How ASP.net webform can be developed by retrieving data from two or more then two tables while using LINQ to SQL. can any body give some code sample for that ----- Thanks!
This is a simple snippet of an application that retrieves data from two tables in LINQ to SQL. If you want something more specific, please ask a more specific question.
DataContext db = new DataContext();
var users = db.Users.OrderByDescending(u => u.LastLogin).Take(5);
var messages = db.Messags.OrderByDescending(m => m.PublishDate).Take(5);
Response.Write("<b>Last 5 users logged in:</b><br/>");
foreach(User u in users) {
Response.Write(u.Username + "<br/>");
}
Response.Write("<b>Last 5 messages:</b><br/>");
foreac(Message m in messages) {
Response.Write(m.Title + "<br/>");
}
Related
I have created an application using Serenity framework. I have completed basic functionality for CRUD using serenity. Based on my tables I need to have graphical representations, any charts like high charts, D3 charts or any .
1. How can I get data from the tables using Linq in serenity
Finally I have found the answer for this. We can use sql queries as well as stored procedure to fetch data from DB. I have used stored procedure and linq to get the data from db.
In repository page you can add Linq,
public ListResponse<MyRow> GetUsers(IDbConnection connection)
{
// This must be te Repository
var myRepos = new UserRepository();
// This must be a type of Request (in this case ListRequest)
var request = new ListRequest();
request.Take = 100;
// This must be a type of Response (in this case ListResponse)
var response = new ListResponse<MyRow>();
// This must be called on Repository
var result = myRepos.List(connection, request);
// Data
var data = result.Entities.Where(r => r.Name != "").ToList();
response.Entities = data;
return response;
}
I have already defined MyRow as UserRow like this using MyRow = Entities.UserRow;.
Hope this will help you.
I am just starting with linq and entity framework in general and I have a question that may seem naive to all of the advanced users!
I have the following code :
var allDocuments = (from i in companyData.IssuedDocuments select i.IssuedDocumentId).ToList<int>();
var deletedDocuments = allDocuments.Except(updatedDocuments);
and I need to delete all the entities in companyData that their id is stored in deletedDocuments in a disconnected scenario.
Could you please show me a way to do this in an efficient manner?
You could avoid fetching all the ids by specifying you only want deleted ids like this:
var deletedIds = from i in companyData.IssuedDocuments
where !updatedIds.Contains(i.IssuedDocumentId)
select i.IssuedDocumentId
Now if companyData.IssuedDocuments is a DbSet you can tell EF to delete them like this:
foreach (var id in deletedIds)
{
var entity = new MyEntity { Id = id };
companyData.IssuedDocuments.Attach(entity);
companyData.IssuedDocuments.Remove(entity);
}
dbContext.SaveChanges();
This will issue multiple DELETE statements to the database without fetching the full entities into memory.
If companyData.IssuedDocuments is your repository then you could load the full entities instead of just the ids:
var deleted = from i in companyData.IssuedDocuments
where !updatedIds.Contains(i.IssuedDocumentId)
select i
foreach (var entity in deleted)
companyData.IssuedDocuments.Delete(entity);
dbContext.SaveChanges();
Again EF issues multiple DELETE statements to the database
If you can upgrade then EF6 has introduced a RemoveRange method on the DbSet that at you could look at. It may send a single DELETE statement to the database - I haven't tried it yet.
If performance is still an issue then you have to execute sql.
References:
RemoveRange
Deleting an object without retrieving it
How should I remove all elements in a DbSet?
companyData.RemoveAll(x=>deletedDocuments.Contains(x.Id));
I suppose the companyData is a IEnumerable type. The type T contains an Id property, which is the Id of the data. Then deletedDocuments contains the ids of all the documents that we want to remove.
One thing that's important and I should note it here is that the deletion of the documents happens in memory and it doesn't execute it in a db. Otherwise you should provide us with the version of entity framework you use and how you access you implelemnt your CRUD operations against your db.
Firstly I would like to thank you all for your suggestions.
I followed Christos Paisios suggestion but I was getting all kinds of exceptions when I was trying to persist the changes to the DB and the way that I finally managed to solve the issues was by adding the following override in my DbContext class
public override int SaveChanges()
{
var orphanedResponses = ChangeTracker.Entries().Where(
e => (e.State == EntityState.Modified || e.State == EntityState.Added) &&
e.Entity is IssuedDocument &&
e.Reference("CompanyData").CurrentValue == null);
foreach (var orphanedResponse in orphanedResponses)
{
IssuedDocuments.Remove(orphanedResponse.Entity as IssuedDocument);
}
return base.SaveChanges();
}
Hi I'm trying to do a basic update based on an id using Linq and the entity framework. I'm very new to this but I do not see the problem.
My entity class object is declared at the controller level.
gwwbnEntities db = new gwwbnEntities();
The Method grabs a querystring id and updates the user's registration status who is represented by that id.
public ActionResult ConfirmedAccount(int id)
{
var q = from u in db.user_registration
where u.id == id && u.reg_status == null
select u;
if (q.Any())
{
foreach(var item in q){
user_registration user = item;
user.reg_status = 202;
db.Entry(user).State = EntityState.Modified;
db.SaveChanges();
}
return View();
}
else
{
return RedirectToAction("RegistrationError");
}
}
Any help would be greatly appreciated! Again everything works and populates correctly, but the context object.SaveChanges() method fails everytime.
Thanks guys!
The exception you are seeing is because you have an open data reader (foreach) and you are trying to create transaction (EF does it for you) in SaveChanges(). Call SaveChanges outside the loop.
In addtion: Don't set the state to Modified - EF will detect that properties changed and will automatically set the state accordingly. You may want to do .ToList() on the q before doing anything. At the moment you are sending to queries to the database (one for .Any() and one to get entities). If you do .ToList() you will send only one query that brings entities but .Any() would be called on the list not on the database so it will be much faster and there is no trip to the database. Also ToList() force query evaluation so your foreach loop will not keep the data reader open as it will iterate on the list.
I am working with code first approach in EDM and facing an error for which I can't the solution.Pls help me
LINQ to Entities does not recognize the method 'Boolean
CheckMeetingSettings(Int64, Int64)' method, and this method cannot be
translated into a store expression.
My code is following(this is the query which I have written
from per in obj.tempPersonConferenceDbSet
where per.Conference.Id == 2
select new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327,per.Person.Id)
}
public bool CheckMeetingSettings(int,int)
{
///code I have written.
}
Please help me out of this.
EF can not convert custom code to SQL. Try iterating the result set and assigning the property outside the LINQ query.
var people = (from per in obj.tempPersonConferenceDbSet
where per.Conference.Id == 2
order by /**/
select new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
}).Skip(/*records count to skip*/)
.Take(/*records count to retrieve*/)
.ToList();
people.ForEach(p => p.CanSendMeetingRequest = CheckMeetingSettings(6327, p.Id));
With Entity Framework, you cannot mix code that runs on the database server with code that runs inside the application. The only way you could write a query like this, is if you defined a function inside SQL Server to implement the code that you've written.
More information on how to expose that function to LINQ to Entities can be found here.
Alternatively, you would have to call CheckMeetingSettings outside the initial query, as Eranga demonstrated.
Try:
var personDetails = obj.tempPersonConferenceDbSet.Where(p=>p.ConferenceId == 2).AsEnumerable().Select(p=> new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327,per.Person.Id)
});
public bool CheckMeetingSettings(int,int)
{
///code I have written.
}
You must use AsEnumerable() so you can preform CheckMeetingSettings.
Linq to Entities can't translate your custom code into a SQL query.
You might consider first selecting only the database columns, then add a .ToList() to force the query to resolve. After you have those results you van do another select where you add the information from your CheckMeetingSettings method.
I'm more comfortable with the fluid syntax so I've used that in the following example.
var query = obj.tempPersonConferenceDbSet
.Where(per => per.Conference.Id == 2).Select(per => new { Id = per.Person.Id, JobTitle = per.Person.JobTitle })
.ToList()
.Select(per => new PersonDetails { Id = per.Id,
JobTitle = per.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327, per.Person.Id) })
If your CheckMeetingSettings method also accesses the database you might want to consider not using a seperate method to prevent a SELECT N+1 scenario and try to express the logic as part of the query in terms that the database can understand.
I have the following code -
public void LoadAllContacts()
{
var db = new ContextDB();
var contacts = db.LocalContacts.ToList();
grdItems.DataSource = contacts.OrderBy(x => x.Areas.OrderBy(y => y.Name));
grdItems.DataBind();
}
I'm trying to sort the list of the contacts according to the area name that is contained within each contact. When I tried the above, I get "At least one object must implement IComparable.". Is there an easy way instead of writing a custom IComparer?
Thanks!
try this:
public void LoadAllContacts()
{
var db = new ContextDB();
var contacts = db.LocalContacts.ToList();
grdItems.DataSource = contacts.OrderBy(x => x.Areas.OrderBy(y => y.Name).First().Name);
grdItems.DataBind();
}
this will order the contacts by the first area name, after ordering the areas by name.
Hope this helps :)
Edit: fixed error in code. (.First().Name)
I was in a discussion with #AbdouMoumen but in the end I thought I'd provide my own answer :-)
His answer works, but there two performance issues in this code (both in the answer as in the original question).
First, the code loads ALL contacts in the db. This may or may not be a problem, but in general I would recommend NOT to do this. Many modern controls support paging/filtering out of the box, so you'd be better off supplying an not-yet-evaluated IQueryable<T> instead of List<T>. If however you need everything in memory, you should delay the ToList to the last possible moment.
Second, in AbdouMoumen's answer, there is a so-called 'SELECT N+1' problem. Entity Framework will by default use lazy loading to fetch additional properties. I.e. the Areas property will not be fetched from the database until it's accessed. In this case this will happen in the controls 'for loop', while it's ordering the result set by name.
Open up SQL Server Profiler to see what I mean: you will see a SELECT statement for all the contacts, and an additional SELECT statement for each contact that fetches the Areas for that contact.
A much better solution would be the following:
public void LoadAllContacts()
{
using (var db = new ContextDB())
{
// note: no ToList() yet, just defining the query
var contactsQuery = db.LocalContacts
.OrderBy(x => x.Areas
.OrderBy(y => y.Name)
.First().Name);
// fetch all the contacts, correctly ordered in the DB
grdItems.DataSource = contactsQuery.ToList();
grdItems.DataBind();
}
}
Is it one to one relation (Contact->Area)?
if yeah then try the following :
public partial class Contact
{
public string AreaName
{
get
{
if (this.Area != null)
return this.Area.Name;
return string.Empty;
}
}
}
then
grdItems.DataSource = contacts.OrderBy(x => x.AreaName);