I need to store user queries in a database. The queries will then run against an entity frameworks structure. So an example would be :
using(AdventureWorksDB aw = new
AdventureWorksDB(Settings.Default.AdventureWorks)) {
var newSalesPeople = from p in aw.SalesPeople
where p.HireDate > hireDate
orderby p.HireDate, p.FirstName
select new { Name = p.FirstName + " " + p.LastName,
HireDate = p.HireDate };
foreach(SalesPerson p in newSalesPeople) {
Console.WriteLine("{0}\t{1}", p.FirstName, p.LastName);
}
}
How could I convert the above, if it was stored as text, to become an entity framework query that gets run? Also is there another recommended way of storing the query eg xml
Thanks for reading
It appears ObjectContext.CreateQuery is the best solution. As it says it enables for queries to be added to a context at runtime - http://msdn.microsoft.com/en-us/library/bb339670(v=vs.110).aspx . Also, storing the queries as a string in a database would be perfectly adequate.
Related
This question already has an answer here:
Can I reuse code for selecting a custom DTO object for a child property with EF Core?
(1 answer)
Closed 1 year ago.
I have a rather theoretical issue with Entity Framework Core on SQLite.
I have an entity - Person { ID, FirstName, LastName, ... }, class PersonReference { ID, Representation : string }, extension method with argument of type Person that composes reference out of Person like this:
public static PersonReference ComposeReference(this Person from) => new PersonReference
{
ID = from.ID,
Representation = from.FirstName + " " + from.LastName
};
I need to compose references on the sql side. So I do the following:
var result = dbContext.People.Select(p => p.ComposeReference());
Result is IQueriable and program goes beyond that line and materializes the collection successfully. But when I look at the query I see it selects everything of Person and then query text ends.
If I rewrite EF expression to direct
var result = dbContext.People.Select(p => new PersonReference
{
ID = from.ID,
Representation = from.FirstName + " " + from.LastName
});
it gives me the satisfying expression with compact select and string concatenations.
Is there a way to keep the composition logic in extension method but still do calculations on the SQL side?
The trick is about using System.Linq.Expressions.Expression.
I met it at work and didn't get what it was for at first, but it is designed right for the purpose I required.
Declaration:
Expression<Func<Person, PersonReference>> ComposeReference => from => new
PersonReference
{
ID = from.ID,
Representation = from.FirstName + " " + from.LastName
};
Usage:
var result = dbContext.People.Select(ComposeReference);
Pay attention, that expressions can be compiled, but for this case never do it, or it will treat your DbSet as IEnumerable.
The answer from Svyatoslav's comment referred to some libraries, but I think vanilla EF does well enough on its own.
Hi I am using a Jqwidgets Grid to display my data. It has a build in possibility to use filters but if you filter your records on the server side you have to build your own query. As I am working with Linq I thought to use the Dynamic Linq Library for Asp net core. Problem is there are not many examples or explanations how to do this. But I am busy for days now and not getting very far.The way I am setup; I have a normal Linq query:
var Mut = from M in _DB.Mutations
join S in _DB.Shifts on M.ShiftId equals S.ShiftId
join U in _DB.RoosterUsers on M.UserId equals U.RoosterUserId
join D in deps on M.UserId equals D.UserId
join DD in _DB.Departements on D.DepartementID equals DD.DepartementId
select new MutationModel
{
MutId=M.MutationId,
Naam=U.FirstName + " " + U.LastName,
UserId=M.UserId,
Departement= DD.DepartementName,
MutationType = S.publicName,
MutationGroup = S.ShiftType.ToString(),
DateTot =M.DateTill,
TijdVan=M.DateStartOn,
TijdTot=M.DateTill,
Status=CreateStatus(M.Tentative, M.ApprovedOn, M.Processed, M.CancelRefId, M.Deleted)
};
This query is running OK and gives me all the data I need for the Grid.
Then for the filter I would like to add a dynamic Linq Query using the System.Linq.Dynamic.Core library
But this is as far as I get things working until now:
var outQuery = Mut.Where("Status = #0 and UserId = #1", "Nieuw", "KLM22940").Select("Status");
My questions now :
1. In the where clause If I make the fieldname variable I get an error. how to do this??
2. In the Select Clause, how to add multiple Columns? (actually I just like to output all columns.)
Best would be to see an example. has somebody used Dynamic Linq to build a dynamic linq query for the JQWidgets Grid?
Thank you very much.
In what way you are trying to use fieldname variable in where clause ?
If you want to output all columns you can use ToList()
like
var outQuery = Mut.Where("Status = #0 and UserId = #1", "Nieuw", "KLM22940").ToList();
If you want to get some specific columns you can use Select clause like this
var outQuery = Mut.Where("Status = #0 and UserId = #1", "Nieuw", "KLM22940").Select("new(Status,UserId )");
This Select clause creates data class which contains Status and UserId properties and returns a sequence of instances of that data class.
An old question for Linq 2 Entities. I'm just asking it again, in case someone has came up with the solution.
I want to perform query that does this:
UPDATE dbo.Products WHERE Category = 1 SET Category = 5
And I want to do it with Entity Framework 4.3.1.
This is just an example, I have a tons of records I just want 1 column to change value, nothing else. Loading to DbContext with Where(...).Select(...), changing all elements, and then saving with SaveChanges() does not work well for me.
Should I stick with ExecuteCommand and send direct query as it is written above (of course make it reusable) or is there another nice way to do it from Linq 2 Entities / Fluent.
Thanks!
What you are describing isnt actually possible with Entity Framework. You have a few options,
You can write it as a string and execute it via EF with .ExecuteSqlCommand (on the context)
You can use something like Entity Framework Extended (however from what ive seen this doesnt have great performance)
You can update an entity without first fetching it from db like below
using (var context = new DBContext())
{
context.YourEntitySet.Attach(yourExistingEntity);
// Update fields
context.SaveChanges();
}
If you have set-based operations, then SQL is better suited than EF.
So, yes - in this case you should stick with ExecuteCommand.
I don't know if this suits you but you can try creating a stored procedure that will perform the update and then add that procedure to your model as a function import. Then you can perform the update in a single database call:
using(var dc = new YourDataContext())
{
dc.UpdateProductsCategory(1, 5);
}
where UpdateProductsCategory would be the name of the imported stored procedure.
Yes, ExecuteCommand() is definitely the way to do it without fetching all the rows' data and letting ChangeTracker sort it out. Just to provide an example:
Will result in all rows being fetched and an update performed for each row changed:
using (YourDBContext yourDB = new YourDBContext()) {
yourDB.Products.Where(p => p.Category = 1).ToList().ForEach(p => p.Category = 5);
yourDB.SaveChanges();
}
Just a single update:
using (YourDBContext yourDB = new YourDBContext()) {
var sql = "UPDATE dbo.Products WHERE Category = #oldcategory SET Category = #newcategory";
var oldcp = new SqlParameter { ParameterName = "oldcategory", DbType = DbType.Int32, Value = 1 };
var newcp = new SqlParameter { ParameterName = "newcategory", DbType = DbType.Int32, Value = 5 };
yourDB.Database.ExecuteSqlCommand(sql, oldcp, newcp);
}
Is it possible to generate arbitrary where condtions SQL query through Jdbc template:
example:
If i pass value for 1 parameter (only name) : search by name
"select * from address where shopname = ?";
If i pass value for 2 parameter (name and city) - search by shopname and city:
"select * from address where shopname = ? and city = ?";
I have mupliple search fields. 7 fields. If user enters any combination. i have search only based on parameter. How to dynamically pass the parameters to the sql. Need snippet/Example how to achieve this.
What you want is some sort of criteria building api, which Hibernate has. Unfortunately, I don't think Spring's JdbcTemplate has any such facility. Others will correct me if I'm wrong...
Though as some guys already suggested that Hibernate is the best way of doing this, but still i think you can try this approach-
String sql = "select * from address where 1 = 1";
if(shopname != null)
sql += "and shopname = :shopname";
if(city!= null)
sql += "and city = :city";
and so on..and use NamedParameterJdbcTemplate
Spring Data and Hibernate have that kind of functionality. Though it might not be worth dragging in such big framework for your app.
You can try to check out SimpleJdbcInsert
http://docs.spring.io/spring/docs/current/spring-framework-reference/html/jdbc.html
Edit:
Alternatively you can try to fix it in SQL with checking on empty, but if you have lots of data to go through, this technique will slow down your request.
"select * from address
where (shopname = ? or shopname = null)
and (city = ? or city = null)";
If Scala is an option to you, the query could be constructed with something like this:
case class Search(shopname:String, city:String = None) {
def sql = "select * from address where shopname = '"+shopname+"'" + city.map(" and city = '"+
_ +"'").getOrElse("")
}
Example usage:
Search("lloh").sql
Search("lloh", Some("Austin")).sql
I have a combo box in Silverlight. It has a collection of values built out of the properties of one of my LINQ-to-SQL objects (ie Name, Address, Age, etc...). I would like to filter my results based off the value selected in a combo box.
Example: Say I want everyone with a last name "Smith". I'd select 'Last Name' from the drop down list and enter smith into a textbox control. Normally I would write a LINQ query similar to...
var query = from p in collection where p.LastName == textbox.Text select p;
Is it possible to decide the property dynamically, maybe using Reflection? Something like
var query = from p in collection where p.(DropDownValue) == textbox.Text select p;
Assuming:
public class Person
{
public string LastName { get; set; }
}
IQueryable<Person> collection;
your query:
var query =
from p in collection
where p.LastName == textBox.Text
select p;
means the same as:
var query = collection.Where(p => p.LastName == textBox.Text);
which the compiler translates from an extension method to:
var query = Queryable.Where(collection, p => p.LastName == textBox.Text);
The second parameter of Queryable.Where is an Expression<Func<Person, bool>>. The compiler understands the Expression<> type and generates code to build an expression tree representing the lambda:
using System.Linq.Expressions;
var query = Queryable.Where(
collection,
Expression.Lambda<Func<Person, bool>>(
Expression.Equal(
Expression.MakeMemberAccess(
Expression.Parameter(typeof(Person), "p"),
typeof(Person).GetProperty("LastName")),
Expression.MakeMemberAccess(
Expression.Constant(textBox),
typeof(TextBox).GetProperty("Text"))),
Expression.Parameter(typeof(Person), "p"));
That is what the query syntax means.
You are free to call these methods yourself. To change the compared property, replace this:
typeof(Person).GetProperty("LastName")
with:
typeof(Person).GetProperty(dropDown.SelectedValue);
Scott Guthrie has a short series on dyamically built LINQ to SQL queries:
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
That's the easy way...then there's another way that's a bit more involved:
http://www.albahari.com/nutshell/predicatebuilder.aspx
You can also use the library I created: http://tomasp.net/blog/dynamic-linq-queries.aspx. You would store the properties in ComboBox as lambda expressions and then just write:
var f = (Expression<Func<Product, string>>)comboBox.SelectedValue;
var query =
from p in collection
where f.Expand(textBox.Text)
select p;