I have a need to replace multiple strings with others in a query
from p in dx.Table
where p.Field.Replace("A", "a").Replace("B", "b").ToLower() = SomeVar
select p
Which provides a nice single SQL statement with the relevant REPLACE() sql commands.
All good :)
I need to do this in a few queries around the application... So i'm looking for some help in this regard; that will work as above as a single SQL hit/command on the server
It seems from looking around i can't use RegEx as there is no SQL eq
Being a LINQ newbie is there a nice way for me to do this?
eg is it possible to get it as a IQueryable "var result" say and pass that to a function to add needed .Replace()'s and pass back? Can i get a quick example of how if so?
EDIT: This seems to work! does it look like it would be a problem?
var data = from p in dx.Videos select p;
data = AddReplacements(data, checkMediaItem);
theitem = data.FirstOrDefault();
...
public IQueryable<Video> AddReplacements(IQueryable<Video> DataSet, string checkMediaItem)
{
return DataSet.Where(p =>
p.Title.Replace(" ", "-").Replace("&", "-").Replace("?", "-") == checkMediaItem);
}
Wouldn't it be more performant to reverse what you are trying to do here, ie reformat the string you are checking against rather than reformatting the data in the database?
public IQueryable<Video> AddReplacements(IQueryable<Video> DataSet, string checkMediaItem)
{
var str = checkMediaItem.Replace("-", "?").Replace("&", "-").Replace("-", " "));
return DataSet.Where(p => p.Title == str);
}
Thus you are now comparing the field in the database with a set value, rather than scanning the table and transforming the data in each row and comparing that.
Related
Let's say I have a table that I can query with EF + LINQ like this:
var results = dbContext.MyTable.Where(q => q.Flag = true);
Then, I know that if I want to limit the columns returned, I can just add a select in that line like this:
var results = dbContext.MyTable
.Select(model => new { model.column2, model.column4, model.column9 })
.Where(q => q.Flag == true);
The next step that I need to figure out, is how to select those columns dynamically. Said another way, I need to be able to select columns in a table withoutknowing what they are at compile time. So, for example, I need to be able to do something like this:
public IEnumerable<object> GetWhateverColumnsYouWant(List<string> columns = new List<string{ "column3", "column4", "column999"})
{
// automagical stuff goes here.
}
It is important to keep the returned record values strongly typed, meaning the values can't just be dumped into a list of strings. Is this something that can be accomplished with reflection? Or would generics fit this better? Honestly, I'm not sure where to start with this.
Thanks for any help.
I think you want some dynamic linq, Im no expert on this but i THINK it will go something like this
public static IEnumerable<object> GetWhateverColumnsYouWant(this IQueriable<T> query, List<string> columns = new List<string{ "column3", "column4", "column999"})
{
return query.Select("new (" + String.Join(", ", columns) + ")");
}
See scott Gu's blog here http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx and this question System.LINQ.Dynamic: Select(" new (...)") into a List<T> (or any other enumerable collection of <T>)
You could probably also do this by dynamically composing an expression tree of the columns youre wanting to select but this would be substantially more code to write.
I am trying to select some records using LINQ for Entities (EF4 Code First).
I have a table called Monitoring with a field called AnimalType which has values such as
"Lion,Tiger,Goat"
"Snake,Lion,Horse"
"Rattlesnake"
"Mountain Lion"
I want to pass in some values in a string array (animalValues) and have the rows returned from the Monitorings table where one or more values in the field AnimalType match the one or more values from the animalValues. The following code ALMOST works as I wanted but I've discovered a major flaw with the approach I've taken.
public IQueryable<Monitoring> GetMonitoringList(string[] animalValues)
{
var result = from m in db.Monitorings
where animalValues.Any(c => m.AnimalType.Contains(c))
select m;
return result;
}
To explain the problem, if I pass in animalValues = { "Lion", "Tiger" } I find that three rows are selected due to the fact that the 4th record "Mountain Lion" contains the word "Lion" which it regards as a match.
This isn't what I wanted to happen. I need "Lion" to only match "Lion" and not "Mountain Lion".
Another example is if I pass in "Snake" I get rows which include "Rattlesnake". I'm hoping somebody has a better bit of LINQ code that will allow for matches that match the exact comma delimited value and not just a part of it as in "Snake" matching "Rattlesnake".
This is a kind of hack that will do the work:
public IQueryable<Monitoring> GetMonitoringList(string[] animalValues)
{
var values = animalValues.Select(x => "," + x + ",");
var result = from m in db.Monitorings
where values.Any(c => ("," + m.AnimalType + ",").Contains(c))
select m;
return result;
}
This way, you will have
",Lion,Tiger,Goat,"
",Snake,Lion,Horse,"
",Rattlesnake,"
",Mountain Lion,"
And check for ",Lion," and "Mountain Lion" won't match.
It's dirty, I know.
Because the data in your field is comma delimited you really need to break those entries up individually. Since SQL doesn't really support a way to split strings, the option that I've come up with is to execute two queries.
The first query uses the code you started with to at least get you in the ballpark and minimize the amount of data you're retrieving. It converts it to a List<> to actually execute the query and bring the results into memory which will allow access to more extension methods like Split().
The second query uses the subset of data in memory and joins it with your database table to then pull out the exact matches:
public IQueryable<Monitoring> GetMonitoringList(string[] animalValues)
{
// execute a query that is greedy in its matches, but at least
// it's still only a subset of data. The ToList()
// brings the data into memory, so to speak
var subsetData = (from m in db.Monitorings
where animalValues.Any(c => m.AnimalType.Contains(c))
select m).ToList();
// given that subset of data in the List<>, join it against the DB again
// and get the exact matches this time
var result = from data in subsetData
join m in db.Monitorings on data.ID equals m.ID
where data.AnimalType.Split(',').Intersect(animalValues).Any ()
select m;
return result;
}
I'm programming a search for a SQLite-database using C# and LINQ.
The idea of the search is, that you can provide one or more keywords, any of which must be contained in any of several column-entries for that row to be added to the results.
The implementation consists of several linq-queries which are all put together by union. More keywords and columns that have to be considered result in a more complicated query that way. This can lead to SQL-code, which is to long for the SQLite-parser.
Here is some sample code to illustrate:
IQueryable<Reference> query = null;
if (searchAuthor)
foreach (string w in words)
{
string word = w;
var result = from r in _dbConnection.GetTable<Reference>()
where r.ReferenceAuthor.Any(a => a.Person.LastName.Contains(word) || a.Person.FirstName.Contains(word))
orderby r.Title
select r;
query = query == null ? result : query.Union(result);
}
if (searchTitle)
foreach (string word in words)
{
var result = from r in _dbConnection.GetTable<Reference>()
where r.Title.Contains(word)
orderby r.Title
select r;
query = query == null ? result : query.Union(result);
}
//...
Is there a way to structure the query in a way that results in more compact SQL?
I tried to force the creation of smaller SQL-statments by calling GetEnumerator() on the query after every loop. But apparently Union() doesn't operate on data, but on the underlying LINQ/SQL statement, so I was generating to long statements regardless.
The only solution I can think of right now, is to really gather the data after every "sub-query" and doing a union on the actual data and not in the statement. Any ideas?
For something like that, you might want to use a PredicateBuilder, as shown in the chosen answer to this question.
I've been following with great interest the converstaion here:
Construct Query with Linq rather than SQL strings
with regards to constructing expression trees where even the table name is dynamic.
Toward that end, I've created a Extension method, addWhere, that looks like:
static public IQueryable<TResult> addWhere<TResult>(this IQueryable<TResult> query, string columnName, string value)
{
var providerType = query.Provider.GetType();
// Find the specific type parameter (the T in IQueryable<T>)
var iqueryableT = providerType.FindInterfaces((ty, obj) => ty.IsGenericType && ty.GetGenericTypeDefinition() == typeof(IQueryable<>), null).FirstOrDefault();
var tableType = iqueryableT.GetGenericArguments()[0];
var tableName = tableType.Name;
var tableParam = Expression.Parameter(tableType, tableName);
var columnExpression = Expression.Equal(
Expression.Property(tableParam, columnName),
Expression.Constant(value));
var predicate = Expression.Lambda(columnExpression, tableParam);
var function = (Func<TResult, Boolean>)predicate.Compile();
var whereRes = query.Where(function);
var newquery = whereRes.AsQueryable();
return newquery;
}
[thanks to Timwi for the basis of that code]
Which functionally, works.
I can call:
query = query.addWhere("CurUnitType", "ML 15521.1");
and it's functionally equivalent to :
query = query.Where(l => l.CurUnitType.Equals("ML 15521.1"));
ie, the rows returned are the same.
However, I started watching the sql log, and I noticed with the line:
query = query.Where(l => l.CurUnitType.Equals("ML 15521.1"));
The Query generated is:
SELECT (A bunch of columns)
FROM [dbo].[ObjCurLocView] AS [t0]
WHERE [t0].[CurUnitType] = #p0
whereas when I use the line
query = query.addWhere("CurUnitType", "ML 15521.1");
The query generated is :
SELECT (the same bunch of columns)
FROM [dbo].[ObjCurLocView] AS [t0]
So, the comparison is now happening on the client side, instead of being added to the sql.
Obviously, this isn't so hot.
To be honest, I mostly cut-and-pasted the addWhere code from Timwi's (slightly different) example, so some of it is over my head. I'm wondering if there's any adjustment I can make to this code, so the expression is converted into the SQL statement, instead of being determined client-side
Thanks for taking the time to read through this, I welcome any comments, solutions, links, etc, that could help me with this. And of course if I find the solution through other means, I'll post the answer here.
Cheers.
The big problem is that you're converting the expression tree into a delegate. Look at the signature of Queryable.Where - it's expressed in expression trees, not delegates. So you're actually calling Enumerable.Where instead. That's why you need to call AsQueryable afterwards - but that doesn't do enough magic here. It doesn't really put it back into "just expression trees internally" land, because you've still got the delegate in there. It's now wrapped in an expression tree, but you've lost the details of what's going on inside.
I suspect what you want is this:
var predicate = Expression.Lambda<Func<TResult, Boolean>>
(columnExpression, tableParam);
return query.Where(predicate);
I readily admit that I haven't read the rest of your code, so there may be other things going on... but that's the core bit. You want a strongly typed expression tree (hence the call to the generic form of Expression.Lambda) which you can then pass into Queryable.Where. Give it a shot :)
Maybe a simple question, I'm trying to get a result from a table where the Name column contains all of an array of search terms. I'm creating a query and looping through my search strings, each time assigning the query = query.Where(...);. It appears that only the last term is being used, I supposed because I am attempting to restrict the same field each time. If I call .ToArray().AsQueryable() with each iteration I can get the cumlative restrinction behavior I'm looking for, but it there an easy way to do this using defered operators only?
Thanks!
If you're doing something like:
foreach (int foo in myFooArray)
{
query = query.where(x => x.foo == foo);
}
...then it will only use the last one since each where criteria will contain a reference to the 'foo' loop variable.
If this is what you're doing, change it to:
foreach (int foo in myFooArray)
{
int localFoo = foo;
query = query.where(x => x.foo == localFoo);
}
...and everything should be fine again.
If this is not what is happening, please provide a code sample of what you're doing...