I am having trouble wrapping my head around the Dynamic LinQ select. I have read Scott Gu's blog and each of the suggested posts but still cannot get my little brain to understand. I need to query an entity object passing in a variable for the column name. Assume an entity (table) containing 3 columns named: Mon, Tue & Wed. These contain data for each record. I do not know the column to hit at compile time so I need to pass a column name into the query. How can I accomplish this either using DLink or directly using Expression Trees?
Don't need Dynamic LINQ for that...
var columniwant="Mon"; // Or "Tue" or "Wed"
var query=db.stuff here;
IQueryable<string> result;
switch(columniwant)
{
case "Mon": result=query.Select(q=>q.Mon); break;
case "Tue": result=query.Select(q=>q.Tue); break;
case "Wed": result=query.Select(q=>q.Wed); break;
}
That said, I would think this would work:
var result=db.stuff here.Select(columniwant);
Related
everyone! ))
Here is code.
var existingEntities = (from record in globalOne.serviceContext.records_out
where record.timestamp.Date == DateTime.Now.Date
select record ).ToList();
It doesn't work.
Another code:
var existingEntities = (from record in globalOne.serviceContext.records_out
where record.timestamp.Day == DateTime.Now.Day
select record ).ToList();
It does work.
So, problem id in next string:
where record.timestamp.**Date** == DateTime.Now.Date
also won't do
where record.timestamp.Date.Equals(DateTime.Now.Date)
But why? I have no clue. "Timestamp" field is dateTime field in MS SQL SERVER.
And - there is NO records in table.
And I almost forgot - what does it mean - "doesn't work".
App just will not reach the breakpoint after that query(first), without any error, without anything.
Thanks.
You can call record.timestamp.Date because EF can't convert it to required expression tree (then convert it to sql command). In fact EF supports limited number of functions and properties, But for DateTime, EF has some good Canonical functions. You can use them in your case, e.g you can use Day(),Month(),Year() functions to solve your problem (see the link).
I'm using OpenXML to open a spreadsheet and loop through the rows of a spreadsheet. I have a linq query that returns all cells within a row. The linq query was ripped straight from a demo on the MSDN.
IEnumerable<String> textValues =
from cell in row.Descendants<Cell>()
where cell.CellValue != null
select (cell.DataType != null
&& cell.DataType.HasValue
&& cell.DataType == CellValues.SharedString
? sharedString.ChildElements[int.Parse(cell.CellValue.InnerText)].InnerText
: cell.CellValue.InnerText);
The linq query is great at returning all cells that have a value, but it doesn't return cells that don't have a value. This in turn makes it impossible to tell which cell is which. Let me explain a little more. Say for instance we have three columns in our spreadsheet: Name, SSN, and Address. The way this linq query works is it only returns those cells that have a value for a given row. So if there is a row of data that has "John", "", "173 Sycamore" then the linq query only returns "John" and "173 Sycamore" in the enumeration, which in turn makes it impossible for me to know if "173 Sycamore" is the SSN or the Address field.
Let me reiterate here: what I need is for all cells to be returned, and not just cells that contain a value.
I've tried to monkey the linq query in every way that I could think of, but I had no luck whatsoever (ie - removing the where clause isn't the trick). Any help would be appreciated. Thanks!
The OpenXML standard does not define placeholders for cells that don't have data. In other words, it's underlying storage in XML is sparse. You could work round this on one of two ways:
Create a list of all "available" or "possible" cells (probably by using a CROSS JOIN type of operation) then "left" joining to the row.Descendants<Cell>() collection to see if the cell reference has a value
Utilize a 3rd party tool such as ClosedXML or EPPlus as a wrapper around the Excel data and query their interfaces, which are much more developer-friendly.
With ClosedXML:
var wb = new XLWorkbook("YourWorkbook.xlsx");
var ws = wb.Worksheet("YourWorksheetName");
var range = ws.RangeUsed();
foreach(var row in range.Rows())
{
// Do something with the row...
// ...
foreach(var cell in row.Cells())
{
// Now do something with every cell in the row
// ...
}
}
The one way I recommend is to fill in all the null cells with blank data so they will be returned by your linq statement. See this answer for how to do that.
I'm wondering about the best usage of the delete method in nhibernate.
If you go the entity than just call delete and send it, but if not you need to query it or write a query and send it to delete method.
I'm wondering if its possible to write a linq expression and send it to delete.
Is it possible to perform a Linq transformation to hql and than call session.Delete(query)
with the generated hql?
I want to call Session.Delete, and give it a linq so it can know what to delete without selecting the data. Do you know a class that can convert linq expression to hql?
You now can directly in linq with NHibernate 5.0
//
// Summary:
// Delete all entities selected by the specified query. The delete operation is
// performed in the database without reading the entities out of it.
//
// Parameters:
// source:
// The query matching the entities to delete.
//
// Type parameters:
// TSource:
// The type of the elements of source.
//
// Returns:
// The number of deleted entities.
public static int Delete<TSource>(this IQueryable<TSource> source);
Exemple:
var tooOldDate = System.DateTime.Now.AddYears(5);
session.Query<User>()
.Where(u => u.LastConnection <= tooOldDate)
.Delete();
The Q in LINQ stands for "Query". So, no, you can't use a LINQ expression for delete.
That said, NH's query language, HQL, does support that.
In the same way that you can say "from Foo where Bar = :something" to get all the foos matching a condition, you can do this:
session.CreateQuery("delete Foo where Bar = :something")
.SetParameter("something", ...)
.ExecuteUpdate();
I have submitted a pull request for NH-3659 - Strongly Typed Delete. The link is available at nhibernate.jira.com/browse/NH-3659.
I know this is an old question but for those reading this now. NHibernate 5 released Oct 10, 2017 has added a Delete Linq extension
from documentation 17.6.3. Deleting entities
Delete method extension expects a queryable defining the entities to delete. It immediately deletes them.
session.Query<Cat>()
.Where(c => c.BodyWeight > 20)
.Delete();
I'm sure it would be possible to do what you want but the bottom line is that it doesn't make a lot of sense (not sure why you want to give NHibernate the select criteria when you can do it in a single statement, your approach would end up causing 2 hits to the database), having said that, one easy option you could do, is query the IDs using LINQ and pass those to NHibernate
int[] deleteIds = (from c in Customer where {some condition} select c.Id).ToArray<int>();
session.CreateQuery("delete Customer c where c.id in (:deleteIds)")
.SetParameterList("deleteIds", deleteIds)
.ExecuteUpdate();
I'm trying to do a query like so...
query.Where(x => !string.IsNullOrEmpty(x.PropertyName));
but it fails...
so for now I have implemented the following, which works...
query.Where(x => (x.PropertyName ?? string.Empty) != string.Empty);
is there a better (more native?) way that LINQ handles this?
EDIT
apologize! didn't include the provider... This is using LINQ to SQL
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=367077
Problem Statement
It's possible to write LINQ to SQL that gets all rows that have either null or an empty string in a given field, but it's not possible to use string.IsNullOrEmpty to do it, even though many other string methods map to LINQ to SQL.
Proposed Solution
Allow string.IsNullOrEmpty in a LINQ to SQL where clause so that these two queries have the same result:
var fieldNullOrEmpty =
from item in db.SomeTable
where item.SomeField == null || item.SomeField.Equals(string.Empty)
select item;
var fieldNullOrEmpty2 =
from item in db.SomeTable
where string.IsNullOrEmpty(item.SomeField)
select item;
Other Reading:
1. DevArt
2. Dervalp.com
3. StackOverflow Post
This won't fail on Linq2Objects, but it will fail for Linq2SQL, so I am assuming that you are talking about the SQL provider or something similar.
The reason has to do with the way that the SQL provider handles your lambda expression. It doesn't take it as a function Func<P,T>, but an expression Expression<Func<P,T>>. It takes that expression tree and translates it so an actual SQL statement, which it sends off to the server.
The translator knows how to handle basic operators, but it doesn't know how to handle methods on objects. It doesn't know that IsNullOrEmpty(x) translates to return x == null || x == string.empty. That has to be done explicitly for the translation to SQL to take place.
This will work fine with Linq to Objects. However, some LINQ providers have difficulty running CLR methods as part of the query. This is expecially true of some database providers.
The problem is that the DB providers try to move and compile the LINQ query as a database query, to prevent pulling all of the objects across the wire. This is a good thing, but does occasionally restrict the flexibility in your predicates.
Unfortunately, without checking the provider documentation, it's difficult to always know exactly what will or will not be supported directly in the provider. It looks like your provider allows comparisons, but not the string check. I'd guess that, in your case, this is probably about as good of an approach as you can get. (It's really not that different from the IsNullOrEmpty check, other than creating the "string.Empty" instance for comparison, but that's minor.)
... 12 years ago :) But still, some one may found it helpful:
Often it is good to check white spaces too
query.Where(x => !string.IsNullOrWhiteSpace(x.PropertyName));
it will converted to sql as:
WHERE [x].[PropertyName] IS NOT NULL AND ((LTRIM(RTRIM([x].[PropertyName])) <> N'') OR [x].[PropertyName] IS NULL)
or other way:
query.Where(x => string.Compare(x.PropertyName," ") > 0);
will be converted to sql as:
WHERE [x].[PropertyName] > N' '
If you want to go change the type of the collection from nullable type IEnumerable<T?> to non-null type IEnumerable<T> you can use .OfType<T>().
.OfType<T>() will remove null values and return a list of the type T.
Example: If you have a list of nullable strings: List<string?> you can change the type of the list to string by using OfType<string() as in the below example:
List<string?> nullableStrings = new List<string?> { "test1", null, "test2" };
List<string> strings = nullableStrings.OfType<string>().ToList();
// strings now only contains { "test1", "test2" }
This will result in a list of strings only containing test1 and test2.
Why do I get the error:
Unable to create a constant value of type 'Closure type'. Only
primitive types (for instance Int32, String and Guid) are supported in
this context.
When I try to enumerate the following Linq query?
IEnumerable<string> searchList = GetSearchList();
using (HREntities entities = new HREntities())
{
var myList = from person in entities.vSearchPeople
where upperSearchList.All( (person.FirstName + person.LastName) .Contains).ToList();
}
Update:
If I try the following just to try to isolate the problem, I get the same error:
where upperSearchList.All(arg => arg == arg)
So it looks like the problem is with the All method, right? Any suggestions?
It looks like you're trying to do the equivalent of a "WHERE...IN" condition. Check out How to write 'WHERE IN' style queries using LINQ to Entities for an example of how to do that type of query with LINQ to Entities.
Also, I think the error message is particularly unhelpful in this case because .Contains is not followed by parentheses, which causes the compiler to recognize the whole predicate as a lambda expression.
I've spent the last 6 months battling this limitation with EF 3.5 and while I'm not the smartest person in the world, I'm pretty sure I have something useful to offer on this topic.
The SQL generated by growing a 50 mile high tree of "OR style" expressions will result in a poor query execution plan. I'm dealing with a few million rows and the impact is substantial.
There is a little hack I found to do a SQL 'in' that helps if you are just looking for a bunch of entities by id:
private IEnumerable<Entity1> getByIds(IEnumerable<int> ids)
{
string idList = string.Join(",", ids.ToList().ConvertAll<string>(id => id.ToString()).ToArray());
return dbContext.Entity1.Where("it.pkIDColumn IN {" + idList + "}");
}
where pkIDColumn is your primary key id column name of your Entity1 table.
BUT KEEP READING!
This is fine, but it requires that I already have the ids of what I need to find. Sometimes I just want my expressions to reach into other relations and what I do have is criteria for those connected relations.
If I had more time I would try to represent this visually, but I don't so just study this sentence a moment: Consider a schema with a Person, GovernmentId, and GovernmentIdType tables. Andrew Tappert (Person) has two id cards (GovernmentId), one from Oregon (GovernmentIdType) and one from Washington (GovernmentIdType).
Now generate an edmx from it.
Now imagine you want to find all the people having a certain ID value, say 1234567.
This can be accomplished with a single database hit with this:
dbContext context = new dbContext();
string idValue = "1234567";
Expression<Func<Person,bool>> expr =
person => person.GovernmentID.Any(gid => gid.gi_value.Contains(idValue));
IEnumerable<Person> people = context.Person.AsQueryable().Where(expr);
Do you see the subquery here? The generated sql will use 'joins' instead of sub-queries, but the effect is the same. These days SQL server optimizes subqueries into joins under the covers anyway, but anyway...
The key to this working is the .Any inside the expression.
I have found the cause of the error (I am using Framework 4.5). The problem is, that EF a complex type, that is passed in the "Contains"-parameter, can not translate into an SQL query. EF can use in a SQL query only simple types such as int, string...
this.GetAll().Where(p => !assignedFunctions.Contains(p))
GetAll provides a list of objects with a complex type (for example: "Function"). So therefore, I would try here to receive an instance of this complex type in my SQL query, which naturally can not work!
If I can extract from my list, parameters which are suited to my search, I can use:
var idList = assignedFunctions.Select(f => f.FunctionId);
this.GetAll().Where(p => !idList.Contains(p.FunktionId))
Now EF no longer has the complex type "Function" to work, but eg with a simple type (long). And that works fine!
I got this error message when my array object used in the .All function is null
After I initialized the array object, (upperSearchList in your case), the error is gone
The error message was misleading in this case
where upperSearchList.All(arg => person.someproperty.StartsWith(arg)))