There's a many-to-many UserFeed table that stands between User and Feed and denotes a twitter-like follow relationship.
It only has two fields, which form a composite key: UserID and FeedID.
I need to write a method that will subscribe or unsubscribe a user from a feed based on a boolean flag.
public void SetSubscriptionFlag (int userId, int storeId, bool subscribe)
{
}
I'm new to Entity Framework so I'm trying to find and follow an "EF-ish" way to accomplish this.
My initial thoughts are:
Instead of working with the middle UserFeed class, I should create a many-to-many Subscriptions property (EDIT: hit limitations here);
After I've done so, I'll need to fetch a User instance by ID, check whether it has given Feed in its Subscriptions and add/delete it depending on the flag and current existence;
Figure out how to avoid racing conflicts when there is a time interval before the check and adding/deleting and user manages to submit two adding or deletion requests;
Optimize my code as to avoid unneccessary SELECTs, if any occur, because all I really want to do is a single SELECT and single INSERT/DELETE.
A relevant code snippet and comment on my points is highly appreciated.
Thanks!
You can use dummy objects - it definitely works for insert and I hope it can be used for delete as well:
Create new relation:
var user = new User() { Id = userId };
context.Users.Attach(user);
var store = new Store() { Id = storeId };
context.Stores.Attach(store);
// Now context trackes both entities as "existing"
// and you can build a new relation
user.Subscriptions.Add(store);
context.SaveChanges();
Remove existing relation:
var user = new User() { Id = userId };
var store = new Store() { Id = storeId };
user.Subscriptions.Add(store);
context.Users.Attach(user);
// Now context trackes both entities as "existing"
// with "existing" relation so you can try to remove it
user.Subscriptions.Remove(store);
context.SaveChanges();
i have following trouble with LINQ to SQL entities:
// Context is DataContext that was auto genereted when i create my .dbml file
var cl = Context.Classes.ToArray();
var rm = Context.Rooms.ToArray();
List<DaySchedule> s = new List<DaySchedule>();
s.Add(new DaySchedule()
{
Class = cl[0],
DayOfWeek = 0,
Pair = 1,
Room = rm[0]
});
Context.SubmitChanges();
so, after "SubmitChanges" new DaySchedules will be saved to db. BUT i didn't call InsertOnSubmit function and i don't want to save this DaySchedule.
BTW,
if i will using following code:
s.Add(new Acceron.University.DBAccess.DaySchedule()
{
Class_id = cl[0].Class_ID,
DayOfWeek = 0,
Pair = 1,
Room_id = rm[0].Room_ID
});
It will not be auto saved to db.
Could you explain is it bug or feature and how i can solve it?
It is by design. Class and Room are context-aware entities, since they were queried against the context. Anytime a context-aware entity adds children, it queues up those changes automatically to the context and marks it as inserted. So you cannot add new entities without the auto-queuing feature. I'd highly recommend not calling save changes later on.
I've a Entity Framework 4.0, with poco object. the edmx model file is generated from the database.
This datacontext is accessed through WCF service, it's only mean that I receive some objects and I need to attach them to the current datacontext(or reload them with the key correspondance).
Everything seems to work fine, except for one case:
I've a N-N relationship between two table, so I've an association table, without any field other than ID of two tables:
LINQ transform this into the following schema, this seems to be right.
When I retrieve data, there is no problem, data I've inserted myself in the Right_group are correctly transformed into "new object in my collection of Rights/Groups".
But if I try to modify something and save, it doesn't work
public void SaveRights(Group group, List<Rights> rights){
//here, group and rights are objects attached to the database
group.Rights.Clear();
group.Rights.AddRange(rights);
_dataContext.SaveChanges();
}
So my question is: How to save this "relationship" of two objects ?
Thank you!
If you want to avoid loading the objects from the database first you can do it like this(Code taken from one of my aplications so you will have to adapt it):
public void AddAndRemovePersons(int id, int[] toAdd, int[] toDelete)
{
var mailList = new MailList { ID = id, ContactInformations = new List<ContactInformation>() };
this.db.MailLists.Attach(mailList);
foreach (var item in toAdd)
{
var ci = new ContactInformation { ID = item };
this.db.ContactInformations.Attach(ci);
this.db.ObjectStateManager.ChangeRelationshipState(mailList, ci, ml => ml.ContactInformations, System.Data.EntityState.Added);
}
foreach (var item in toDelete)
{
var ci = new ContactInformation { ID = item };
this.db.ContactInformations.Attach(ci);
this.db.ObjectStateManager.ChangeRelationshipState(mailList, ci, ml => ml.ContactInformations, System.Data.EntityState.Deleted);
}
}
I found deleting the relationship as hard as creating it so I left that code in there. One thing about this solution is that both the maillist and the contacts exist prior to this function being run. I attach them to make the state manager track them.
If you are adding new objects that you also want to save you would use the
this.db.MailLists.AddObject(you new item here)
I hope that helps!
Just a thought... how are the keys setup in the Right_Group table? If you use both IDRight and IDGroup together as primary key - this problem might occur. One suggetion is to add a new column (ID) into the Right_Group table, and having this ID as the primary key. Then use foreign keys on the other columns (IDRight, IDGroup) respectivly.
Lets say I query the database and load a list of items. Then I open one of the items in a detail view form, and instead of re-querying the item out of the database, I create an instance of the item from the datasource in the list.
Is there a way I can update the database record without fetching the record of the individual item?
Here is a sample how I am doing it now:
dataItem itemToUpdate = (from t in dataEntity.items
where t.id == id
select t).FirstOrDefault();
Then after pulling the record I update some values in the item and push the record back:
itemToUpdate.itemstatus = newStatus;
dataEntity.SaveChanges();
I would think there would be a better way to do this, any ideas?
You should use the Attach() method.
Attaching and Detaching Objects
You can also use direct SQL against the database using the context of the datastore. Example:
dataEntity.ExecuteStoreCommand
("UPDATE items SET itemstatus = 'some status' WHERE id = 123 ");
For performance reasons, you may want to pass in variables instead of a single hard coded SQL string. This will allow SQL Server to cache the query and reuse with parameters. Example:
dataEntity.ExecuteStoreCommand
("UPDATE items SET itemstatus = 'some status' WHERE id = {0}", new object[] { 123 });
UPDATE - for EF 6.0
dataEntity.Database.ExecuteSqlCommand
("UPDATE items SET itemstatus = 'some status' WHERE id = {0}", new object[] { 123 });
The code:
ExampleEntity exampleEntity = dbcontext.ExampleEntities.Attach(new ExampleEntity { Id = 1 });
exampleEntity.ExampleProperty = "abc";
dbcontext.Entry<ExampleEntity>(exampleEntity).Property(ee => ee.ExampleProperty).IsModified = true;
dbcontext.Configuration.ValidateOnSaveEnabled = false;
dbcontext.SaveChanges();
The result TSQL:
exec sp_executesql N'UPDATE [dbo].[ExampleEntities]
SET [ExampleProperty ] = #0
WHERE ([Id] = #1)
',N'#0 nvarchar(32),#1 bigint',#0='abc',#1=1
Note:
The "IsModified = true" line, is needed because when you create the new ExampleEntity object (only with the Id property populated) all the other properties has their default values (0, null, etc). If you want to update the DB with a "default value", the change will not be detected by entity framework, and then DB will not be updated.
In example:
exampleEntity.ExampleProperty = null;
will not work without the line "IsModified = true", because the property ExampleProperty, is already null when you created the empty ExampleEntity object, you needs to say to EF that this column must be updated, and this is the purpose of this line.
If the DataItem has fields EF will pre-validate (like non-nullable fields), we'll have to disable that validation for this context:
DataItem itemToUpdate = new DataItem { Id = id, Itemstatus = newStatus };
dataEntity.Entry(itemToUpdate).Property(x => x.Itemstatus).IsModified = true;
dataEntity.Configuration.ValidateOnSaveEnabled = false;
dataEntity.SaveChanges();
//dataEntity.Configuration.ValidateOnSaveEnabled = true;
Otherwise we can try satisfy the pre-validation and still only update the single column:
DataItem itemToUpdate = new DataItem
{
Id = id,
Itemstatus = newStatus,
NonNullableColumn = "this value is disregarded - the db original will remain"
};
dataEntity.Entry(itemToUpdate).Property(x => x.Itemstatus).IsModified = true;
dataEntity.SaveChanges();
Assuming dataEntity is a System.Data.Entity.DbContext
You can verify the query generated by adding this to the DbContext:
/*dataEntity.*/Database.Log = m => System.Diagnostics.Debug.Write(m);
Now native support for this in EF Core 7 — ExecuteUpdate:
Finally! After a long wait, EF Core 7.0 now has a natively supported way to run UPDATE (and also DELETE) statements while also allowing you to use arbitrary LINQ queries (.Where(u => ...)), without having to first retrieve the relevant entities from the database: The new built-in method called ExecuteUpdate — see "What's new in EF Core 7.0?".
ExecuteUpdate is precisely meant for these kinds of scenarios, it can operate on any IQueryable instance, and lets you update specific columns on any number of rows, while always issuing a single UPDATE statement behind the scenes, making it as efficient as possible.
Usage:
Imagine you wanted to update the Email column of a specific user:
dbContext.Users
.Where(u => u.Id == someId)
.ExecuteUpdate(b =>
b.SetProperty(u => u.Email, "NewEmail#gmail.com")
);
As you can see, calling ExecuteUpdate requires you to make calls to the SetProperty method, to specify which property to update, and also what new value to assign to it.
EF Core will translate this into the following UPDATE statement:
UPDATE [u]
SET [u].[Email] = "NewEmail#gmail.com"
FROM [Users] AS [u]
WHERE [u].[Id] = someId
Also, ExecuteDelete for deleting rows:
There's also a counterpart to ExecuteUpdate called ExecuteDelete, which, as the name implies, can be used to delete a single or multiple rows at once without having to first fetch them.
Usage:
// Delete all users that haven't been active in 2022:
dbContext.Users
.Where(u => u.LastActiveAt.Year < 2022)
.ExecuteDelete();
Similar to ExecuteUpdate, ExecuteDelete will generate DELETE SQL statements behind the scenes — in this case, the following one:
DELETE FROM [u]
FROM [Users] AS [u]
WHERE DATEPART(year, [u].[LastActiveAt]) < 2022
Other notes:
Keep in mind that both ExecuteUpdate and ExecuteDelete are "terminating", meaning that the update/delete operation will take place as soon as you call the method. You're not supposed to call dbContext.SaveChanges() afterwards.
If you're curious about the SetProperty method, and you're confused as to why ExectueUpdate doesn't instead receive a member initialization expression (e.g. .ExecuteUpdate(new User { Email = "..." }), then refer to this comment (and the surrounding ones) on the GitHub issue for this feature.
Furthermore, if you're curious about the rationale behind the naming, and why the prefix Execute was picked (there were also other candidates), refer to this comment, and the preceding (rather long) conversation.
Both methods also have async equivalents, named ExecuteUpdateAsync, and ExecuteDeleteAsync respectively.
I recommend using Entity Framework Plus
Updating using Entity Framework Core can be very slow if you need to update hundreds or thousands of entities with the same expression. Entities are first loaded in the context before being updated which is very bad for the performance and then, they are updated one by one which makes the update operation even worse.
EF+ Batch Update updates multiple rows using an expression in a single database roundtrip and without loading entities in the context.
// using Z.EntityFramework.Plus; // Don't forget to include this.
// UPDATE all users inactive for 2 years
var date = DateTime.Now.AddYears(-2);
ctx.Users.Where(x => x.LastLoginDate < date)
.Update(x => new User() { IsSoftDeleted = 1 });
Simple and elegant extension method:
I've written an extension method for DbContext that does exactly what the OP asked for.
In addition to that, it only requires you to provide a member initialization expression (e.g. new User { ... }), and it then figures out on its own what properties you've changed, so you won't have to specify them by hand:
public static void UpdateEntity<TEntity>(
this DbContext context,
int id,
Expression<Func<TEntity>> updateExpression
) where TEntity : BaseEntity, new()
{
if (updateExpression.Body is not MemberInitExpression memberInitExpr)
throw new ArgumentException("The update expression should be a member initialization.");
TEntity entityToUpdate = updateExpression.Compile().Invoke();
entityToUpdate.Id = id;
context.Attach(entityToUpdate);
var updatedPropNames = memberInitExpr.Bindings.Select(b => b.Member.Name);
foreach (string propName in updatedPropNames)
context.Entry(entityToUpdate).Property(propName).IsModified = true;
}
You also need a BaseEntity class or interface that has your primary key in it, like:
public abstract class BaseEntity
{
public int Id { get; set; }
}
Usage:
Here's how you'd use the method:
dbContext.UpdateEntity(1234 /* <- this is the ID */, () => new User
{
Name = "New Name",
Email = "TheNewEmail#gmail.con",
});
dbContext.SaveChanges();
Nice and simple! :D
And here's the resulting SQL that gets generated by Entity Framework:
UPDATE [Users]
SET [Name] = #p0, [Email] = #p1
WHERE [Id] = #p2;
Limitation:
This method only allows you to update a single row using its primary key.
So, it doesn't work with .Where(...), IQueryable<...>, and so on. If you don't have the PK, or you want to bulk-update, then this wouldn't be your best option. In general, if you have more complex update operations, then I'd recommend you use Entity Framework Plus, or similar libraries.
It works somewhat different in EF Core:
There may be a faster way to do this in EF Core, but the following ensures an UPDATE without having to do a SELECT (tested with EF Core 2 and JET on the .NET Framework 4.6.2):
Ensure your model does not have IsRequired properties
Then use the following template (in VB.NET):
Using dbContext = new MyContext()
Dim bewegung = dbContext.MyTable.Attach(New MyTable())
bewegung.Entity.myKey = someKey
bewegung.Entity.myOtherField = "1"
dbContext.Entry(bewegung.Entity).State = EntityState.Modified
dbContext.Update(bewegung.Entity)
Dim BewegungenDescription = (From tp In dbContext.Model.GetEntityTypes() Where tp.ClrType.Name = "MyTable" Select tp).First()
For Each p In (From prop In BewegungenDescription.GetProperties() Select prop)
Dim pp = dbContext.Entry(bewegung.Entity).Property(p.Name)
pp.IsModified = False
Next
dbContext.Entry(bewegung.Entity).Property(Function(row) row.myOtherField).IsModified = True
dbContext.SaveChanges()
End Using
ef core 7 :
public async Task<int> Update(UpdateLevelVm vm)
{
return await _db.Levels.Where(l => l.Id == vm.LevelId)
.ExecuteUpdateAsync(u => u
.SetProperty(l => l.GradeId, vm.GradeId)
.SetProperty(l => l.Title, vm.Title)
);
}
this has worked for me in EF core 3.1
await _unitOfWork.Context.Database.ExecuteSqlRawAsync("UPDATE Student SET Age = 22 Where StudentId = 123");
Generally speaking, if you used Entity Framework to query all the items, and you saved the entity object, you can update the individual items in the entity object and call SaveChanges() when you are finished. For example:
var items = dataEntity.Include("items").items;
// For each one you want to change:
items.First(item => item.id == theIdYouWant).itemstatus = newStatus;
// After all changes:
dataEntity.SaveChanges();
The retrieval of the one item you want should not generate a new query.
with my Repository classes, I use LinqToSql to retrieve the data from the repository (eg. Sql Server 2008, in my example). I place the result data into a POCO object. Works great :)
Now, if my POCO object has a child property, (which is another POCO object or an IList), i'm trying to figure out a way to populate that data. I'm just not too sure how to do this.
Here's some sample code i have. Please note the last property I'm setting. It compiles, but it's not 'right'. It's not the POCO object instance .. and i'm not sure how to code that last line.
public IQueryable<GameFile> GetGameFiles(bool includeUserIdAccess)
{
return (from q in Database.Files
select new Core.GameFile
{
CheckedOn = q.CheckedOn.Value,
FileName = q.FileName,
GameFileId = q.FileId,
GameType = (Core.GameType)q.GameTypeId,
IsActive = q.IsActive,
LastFilePosition = q.LastFilePosition.Value,
UniqueName = q.UniqueName,
UpdatedOn = q.UpdatedOn.Value,
// Now any children....
// NOTE: I wish to create a POCO object
// that has an int UserId _and_ a string Name.
UserAccess = includeUserIdAccess ?
q.FileUserAccesses.Select(x => x.UserId).ToList() : null
});
}
Notes:
Database.Files => The File table.
Database.FilesUserAccess => the FilesUserAccess table .. which users have access to the GameFiles / Files table.
Update
I've now got a suggestion to extract the children results into their respective POCO classes, but this is what the Visual Studio Debugger is saying the class is :-
Why is it a System.Data.Linq.SqlClient.Implementation.ObjectMaterializer<..>
.Convert<Core.GameFile> and not a List<Core.GameFile> containing the POCO's?
Any suggestions what that is / what I've done wrong?
Update 2:
this is what i've done to extract the children data into their respective poco's..
// Now any children....
UserIdAccess = includeUserIdAccess ?
(from x in q.FileUserAccesses
select x.UserId).ToList() : null,
LogEntries = includeUserIdAccess ?
(from x in q.LogEntries
select new Core.LogEntry
{
ClientGuid = x.ClientGuid,
ClientIpAndPort = x.ClientIpAndPort,
// ... snip other properties
Violation = x.Violation
}).ToList() : null
I think that all you need to do is to put another Linq query in here:
q.FileUserAccesses.Select(x => x.UserId).ToList()
i.e. You want to select data from the FileUserAccess records - which I'm assuming are Linq to SQL classes, so to do this you can have something like:
(from fua in q.FileUserAccesses
select new PocoType
{
UserID = fua.UserID,
Name = fua.User.UserName // Not sure at this point where the name comes from
}).ToList()
That should get you pointed in the right direction at least.
What is the type of UserIdAccess? How is it not 'right'? Are you getting the 'wrong' data? if so have you checked your database directly to make sure the 'right' data is there?