I need to update all rows of my table when user visits one specific page.
All fields need to be updated to "false".
I tried this code:
var history = db.UserHistory.Where(m => m.UserID == id);
TryUpdateModel(history);
history.IsActive = false;
db.SaveChanges();
But it throws me an error message.
Anyone could help me with this update?
I couldn't get at first that LINQ didn`t update many rows at once, I have to make a loop and update each one. My final code is this one:
var history = db.UserHistory.Where(m => m.UserID == id).ToList();
TryUpdateModel(history);
history.ForEach(m => m.IsActive = false);
db.SaveChanges();
Since it seems like you're just selecting a single entry you want to probably want to append .FirstOrDefault() to your first line, which should appear with intellisense.
var history = db.UserHistory.Where(m => m.UserID == id).FirstOrDefault();
TryUpdateModel(history);
history.IsActive = false;
db.SaveChanges();
Right now you're returning an IQueryable as opposed to a single entry. FirstOrDefault will return a single UserHistory entity, so at least you'd be passing a single item to TryUpdateModel.
Related
I am working on a web service. I am using linq to query a database. Seemingly simple, but I've run into an issue. Here is my code for reference:
List<Comment> res = new List<Comment>();
using (ApplicationHistoryEntities ahe = new ApplicationHistoryEntities())
{
res = (from columns in ahe.Comments
where columns.NetforumId == actionuniqueid
select columns).ToList();
}
If I have no entries in the database, will my .ToList() throw an error? I could deploy it, and just try it out but I want to know more about the mechanism that my linq is using. If ahe.Comments database has no rows... what will the (from...) section return?
I could just add a null reference check, use dynamics etc but I want to really understand it.
I found this Q: how to know if my linq query returns null but it seems like all of the answers are in conflict on how it really should be done...
example answers:
Either you can convert it to list and then check the count
Best approach is to check there is null(no items) in list use Any() instead of count()
LINQ queries should never return null and you should not get an exception if the result is empty. You probably have an error in your code.
You can realise the result as a list then check the items.
You can see why I question how it works.
Edit:
Final code looks like this:
List<Comment> res;
using (ApplicationHistoryEntities ahe = new ApplicationHistoryEntities())
{
res = ahe.Comments?.Where(rowItem => rowItem.NetforumId == actionuniqueid).ToList() ??
new List<Comment>().ToList();
}
Look at this example:
List<string> test = new List<string>();
var test1 = test.Where(x => x == "a").ToList();
If test exists but is empty the query returns an empty list. If test is null the query throws an error. So you can adapt the query as follows
List<string> test = new List<string>();
test = null;
var test1 = test?.Where(x => x == "a") ?? new List<string>().ToList();
The query is now 'safe'. Both of the above examples return an empty list i.e. test1.Count() will return zero but will be usable.
You can also look at the definitions of Where and ToList
I am attempting to use the following code to do an update on a table with entity framework. The where statement will only work if I remove the and, either side of the and will bring results, but the and results in a null. I know that the value I am searching for exists.
foreach (fdd element in FddList)
{
var slist = context.ResidenceFDDs.ToList<ResidenceFDD>();
ResidenceFDD fddtoupdate = slist
.Where(s =>
s.StName.Contains("Adrienne") &&
s.StNum == element.addressnumb.ToString())
.FirstOrDefault<ResidenceFDD>();
fddtoupdate.Comments = "Comment newly added.";
context.SaveChanges();
}
if you are using .toString() then use as below
ResidenceFDD fddtoupdate = slist.Where(
s =>
s.StName.Contains("Adrienne") &&
s.StNum == element.addressnumb.ToString())
.FirstOrDefault<ResidenceFDD>().AsEnumerable();
I'm trying to add a column to the following LINQ expression. I want the column to contain a string concatenation of a text value in a many table called WasteItems. The join would be on "Waste.WasteId = WasteItem.WasteId". My problem is I need to display in a single dynamic column a string such as "EW (5); EX (3)" if there was 8 records in WasteItem and the column containing the 2 character string was called WasteItem.EWC. Hope that makes sense, there must be an efficient way since I realise LINQ is very powerfull. I'm new to it and not sure how to start or go about this:
return from waste in this._db.Wastes
where (from u in _db.UsersToSites.Where(p => p.UserId == userId && p.SystemTypeId == SystemType.W)
select u.SiteId)
.Contains(waste.SiteId)
orderby waste.Entered descending select waste;
THANKS IN ADVANCE
Something like this should do:
wastes.GroupJoin(db.WasteItems, w => w.WastId, wi => wi.WasteId, (w,wi) => new { w, wi })
.AsEnumerable()
.Select(x => new
{
x.w.Name,
Items = string.Join(", ", x.wi.GroupBy(wi => wi.EWC).Select(g => string.Format("{0} ({1})", g.Key, g.Count())))
})
Where wastes is the result from your query. The AsEnumerable() is necessary because Entity Framework can not handle string.Join, so that part must be dealt with in memory.
I could not check the syntax, obviously, but at least it may show you the way to go.
private string FindTaxItemLocation(string taxItemDescription)
{
if (!templateDS.Tables.Contains(cityStateTaxesTable.TableName))
throw new Exception("The schema dos not include city state employee/employer taxes table");
var cityStateTaxes =
templateDS.Tables[cityStateTaxesTable.TableName].AsEnumerable().FirstOrDefault(
x => x.Field<string>(Fields.Description.Name) == taxItemDescription);//[x.Field<string>(Fields.SteStateCodeKey.Name)]);
if (cityStateTaxes != null)
return cityStateTaxes[Fields.SteStateCodeKey.Name].ToString();
return null;
}
cityStateTaxes is a DataRow, why/how I cannot get the column value inside FirstOrDefault()?
Thanks,
FirstOrDefault() selects the first item in the collection (optionally that satisfies a predicate) or returns null in the case there it is empty (or nothing satisfies the predicate). It will not do projections for you. So if you use it, it can be awkward to access a field of the item since you must include default value checks.
My suggestion is to always project to your desired field(s) first before using FirstOrDefault(), that way you get your field straight without needing to perform the check.
var cityStateTaxes = templateDS.Tables[cityStateTaxesTable.TableName]
.AsEnumerable()
.Where(row => row.Field<string>(Fields.Description.Name) == taxItemDescription) // filter the rows
.Select(row => row.Field<string>(Fields.SteStateCodeKey.Name)) // project to your field
.FirstOrDefault(); // you now have your property (or the default value)
return cityStateTaxes;
I want to do something like DELETE FROM TABLE WHERE ID NOT IN (1,2,3) AND PAGEID = 9
I have a List of IDS but that could be changed if needs be. I can't work out how to get a boolean result for the LINQ parser.
Here is what Subsonic expects I think.
db.Delete(content => content.PageID == ID).Execute();
I can't work out how to do the NOT IN statement. I've tried the List.Contains method but something not quite right.
UPDATE: One alternative is to do:
var items = TABLE.Find(x => x.PageID == ID)'
foreach(var item in items)
{
item.Delete();
}
This hits the database a lot more though
When you say "something not quite right" what exactly do you mean?
I'd expect to write:
List<int> excluded = new List<int> { 1, 2, 3 };
db.Delete(content => !excluded.Contains(content.PageID)).Execute();
Note that you need to call Contains on the array of excluded values, not on your candidate. In other words, instead of saying "item not in collection" you're saying "collection doesn't contain item."
Try .Contains:
db.Delete(content => content.PageID.Contains(<Array containing ID's>).Execute();
(the above is just an example, might need some polishing for your specific situation)
I have found that this works but its not via LINQ
var table = new WebPageContentTable(_db.DataProvider);
var g = new SubSonic.Query.Delete<WebPageContent(_db.DataProvider)
.From(table)
.Where(table.ID)
.NotIn(usedID)
.Execute();
I have found that this does work and via LINQ - however it hits the database multiple times.
var f = WebPageContent.Find(x => !usedID.Any(e => e == x.ID));
if (f.Count > 0)
{
var repo = WebPageContent.GetRepo();
repo.Delete(f);
}
This I imagine would work in one hit to the database but I get an exception thrown in QueryVisitor::VisitUnary
WebPageContent.Delete(x => !usedID.Any(e => e == x.ID));