I'm using a view returning Domains according to an id. The Domains column can be 'Geography' or can be stuffed domains 'Geography,History'. (In any way, the data returned is a VARCHAR)
In my C# code, I have a list containing main domains:
private static List<string> _mainDomains = new List<string>()
{
"Geography",
"Mathematics",
"English"
};
I want to filter my LINQ query in order to return only data related to one or many main Domain:
expression = i => _mainDomains.Any(s => i.Domains.Contains(s));
var results = (from v_lq in context.my_view
select v_lq).Where(expression)
The problem is I can't use the Any key word, nor the Exists keyword, since they aren't available in SQL. I've seen many solutions using the Contains keyword, but it doesn't fit to my problem.
What should I do?
You can use contains:
where i.Domains.Any(s => _mainDomains.Contains<string>(s.xxx))
Notice that the generic arguments are required (even if Resharper might tell you they are not). They are required to select Enumerable.Contains, not List.Contains. The latter one is not translatable (which I consider an error in the L2S product design).
(I might not have gotten the query exactly right for your data model. Please just adapt it to your needs).
I figured it out. Since I can't use the Any keyword, I used this function:
public static bool ContainsAny(this string databaseString, List<string> stringList)
{
if (databaseString == null)
{
return false;
}
foreach (string s in stringList)
{
if (databaseString.Contains(s))
{
return true;
}
}
return false;
}
So then I can use this expression in my Where clause:
expression = i => i.Domains.ContainsAny(_mainDomains);
Update:
According to usr, the query would return all the values and execute the where clause server side. A better solution would be to use a different approach (and not use stuffed/comma-separated values)
Related
I sum myself to the hapless lot that fumbles with custom methods in LINQ to EF queries. I've skimmed the web trying to detect a pattern to what makes a custom method LINQ-friendly, and while every source says that the method must be translatable into a T-SQL query, the applications seem very diverse. So, I'll post my code here and hopefully a generous SO denizen can tell me what I'm doing wrong and why.
The Code
public IEnumerable<WordIndexModel> GetWordIndex(int transid)
{
return (from trindex in context.transIndexes
let trueWord = IsWord(trindex)
join trans in context.Transcripts on trindex.transLineUID equals trans.UID
group new { trindex, trans } by new { TrueWord = trueWord, trindex.transID } into grouped
orderby grouped.Key.word
where grouped.Key.transID == transid
select new WordIndexModel
{
Word = TrueWord,
Instances = grouped.Select(test => test.trans).Distinct()
});
}
public string IsWord(transIndex trindex)
{
Match m = Regex.Match(trindex.word, #"^[a-z]+(\w*[-]*)*",
RegexOptions.IgnoreCase);
return m.Value;
}
With the above code I access a table, transIndex that is essentially a word index of culled from various user documents. The problem is that not all entries are actually words. Nubers, and even underscore lines, such as, ___________,, are saved as well.
The Problem
I'd like to keep only the words that my custom method IsWord returns (at the present time I have not actually developed the parsing mechanism). But as the IsWord function shows it will return a string.
So, using let I introduce my custom method into the query and use it as a grouping parameter, the is selectable into my object. Upon execution I get the omninous:
LINQ to Entities does not recognize the method
'System.String IsWord(transIndex)' method, and this
method cannot be translated into a store expression."
I also need to make sure that only records that match the IsWord condition are returned.
Any ideas?
It is saying it does not understand your IsWord method in terms of how to translate it to SQL.
Frankly it does not do much anyway, why not replace it with
return (from trindex in context.transIndexes
let trueWord = trindex.word
join trans in context.Transcripts on trindex.transLineUID equals trans.UID
group new { trindex, trans } by new { TrueWord = trueWord, trindex.transID } into grouped
orderby grouped.Key.word
where grouped.Key.transID == transid
select new WordIndexModel
{
Word = TrueWord,
Instances = grouped.Select(test => test.trans).Distinct()
});
What methods can EF translate into SQL, i can't give you a list, but it can never translate a straight forward method you have written. But their are some built in ones that it understands, like MyArray.Contains(x) for example, it can turn this into something like
...
WHERE Field IN (ArrItem1,ArrItem2,ArrItem3)
If you want to write a linq compatible method then you need to create an expresion tree that EF can understand and turn into SQL.
This is where things star to bend my mind a little but this article may help http://blogs.msdn.com/b/csharpfaq/archive/2009/09/14/generating-dynamic-methods-with-expression-trees-in-visual-studio-2010.aspx.
If the percentage of bad records in return is not large, you could consider enumerate the result set first, and then apply the processing / filtering?
var query = (from trindex in context.transIndexes
...
select new WordIndexModel
{
Word,
Instances = grouped.Select(test => test.trans).Distinct()
});
var result = query.ToList().Where(word => IsTrueWord(word));
return result;
If the number of records is too high to enumerate, consider doing the check in a view or stored procedure. That will help with speed and keep the code clean.
But of course, using stored procedures has disadvatages of reusability and maintainbility (because of no refactoring tools).
Also, check out another answer which seems to be similar to this one: https://stackoverflow.com/a/10485624/3481183
I don't think is possible but wanted to ask to make sure. I am currently debugging some software someone else wrote and its a bit unfinished.
One part of the software is a search function which searches by different fields in the database and the person who wrote the software wrote a great big case statement with 21 cases in it 1 for each field the user may want to search by.
Is it possible to reduce this down using a case statement within the Linq or a variable I can set with a case statement before the Linq statement?
Example of 1 of the Linq queries: (Only the Where is changing in each query)
var list = (from data in dc.MemberDetails
where data.JoinDate.ToString() == searchField
select new
{
data.MemberID,
data.FirstName,
data.Surname,
data.Street,
data.City,
data.County,
data.Postcode,
data.MembershipCategory,
data.Paid,
data.ToPay
}
).ToList();
Update / Edit:
This is what comes before the case statement:
string searchField = txt1stSearchTerm.Text;
string searchColumn = cmbFirstColumn.Text;
switch (cmbFirstColumn.SelectedIndex + 1)
{
The cases are then done by the index of the combo box which holds the list of field names.
Given that where takes a predicate, you can pass any method or function which takes MemberDetail as a parameter and returns a boolean, then migrate the switch statement inside.
private bool IsMatch(MemberDetail detail)
{
// The comparison goes here.
}
var list = (from data in dc.MemberDetails
where data => this.IsMatch(data)
select new
{
data.MemberID,
data.FirstName,
data.Surname,
data.Street,
data.City,
data.County,
data.Postcode,
data.MembershipCategory,
data.Paid,
data.ToPay
}
).ToList();
Note that:
You may look for a more object-oriented way to do the comparison, rather than using a huge switch block.
An anonymous type with ten properties that you use in your select is kinda weird. Can't you return an instance of MemberDetail? Or an instance of its base class?
How are the different where statements handled, are they mutually excluside or do they all limit the query somehow?
Here is how you can have one or more filters for a same query and materialized after all filters have been applied.
var query = (from data in dc.MemberDetails
select ....);
if (!String.IsNullOrEmpty(searchField))
query = query.Where(pr => pr.JoinDate.ToString() == searchField);
if (!String.IsNullOrEmpty(otherField))
query = query.Where(....);
return query.ToList();
I have a method which takes an array of strings as parameter and queries against a collection property which is also a collection of strings. If that property has one of the values inside the string array passed as parameter, it should be returned.
Here is my code:
public IEnumerable<BlogPost> GetAll(string[] tags,
bool includeUnapprovedEntries = false) {
foreach (var tag in tags) {
foreach (var blogPost in GetAll(includeUnapprovedEntries).
ToList().Where(x => x.Tags.Any(t => t == tag))) {
yield return blogPost;
}
}
}
Note:
Here is the complete code:
https://github.com/tugberkugurlu/MvcBloggy/blob/master/src/MvcBloggy.Data/DataAccess/SqlServer/BlogPostRepository.cs
This does the job but it just doesn't seem right. I could have made this better with some extension methods but couldn't figure out what would do the trick and make this implementation right.
Any idea?
How about this:
public IEnumerable<BlogPost> GetAll(string[] tags,
bool includeUnapprovedEntries = false) {
return GetAll(includeUnapprovedEntries)
.Where(x => x.Tags.Any(t => tags.Contains(t));
}
You may want to call ToList() to materialize the result. Note that this will (hopefully!) result in an IN query in SQL; if you have a large number of tags, I wouldn't be surprised if that failed. (I don't know how the Entity Framework handles that situation.) I believe it should be okay with smaller numbers of tags though.
Note that whether or not this is supported may depend on the version of the entity framework you're using; I seem to remember that some transformations like this (using Contains on a "local" collection) to translate to IN in SQL) have improved over time. Make sure you develop against the same version you'll be deploying against :)
I have a simple issue with Entity Framework syntax for the "not in" SQL equivalent. Essentially, I want to convert the following SQL syntax into Entity Framework syntax:
select ID
from dbo.List
where ID not in (list of IDs)
Here is a method that I use for looking up a single record:
public static List GetLists(int id)
{
using (dbInstance db = new dbInstance())
{
return db.Lists.Where(m => m.ID == id);
}
}
Here is a pseudo-method that I want to use for this:
public static List<List> GetLists(List<int> listIDs)
{
using (dbInstance db = new dbInstance())
{
return db.Lists.Where(**** What Goes Here ****).ToList();
}
}
Can anyone give me pointers as to what goes in the Where clause area? I read some forums about this and saw mention of using .Contains() or .Any(), but none of the examples were a close enough fit.
Give this a go...
public static List<List> GetLists(List<int> listIDs)
{
using (dbInstance db = new dbInstance())
{
// Use this one to return List where IS NOT IN the provided listIDs
return db.Lists.Where(x => !listIDs.Contains(x.ID)).ToList();
// Or use this one to return List where IS IN the provided listIDs
return db.Lists.Where(x => listIDs.Contains(x.ID)).ToList();
}
}
These will turn into approximately the following database queries:
SELECT [Extent1].*
FROM [dbo].[List] AS [Extent1]
WHERE NOT ([Extent1].[ID] IN (<your,list,of,ids>))
or
SELECT [Extent1].*
FROM [dbo].[List] AS [Extent1]
WHERE [Extent1].[ID] IN (<your,list,of,ids>)
respectively.
This one requires you to think backwards a little bit. Instead of asking if the value is not in some list of ids, you have to ask of some list of id's does not contain the value. Like this
int[] list = new int[] {1,2,3}
Result = (from x in dbo.List where list.Contains(x.id) == false select x);
Try this for starters ...
m => !listIDs.Contains(m.ID)
This might be a way to do what you want:
// From the method you provided, with changes...
public static List GetLists(int[] ids) // Could be List<int> or other =)
{
using (dbInstance db = new dbInstance())
{
return db.Lists.Where(m => !ids.Contains(m.ID));
}
}
However I've found that doing so might raise error on some scenarios, specially when the list is too big and connection is somewhat slow.
Remember to check everything else BEFORE so this filter might have less values to check.
Also remember that Linq does not populate the variable when you build your filter/query (at least not by default). If you're going to iterate for each record, remember to call a ToList() or ToArray() method before, unless each record has 500MB or more...
I'm having some trouble figuring out the best way to do this, and I would appreciate any help.
Basically, I'm setting up a filter that allows the user to look at a history of audit items associated with an arbitrary "filter" of usernames.
The datasource is a SQL Server data base, so I'm taking the IQueryable "source" (either a direct table reference from the db context object, or perhaps an IQueryable that's resulted from additional queries), applying the WHERE filter, and then returning the resultant IQueryable object....but I'm a little stumped as to how to perform OR using this approach.
I've considered going the route of Expressions because I know how to OR those, but I haven't been able to figure out quite how to do that with a "Contains" type evaluation, so I'm currently using a UNION, but I'm afraid this might have negative impact on performance, and I'm wondering if it may not give me exactly what I need if other filters (in addition to user name filtering shown here) are added in an arbirary order.
Here is my sample code:
public override IQueryable<X> ApplyFilter<X>(IQueryable<X> source)
{
// Take allowed values...
List<string> searchStrings = new List<string>();
// <SNIP> (This just populates my list of search strings)
IQueryable<X> oReturn = null;
// Step through each iteration, and perform a 'LIKE %value%' query
string[] searchArray = searchStrings.ToArray();
for (int i = 0; i < searchArray.Length; i++)
{
string value = searchArray[i];
if (i == 0)
// For first step, perform direct WHERE
oReturn = source.Where(x => x.Username.Contains(value));
else
// For additional steps, perform UNION on WHERE
oReturn = oReturn.Union(source.Where(x => x.Username.Contains(value)));
}
return oReturn ?? source;
}
This feels like the wrong way to do things, but it does seem to work, so my question is first, is there a better way to do this? Also, is there a way to do a 'Contains' or 'Like' with Expressions?
(Editted to correct my code: In rolling back to working state in order to post it, I apparently didn't roll back quite far enough :) )
=============================================
ETA: Per the solution given, here is my new code (in case anyone reading this is interested):
public override IQueryable<X> ApplyFilter<X>(IQueryable<X> source)
{
List<string> searchStrings = new List<string>(AllowedValues);
// <SNIP> build collection of search values
string[] searchArray = searchStrings.ToArray();
Expression<Func<X, bool>> expression = PredicateBuilder.False<X>();
for (int i = 0; i < searchArray.Length; i++)
{
string value = searchArray[i];
expression = expression.Or(x => x.Username.Contains(value));
}
return source.Where(expression);
}
(One caveat I noticed: Following the PredicateBuilder's example, an empty collection of search strings will return false (false || value1 || ... ), whereas in my original version, I was assuming an empty list should just coallesce to the unfiltered source. As I thought about it more, the new version seems to make more sense for my needs, so I adopted that)
=============================================
You can use the PredicateBuilder from the LINQkit to dynamically construct your query.