I have the following columns in my table
Id (int)
Name (nvarchar) usually < 100 characters
Data (nvarchar) average 1MB
I'm writing a program that will go through each row and perform some operations to the Name field. Since I'm only using the Name field and the Data field is very large, is it possible to direct EF to only load the Id and Name field?
Sure is
ctx.YourDbSet.Select(p=> new { Id = p.Id, Name = p.Name});
this method is selecting into an anonymous class.
if you want to save this back again you can do this with something which I call a dummy entity:
foreach(var thing in ctx.YourDbSet.Select(p=> new { Id = p.Id, Name = p.Name}))
{
var dummy = new YourEntity{Id = thing.Id};
ctx.YourDbSet.Attach(dummy);
dummy.Name = thing.Name + "_";
}
ctx.SaveChanges();
This method works with snapshot tracking as EF only tracks changes made after the attach call to send back in the update statement. This means your query will only contain an update for the name property on that entity (ie it wont touch data)
NOTE: you want to make sure you do this on a context you tightly control as you cant attach an object which is already attached to the EF tracking graph. In the above case the select will not attach entities to the graph as its anonymous (so you are safe using the same context)
Related
I have these two tables:
public class FiscalYear
{
... other fields
public int FiscalYears_Id { get; set; }
}
public class SkipHeader
{
... other fields
public int FiscalYears_Id { get; set; }
public virtual FiscalYear FiscalYear { get; set; }
}
Attempting to create a new SkipHeader like so:
var skipHeader = new SkipHeader()
{
... other fields get assigned to
FiscalYear = Session.FiscalYear,
}
Will cause the database to create a new FiscalYear record instead of using the Session.FiscalYear which is simply a static property that gets assigned to at program start. However, if I assign the FiscalYears_Id instead:
var skipHeader = new SkipHeader()
{
... other fields get assigned to
FiscalYears_Id = Session.FiscalYear.FiscalYears_Id,
}
The program uses the existing record as expected.
This bug eluded me and my colleague for months! Now that I found a solution, I would like to know WHY this is the case?
This bug eluded me and my colleague for months! Now that I found a
solution, I would like to know WHY this is the case?
This occurs because the DbContext doesn't know about your FiscalYear object instance, such as whether it represents a new record or an existing one.
Take the following example:
var fiscalYear = new FiscalYear { Id = 4, Name = "2019/20" };
var skipHeader = new SkipHeader { FiscalYear = fiscalYear };
context.SkipHeaders.Add(skipHeader);
context.SaveChanges();
fiscalYear in this instance is an object instance that has been given an ID and Name. When we associate it to a new SkipHeader and add the SkipHeader to the DbContext, EF will see this fiscalYear. Since it isn't an object tracked by the context, it treats it as a new entity like the SkipHeader.
Depending on how your entities are configured for dealing with the PK will determine what happens.
If your PK (Id) is set up as an Identity column (DB will populate) then the FiscalYear will be inserted and assigned the next available Id value. After the SaveChanges() call, fiscalYear.Id would be "6" or "22" or whatever the next new ID assigned to it would be. (Not "4")
If your PK is not an Identity column (App will populate) and a FiscalYear row already exists in the DB for ID 4, then EF will throw a duplicate key Exception on SaveChanges().
Where people get confused is that they assume that since the FiscalYear was at one point (Say during a web request) loaded from a DbContext, it is still somehow acting as a tracked entity when passed into another method outside of the scope of that DbContext. (During another update web request) It's not. When a web request for instance accepts a FinancialYear as a parameter from the client, it is deserializing a FinancialYear. As far as EF is concerned, that is absolutely no different than the new FinancialYear { } example above. The DbContext is not aware of that entity.
Take the following example:
FiscalYear fiscalYear = null;
using (var context = new AppDbContext())
{
fiscalYear = context.FiscalYears.Single(x => x.Id == 4);
}
using (var context = new AppDbContext())
{
var skipHeader = new SkipHeader { FiscalYear = fiscalYear };
context.SkipHeaders.Add(skipHeader);
context.SaveChanges();
}
This provides a basic outline of a Fiscal Year that was loaded by one instance of a DbContext, but then referenced by another instance of a DbContext. When SaveChanges is called, you will get a behaviour like you are getting now. This is what essentially happens in web requests, as when an entity is returned, the entity definition is merely a contract and the Entity is serialized to send to the client. When it comes back into another request, a new untracked object is deserialized.
As a general rule, Entities should not be passed outside the scope of the DbContext they were read from. EF does support this via detaching and re-attaching entities, but this is honestly more trouble than it is typically worth because you cannot 100% rely on just attaching an entity using DbContext.Attach() as often there can be conditional cases where another entity instance is already being tracked by a context and the Attach will fail. In these cases you'd need to replace references with the already tracked entity. (Messy conditional logic to catch possible scenarios) References are everything when dealing with EF. Two different object references with the same key & values are treated as separate and different objects by EF. Rather than passing references around, it's usually a lot simpler, and better to pass just the FK. This has the benefit of being a smaller payload for web requests.
One option you've found out is to update via the FK:
var skipHeader = new SkipHeader()
{
... other fields get assigned to
FiscalYears_Id = Session.FiscalYear.FiscalYears_Id,
}
This works, however when you have entities that are exposing both FK (FiscalYears_Id) and navigation property (FiscalYear) you can potentially find mismatch scenarios when updating records. This is something to be careful with as an application evolves.
For instance, take an example where you are editing an existing SkipHeader with a FiscalYears_Id of "4". This will have an associated FiscalYear reference available with a PK of "4".
Take the following code:
var skipHeader = context.SkipHeaders.Include(x => x.FiscalYear).Single(x => x.Id == skipHeaderId);
skipHeader.FiscalYears_Id = newFiscalYearId; // update FK from "4" to "6"
var fiscalYearId = skipHeader.FiscalYear.Id; // still returns "6"
context.SaveChanges();
We set the FK value on the skip header, however that does not update the reference for FiscalYear until after we call SaveChanges. This can be an important detail when dealing with FKs alongside navigation properties. Now normally we wouldn't bother going to the Navigation Property to get the ID again, but any code we call that is expecting the new FiscalYear reference to be updated will have a different behavior depending on whether SaveChanges had been called before or after the code in question. If before, all FiscalYear details will be for the old fiscal year even though we changed the FK reference.
This can also lead to odd Lazy Loading errors as well such as:
var skipHeader = context.SkipHeaders.Single(x => x.Id == skipHeaderId);
skipHeader.FiscalYears_Id = newFiscalYearId; // update FK from "4" to "6"
var fiscalYearId = skipHeader.FiscalYear.Id; // NullReferenceException!
context.SaveChanges();
Normally, provided you have lazy loading enabled loading a SkipHeader without eager loading the FiscalYear (.Include(x => x.FiscalYear))and then querying a property from the FiscalYear would lazy load this relative. However, if you change the SkipHeader's FiscalYear_ID FK and then try to access a property off the FiscalYear before calling SaveChanges(), you will get a NullReferenceException on the FiscalYear. EF will NOT lazy load either the old or new FiscalYear entity. Bugs in behaviour like that commonly creep in as applications get developed and code starts calling common functions that assume they are dealing with complete entities.
The alternative to setting updated values for known rows by FK is to load the entity to associate, and associate it by reference:
using (var context = new AppDbContext())
{
var fiscalYear = context.FiscalYears.Single(x => x.Id == fiscalYearId);
var skipHeader = new SkipHeader()
{
... other fields get assigned to
FiscalYear = fiscalYear;
}
context.SaveChanges();
}
This example just uses a locally scoped DbContext. If your method has an injected context then use that instead. The context will return any cached, known instance of the Fiscal Year or retrieve it from the DB. If the FiscalYear ID is invalid then that operation will throw an exception specific to the Fiscal Year not being found due to the Single() call rather than a more vague FK violation on SaveChanges(). (Not an issue when there is only one FK relationship, but in entities that have dozens of relationships...)
The advantage of this approach is that the FiscalYear will be in the scope of the DbContext so any methods/code using it will have a valid reference. The entities can define the navigation properties without exposing the extra FK values,using .Map(x => x.MapKey()) [EF6] or Shadow Properties [EFCore] instead to avoid 2 sources of truth for FK values.
This hopefully will provide some insight into what EF is doing and why it resulted in the behaviour you've seen and/or any errors or buggy behaviour you might have also come across.
Assuming you have pretty standard setup with DbContext being scoped (per request) dependency - the reason is that the new instance of your DbContext does not track the Session.FiscalYear instance - so it creates new. Another way to solve this is using DbContext.Attach:
context.Attach(Session.FiscalYear);
var skipHeader = new SkipHeader()
{
... other fields get assigned to
FiscalYears_Id = Session.FiscalYear.FiscalYears_Id,
}
// save skipHeader
More about change tracker in EF.
I am new to entity framework and I have been searching a while for an answer to this question and I can't find anything that directally addresses this.
Here is the problem. I have a table in Oracle. In this table there are 2 fields(there are more but not important to this question). Card_Transaction_Id and Parent_Card_Transaction_ID. The Parent_Card_Transaction_Id field is constrained by the Card_Transaction_Id field and I am using a Oracle sequence via a trigger to populate the Card_Transaction_Id field.
In my code, I am using Entity Framework(Version 5) to connect using the Code First Approach.
The issue is when I try to create a new record. I need to know what the next sequence value is in order to populate the Parent_Card_Transaction_Id. My mapping for card transactions:
public class CardTransactionMap : EntityTypeConfiguration<CardTransaction>
{
public CardTransactionMap(string schema)
{
ToTable("CARD_TRANSACTION", schema);
// Mappings & Properties
// Primary Key
HasKey(t => t.CardTransactionId);
Property(t => t.CardTransactionId)
.HasColumnName("CARD_TRANSACTION_ID")
.HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
Property(t => t.ParentCardTransactionId)
.HasColumnName("PARENT_CARD_TRANSACTION_ID");
Property(t => t.CardProfileId)
.HasColumnName("CARD_PROFILE_ID");
}
}
The question is - is there any way to get the next sequence number before I save the record?
My current work arround is to use the following method:
public static decimal GetNextCardTransactionSequenceValue()
{
using (var context = new Context(new OracleConnectionFactory().GetConnection()))
{
var sequence = context.Database.SqlQuery<int>("select card_transaction_id from card_transaction").ToList();
return sequence[0];
}
}
Using that method, I get the next value and then just populate my table. This works but I don't like doing it this way. I feel that there must be a better way to do it.
Thanks in advance.
You have to do this by navigation properties.
By fetching the next value from a sequence before actually using it in the same session you create yourself a concurrency issue: another user can increment the index (by an insert) in the time between drawing its next value and assigning it to the child record. Now the child will belong to the other user's record!
If your CardTransaction class has a parent reference like this:
int ParentCardTransaction { get; set; }
[ForeignKey("ParentCardTransaction")]
CardTransaction ParentCardTransaction { get; set; }
you can create a parent and child in one go and call SaveChanges without worrying about setting FK values yourself:
var parent = new CardTransaction { ... };
var child = new CardTransaction { ParentCardTransaction = parent, ... };
SaveChanges();
Now EF wil fetch the new CardTransactionId from the parent and assign it to the FK of the child. So generating and getting the parent Id happens all in one session, so it is guaranteed to be the same value.
Apart from preventing concurrency issues, of course it is much easier anyway to let EF do the heavy lifting of getting and assiging key values.
Create a Stored Procedure or Query that will return you the next Value from the Table here is an Example
SELECT NVL(MAX(card_transaction_id + 1), 0) AS MAX_VAL
FROM card_transaction T;
or Create a Trigger - for OracleDB
Change your table definition to this :
CREATE TABLE t1 (c1 NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
c2 VARCHAR2(10));
as per the information in the link i provided in the comment.
after the update ef will automatically query the value for the id that is inserted, there is no need to fill in the id before the insert. ef will generate an insert sql query without id.
What I want to achieve is to only UPDATE particular property of an entity in the Db.
(To mimic something like UPDATE Foo Set Status = 'kool' WHERE ID = 99).
I try this:
public void SetFooStatus(Foo foo)
{
var fooToUpdate = new Foo()
{
Id = foo.Id,
Status =foo.Status
};
this.Context.Foos.Attach(fooToUpdate);
this.Context.Entry(fooToUpdate).Property("Status").IsModified = true;
this.Context.ValidateOnSaveEnabled = false;
this.dbSet.Attach(entityToUpdate); // This is IDbSet<Foo>
this.dbSet.Entry(entityToUpdate).State = EntityState.Modified;
this.dbSet.SaveChanges();
}
Problem is when I set State to EntityState.Modified, all the properties are marked as Chagned -IsModified returns true-.
When I comment it out, then Update is not accomplished -no changes occur in the Db.
Question:
How can I enforce EF to UPDATE only the Status property of the foo object without touching other fields in the Db?
EF does not allow updating a single column. Instead of attaching an entity you may want to fetch it from the server which will set all the fields to the values you have on the server and therefore you won't change other properties. You probably want to use DbSet.Find() method which will try to find an entity by its id in the context and if one does not exist will fetch it from the database.
I have written test code as bellow:
Entities db = new Entities();
var place = new Place
{
Id = Guid.NewGuid(),
Name = "test",
Address = "address"
};
db.Places.Add(place);
var cachedPlace = db.Places.Where(x => x.Id == place.Id).FirstOrDefault(); \\ null
I expected dbset will return the added entity. But it gives me object only after changes were saved to the real DB.
If you want to access the unsaved query, then you use the Local property of the DbSet.
The reason it doesn't work the way you want is that it must also support autonumbered identities, and that will mean the ID is 0. If you insert multiple records, you would have multiple objects with the same 0 ID. EF won't know what the real ID is until after it's been saved.
Normally when you update an object in linq2sql you get the object from a datacontext and use the same datacontext to save the object, right?
What's the best way to update a object that hasn't been retreived by that datacontext that you use to perform the save operation, i.e. I'm using flourinefx to pass data between flex and asp.net and when object return from the client to be saved I don't know how to save the object?
public static void Save(Client client)
{
CompanyDataContext db = new CompanyDataContext();
Validate(client);
if(client.Id.Equals(Guid.Empty))
{
//Create (right?):
client.Id = Guid.NewGuid();
db.Clients.InsertOnSubmit(client);
db.SubmitChanges();
}
else
{
//Update:
OffertaDataContext db = new OffertaDataContext();
db.Clients.????
}
}
Update: different approaches to use Attach doens't work in this case. So I guess a reflection based approach is required.
To update an existing but disconnected object, you need to "attach" it do the data context. This will re-use the existing primary key etc. You can control how to handle changes- i.e. treat as dirty, or treat as clean and track future changes, etc.
The Attach method is on the table - i.e.
ctx.Customers.Attach(customer); // optional bool to treat as modified
I think you have 2 options here:
1) Attach the object to the DataContext on which you will do your save
2) Using the primary key on your object, grab an instance that is attached to your context (e.g. do a FirstOrDefault()), and then copy the data over from the modified object to the object that has a context (reflection might be useful here).
Rick Strahl has a very good blog article on attaching entities to a context at http://www.west-wind.com/weblog/posts/134095.aspx, particularly in regards to some of the problems you might encounter.
I am hoping you can help. I am developing a tiered website using Linq to Sql. I created a new class(or object) in DBML designer called memberState. This object is not an actual table in the database. I have this method in my middle layer:
public override IEnumerable(memberState) GetMembersByState(string #state)<br/>
{<br/>
using (BulletinWizardDataContext context = DataContext)<br/>
{<br/>
IEnumerable(memberState) mems = (from m in context.Members<br/>
join ma in context.MemberAddresses<br/>
on m.UserId equals ma.UserId<br/>
join s in context.States<br/>
on ma.StateId equals s.StateId<br/>
where s.StateName == #state<br/>
select new memberState<br/>
{<br/>
userId = m.UserID,<br/>
firstName = m.FirstName,<br/>
middleInitial = m.MiddleInitial,<br/>
lastName = m.LastName,<br/>
createDate = m.CreateDate,<br/>
modifyDate = m.ModifyDate<br/>
}).ToArray(memberState)();<br/>
return mems;
}
}
The tables in my joins (Members, States, and MemberAddresses are actual tables in my Database). I created the object memberStates so I could use it in the query above (notice the Select New memberState. When the data is updated on the web page how do I persist the changes back to the Member Table? My Member Table consists of the following columns: UserId, FirstName, MiddleInitial, LastName, CreateDate, ModifyDate. I am not sure how save the changes back to the database.
Thanks,