapper is critical, as this is what allows LINQ to SQL to translate your lambda expression to SQL and execute it on the database server.
However, IQueryable<T> also includes IEnumerable<T>. IEnumerable<T> also defines Where and Any methods, but the IEnumerable version takes a predicate of type Func<T, bool>. Because this is a compiled function and not an expression, it can't be translated to SQL. As a result, this code...
Func<Table1, bool> lambda = x => x.Id > 1000;
var result = dataContext.Table1s.Where(lambda);
...will pull EVERY record out of Table1s into memory, and then filter the records in memory. It works, but it's really bad news if your table is large.
Func<Table1, bool> lambda = x => x.Id > 1000;
var result = dataContext.Table2s.Where(x => x.Table1s.Any(lambda));
This version has two lambda expressions. The second one, being passed directly into Where, is an Expression that includes a reference to a Func. You can't mix the two, and the error message you're getting is telling you that the call to Any is expecting an Expression but you're passing in a Func.
var result = dataContext.Table2s.Where(x => x.Table1s.Any(y => y.Id > 1000));
In this version, your inner lambda is automatically being converted to an Expression because that's the only choice if you want your code to be transformed into SQL by LINQ to SQL. In the other cases, you're forcing the lambda to be a Func instead of an Expression - in this case you're not, so it works.
What's the solution? It's actually pretty simple:
Expression<Func<Table1, bool>> lambda = x => x.Id > 1000;
Does Table1 refer to the same namespace? In the first example you're querying against the Table1 objects that are directly under dataContext, in the second example you're querying against the Table1 objects that is a property of the Table2 objects, and in the last example you're using a anonymous function which fix the issue.
I would look up the type of the Table1 objects that is a property of a Table2 object and compare it to a Table1 object that is connected directly to the dataContext. My guess is that they differ and your lambda expression is using the type of the object that is connected to the dataContext.
Related
I'm Using Scala(2.11) and playframework(2.3) and trying to run a query using a helper function to get results through pattern matching. The function is as follows
def resultsfunc() = {
val gradeRegex = "^Class 5\."
val currRegex = "\.NCERT$"
DB.withConnection{ implicit c =>
val filterQuery = SQL(
"""
select * from tbl_graphs
where graph_name REGEXP '{grade_regex}' and
graph_name REGEXP '{curr_regex}' and org_id = 4
""")
.on("grade_regex" -> gradeRegex,
"curr_regex" -> currRegex)
filterQuery().map{ graphRecord =>
new ResultObj(graphRecord[Long]("id"),
graphRecord[String]("name"))
}.toList
}
}
I don't get any errors but I get empty result even though there are multiple records that match the pattern. The same query works if I try to run in mysql workbench and when I tried to print filterQuery the arguments were also mapped correctly.
Should Pattern matching with regex must be carried out differently in Scala Anorm ?
It has absolutely nothing to do specifically with Anorm.
Make sure that executing manually the query with exactly the same data and parameter, you get result.
When using JDBC (even through Anorm), string parameter must not be quoted in the query statement (... '{grade_regex}' ...).
Since a long time, it's recommended to use Anorm interpolation (SQL"SELECT x FROM y WHERE z = ${v}") rather than SQL(..) function.
I want to execute the following select:
SELECT 0 as Value
What is the correlating syntax in LINQ for SQL?
Edit
I want to use the correlating LINQ for SQL statement in a Concat() call like this
var c = (from a in mytable select a.Value).Concat(select 0).Sum();
As you can see, Concat(select 0) obviously doesn't compile. Any ideas?
Edit 2
David suggested to use a simple collection instead. I've tried
private decimal[] mZeroDecimals = new[] { 0.0m };
...
public void MyFunction()
{
var c = (from a in mytable select a.Value).Concat(mZeroDecimals).Sum();
...
but it throws an exception Local sequence cannot be used in LINQ to SQL implementation of query operators except the Contains() operator.
You're not actually querying anything, so there is no LINQ involved. You're just creating an anonymous object with a single property called Value:
var obj = new { Value = 0 };
Edit: Based on your comment, it sounds like you want this object in a collection. That doesn't make it a LINQ query (since you're still not querying anything), but you can declare a collection just as easily as a single object. Something like this:
var coll = new[] { new { Value = 0 } };
Since this is a collection, it can be used with any of the enumerable extension methods that LINQ uses, which sounds like what you're trying to do.
I'm using EF5.0 in an ASP.NET MVC app. My Entity Model is named 'DataModel'. Included in the model is a table-valued function that exists in my MSSQL database, named MatchingEntries. It returns a table of integer ids.
I've looked at the DataModel.Context.cs file, that gets generated via the .tt (T4) template file. It has the following code in it:
[EdmFunction("DataEntities", "MatchingEntries")]
public virtual IQueryable<Nullable<int>> MatchingEntries(string term)
{
var termParameter = term != null ?
new ObjectParameter("Term", term) :
new ObjectParameter("Term", typeof(string));
return ((IObjectContextAdapter)this).ObjectContext.CreateQuery<Nullable<int>>("[DataEntities].[MatchingEntries](#Term)", termParameter);
}
The error I am getting results from using this method twice within the one query, such as:
IQueryable<int> one = db.MatchingEntries("\"one*\"");
IQueryable<int> two = db.MatchingEntries("\"two*\"");
List<int> both = one.Intersect(two).ToList();
The error is:
A parameter named 'Term' already exists in the parameter collection. Parameter names must be unique in the parameter collection.
Parameter name: parameter
Is this a known limitation of the classes generated from an EDMX for table-valued functions? With LINQ2SQL I am able to execute this a a single query to the database (that does a JOIN between the 2 outputs from MatchingEntries) and it replaces the parameter name #Term with #p0 and #p1 for the two different instances of the call. I'd like to make Entity Framework do the same.
So, my question is, how can I get EF to work in the same manner and avoid the 'Duplicate parameter' error?
My fallback is to evaluate each call to db.MatchingEntries separately, by putting ToList() after them. My other idea has been to replace the ObjectParameter name in the T4 generated Context.cs class with something randomly generated each time. These feel like hacks that I should be able to avoid.
This answer is Linq to Entities specific. This doesn't have to be done in Linq to SQL (Linqpad).
Thanks to this question I got a pointer to a viable solution:
extend the autogenerated DBContext class (partial class)
add a method with two parameters in the partial class
at calling, pass an index as second parameter
Detailed Answer:
DataEntitys.my.cs:
[EdmFunction("DataEntities", "MatchingEntries")]
public virtual IQueryable<Nullable<int>> MatchingEntries(string term, int index)
{
string param_name = String.Format("k_{0}", index);
var termParameter = term != null ?
new ObjectParameter(param_name, term) :
new ObjectParameter(param_name, typeof(string));
return ((IObjectContextAdapter)this).
ObjectContext.CreateQuery<Nullable<int>>(
String.Format("[DataEntities].[MatchingEntries](#{0})", param_name),
termParameter);
}
Call the function:
foreach (string teil in such)
{
index++;
if (teil.Trim() != "")
res = res.Join(db.MatchingEntries("\"" + teil + "*\"", index), l => l.ID, s => s.KEY, (l, s) => l);
}
I am trying to use system.dynamic.linq to create dynamic sorting.
this, is the query that I use:
var query = dalSession.Query<T>().AsQueryable();
var res = (from x in query orderby "x.FirstName" select x)
this is the mysql output:
select
affiliate0_.id as id0_,
affiliate0_.first_name as first6_0_,
from affiliate affiliate0_ order by 'x.FirstName' //FirstName as well
So you can see that the output went to the mysql query is the direct string, and not its reflection, ('x.FirstName') or ('FirstName').
This has no meaning in mysql context, it looks like I need order by affiliate0_.first_name.
Is there anyway to provide the Nhibernate the member itself? So the compiled query will be done normally?
You have to remove the " from you requiry. "x.FirstName" is seen as a string and translated to a sql string.
The "x" has no meaning inside the dynamic string.
Remove the x. (i.e. leave just "FirstName") and it should work.
Using a method call instead of the query comprehension syntax, you'd get:
var res = query.Orderby("FirstName")
Did you try it as follows (as Diego suggested)?
(from x in query select x).OrderBy("FirstName")
Because I think the dynamic linq orderby extension method is not executed when you use (x => "FirstName")
Otherwise try this:
(from x in query select x).OrderBy("it.FirstName")
BTW I renamed the OrderBy method in Dynamic.cs to DynamicOrderBy because I had some situations where the not dynamic linq OrderBy method was executed too:
public static IQueryable<T> DynamicOrderBy<T>(this IQueryable<T> source, string ordering, params object[] values) and use that one instead:
(from x in query select x).DynamicOrderBy("FirstName")
var param = Expression.Parameter("x");
var prop = Expression.Property(param, "FirstName");
var lambda = Expression.Lambda<Func<User, string>>(prop, param);
query.Orderby(lambda);
I have Ienumerable<string> collection that I want to concatenate into a single string with delimitor ;
for instance {"One","Two","Three"} -> "One;Two;Three;"
is it possible to do using the following function?
List<string> list = new List<string>(){"One","Two","Three"};
list.Aggregate<String>((x,y) => x + String.Format("{0};",y));
I have tried also this code:
list.Aggregate<String>((x,y) => String.Format("{0};{1}",x,y));
both samples didn't work.
EDIT: I see that it is not possible to do what I wanted using Linq-2-sql or Aggregate function in Linq-2-sql.
http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/dac496c0-5b37-43ba-a499-bb8eff178706/
EDIT2: the workaround I used is to go over the items returned by the original linq query...and copies them to a new list and do the join as suggested in the answers below on a linq object and not linq-2-sql object.
You can just use String.Join for this. If you're using .NET4 then you can use the overload that takes an IEnumerable<string> directly:
string joined = string.Join(";", list);
If you're using an older version of the framework then you'll need to use the overload that takes a string[] array instead, converting your collection to an array first if necessary:
string joined = string.Join(";", list.ToArray());
EDIT...
Of course, if you really want to use Aggregate for some reason then there's nothing stopping you. If so, it's usually recommended to build your string using a StringBuilder rather than multiple string allocations:
string joined = list.Aggregate(new StringBuilder(),
(sb, s) => sb.Append(s).Append(';'),
sb => (sb.Length > 0) ? sb.ToString(0, sb.Length - 1)
: "");
You can do it using below code
list.Aggregate((i, j) => i + ";" + j);
You'll need to provide an initializer, otherwise the first element will not have a ; added to it:
list.Aggregate<String>("", (x,y) => x + String.Format("{0};",y));