I saw this post that suggests an answer but my situation is a bit different.
// Create a new Lunch entity with these two properties.
Lunch lunchEntity = new LunchEntity();
lunchEntity.UserId = userId;
lunchEntity.MealId = mealId;
// Add the entity to the DbContext and save the changes.
restaurantEntities.Lunches.Add(lunchEntity);
restaurantEntities.SaveChanges();
// Get the Lunch entity that we inserted above.
Lunch mySavedLunchEntity = restaurantEntities.Lunches.Find(lunchEntity.Id);
Now, after inserting the Lunch entity, I need to have its instance with all of its navigation properties included. That's why I use the Find() method to select the newly created entity. The problem is that the User navigation property is null, while the Meal navigation property has a reference to the correct object.
Moreover, if I execute this statement
Lunch mySavedLunchEntity = restaurantEntities.Lunches.Find(lunchId);
separately in another method that is supposed to retrieve a Lunch entity for a particular Id, all navigation properties are included correctly.
So, my question is why all my navigation properties are included when I just query a given element, and some of them are not, if I query the element only after it has been inserted?
You could try:
Lunch mySavedLunchEntity = restaurantEntities.Lunches.Where(l => l.LunchId == lunchId).Include(l => l.User).Include(l => l.Meal)
This forces EF to load the two navigation properties instead of Lazy loading them.
Related
Is it possible in .NET 6 with Entity Framework Core 6 to populate the relationship navigation property by setting the foreign key value and then call SaveChanges?
I tried it but it doesn't seem to work. Although the other way around works perfectly (if I set the navigation property to the related entity).
Screenshots:
setting the foreign key
after save changes, "department" property still null
When trying this, student.department remains null after calling SaveChanges
var student = db.Students.Find(9);
student.departmentId = 1;
db.SaveChanges();
While if I do this, the foreign key student.departmentId gets populated after calling SaveChanges:
var student = db.Students.Find(9);
student.department = db.Departments.Find(1);
db.SaveChanges();
When trying this student.department remains null after savechanges
Setting the foreign key value doesn't load the related department. The use case for setting the foreign key directly is typically to avoid actually loading the related entity.
If you want to load the related entity, you might as well just query it and assign it to the navigation property.
After setting the foreign key property on an entity, you can load the related entity if you want to using explicit loading. eg
db.Entry(student).Reference(b => b.Department).Load();
SaveChanges will not automatically load the relationship data unless context is already tracking the corresponding entity (Change Tracking in EF Core). In addition to using one of the options to load the related data (for example the one suggested by #David Browne in his answer), following things will do the trick:
db.Departments.Find(1);
var student = db.Students.Find(9);
student.departmentId = 1;
db.SaveChanges(); // student.department will be filled here
Or even
var student = db.Students.Find(9);
student.departmentId = 1;
db.SaveChanges();
db.Departments.Find(1); // student.department will be filled here
I have a class Customer. I am trying to clone a Customer object and modify it, then I want those modifications to be reflected in the context (database as well). I am using following code to do that.
Customer old = context.Customers.Where(c=>c.CustomerID ==1 ).SingleOrDefault();
Customer m = CustomExtensions.ShallowCopyEntity<Customer>(old);
m.Name = "Modified";
m.MobileNo = "9999999999";
context.Customers.Attach(m);
But its throwing following exception
Attaching an entity of type 'DataBindingSample.Customer'
failed because another entity of the same type already has the same
primary key value. This can happen when using the 'Attach' method or
setting the state of an entity to 'Unchanged' or 'Modified' if any
entities in the graph have conflicting key values. This may be because
some entities are new and have not yet received database-generated key
values. In this case use the 'Add' method or the 'Added' entity state
to track the graph and then set the state of non-new entities to
'Unchanged' or 'Modified' as appropriate.
I tried changing EntityState to Modified but it didn't work.
Can anyone tell me how to achieve this?
My main goals are
I want to clone (I will use deep clone when necessary) an existing entity
Want to modify the cloned entity (as well as referenced entities - I will use deep clone in this case)
Finally I want to save changes to database
EDIT
As pointed out in this comment i am trying to attach object which aready exists in context. So i can detach it first and then atttach again as shown bellow if attach is compulsory.
Customer old = context.Customers.Where(c=>c.CustomerID ==1 ).SingleOrDefault();
Customer m = CustomExtensions.ShallowCopyEntity<Customer>(old);
m.Name = "Modified789789";
m.MobileNo = "9999999999";
((IObjectContextAdapter)context).ObjectContext.Detach(old);
context.Customers.Attach(m);
context.Entry(m).State = EntityState.Modified;
context.SaveChanges();
Otherwise i can follow 2 options mentioned in this answer.
There are 2 options that I can think of:
Copy the updated values back to the original entity loaded into your DbContext and then save changes.
Updated values of the original entity and then discard them if user canceled the update.
Options 1
Just copy the updated values back to the originally loaded entity. Automapper is your friend in tasks like this. This approach can later be extended to allow user to change a model of your entity and not the data layer object itself (e.g. to expose a limited number of fields that user can edit).
var entity = context.Customers.SingleOrDefault(c => c.CustomerID == 1);
var updatedEntity = CustomExtensions.ShallowCopyEntity<Customer>(old);
updatedEntity.Name = "Modified";
updatedEntity.MobileNo = "9999999999";
entity.Name = updatedEntity.Name;
entity.MobileNo = updatedEntity.MobileNo;
context.SaveChanges();
If you add Automapper nuget, then you mappings (copying) will become much easier:
Mapper.CreateMap<Customer, Customer>();
Mapper.Map(updatedEntity, entity);
And your code will look like:
// Configuring mapping. Needs to be done only once.
Mapper.CreateMap<Customer, Customer>();
var entity = context.Customers.SingleOrDefault(c => c.CustomerID == 1);
// Check if entity is null
var updatedEntity = CustomExtensions.ShallowCopyEntity<Customer>(old);
updatedEntity.Name = "Modified";
updatedEntity.MobileNo = "9999999999";
// Copy the updated values back
Mapper.Map(updatedEntity, entity);
context.SaveChanges();
Options 2
Make changes in the originally loaded entity and discard them if user changed her mind and canceled. See this post and this post on how to do it.
Discarding the whole DbContext might not be a good option in case you still need it (duh).
I have the following tables represented in my Entity Framework diagram (.edmx file)
Users
- UserID
- Username
- UserGroupID
Groups
- GroupID
- GroupName
In my code, I retrieve a valid instance of the User object and I'm trying to traverse the relationship to get to the Groups table, to retrieve the GroupName, however everytime, the Groups object is null. The UserGroupID exists in the Groups table, so i'm not sure why this is.
The Visual Studio intellisense knows the relationship exists and allows me to attempt it, but at runtime, the 'Groups' instance is null.
Users users= (Users)e.Row.DataItem;
string groupName = users.Groups.GroupName;
In that case, Groups is null and i'm not sure why. What are the possible causes?
Thanks
Kevin
Looks like you are trying to access Groups from within a databound event and the Groups data was not loaded before you binded it. You most likely need to "include" Groups like below.
var users = entity.Users.include("Groups").where(x => x.UserID == 20);
You might want to read this, Entity doesn't seem to act the same as Linq-To-SQL when it comes to Lazy Loading
http://www.singingeels.com/Articles/Entity_Framework_and_Lazy_Loading.aspx
How to load related items: http://msdn.microsoft.com/en-us/library/bb896272.aspx
probably the lazy loading is disabled.
add the below line when you initialize the context
context.ContextOptions.LazyLoadingEnabled = true;
anyway you should think about what strategy you want to use to pull date from your DB
Here is some code along with my assumptions based on playing around in LINQPad. Can anyone confirm this is how the lazy loading is working, and perhaps provide any additional insight/links so I can understand how it's working on the back end? Thanks in advance!
// Step 1.
var record = context.MyTable.First();
// Step 2.
var foreignKey = ForeignKeyTable.Where(x => x.Id == record.ForeignKeyId).Single();
// Step 3.
var entry = context.Entry(record);
// Step 4.
trace(entry.Reference(x => x.ForeignKey).IsLoaded);
// Step 5.
trace(record.ForeignKey.SomeProperty);
Retrieve some record (DB is queried).
Retrieve a record that happens to be a foreign key property of record without using lazy loading like record.ForeignKey to retrieve it (DB is queried).
Get the details of the record entity.
This is the part I'm unsure about. In my testing it outputs true. I'm guessing that IsLoaded doesn't know whether or not record.ForeignKey currently has a value, but knows that record.ForeignKey is already being tracked in the context based on it's knowledge of record.ForeignKeyId and the relationships that have been established.
The db doesn't seem to be hit here, and I assume it's for the same reason IsLoaded returns true in 4. It knows that it's tracking the foreignKey object already, so it knows it doesn't have to do the lazy loading.
Edit: The actual problem I'm trying to solve can be illustrated as such:
var record = context.MyTable.First();
var foreignKey = new ForeignKey() { Id = record.ForeignKeyId, SomeProperty = 5 };
context.ForeignKeyTable.Attach(foreignKey);
var entry = context.Entry(record);
// Returns false.
trace(entry.Reference(x => x.ForeignKey).IsLoaded);
// Doesn't query for ForeignKey, so it must know it's `loaded` somehow, and
// gets SomeProperty from my new foreignKey object. What???
trace(record.ForeignKey.SomeProperty);
EF fixes relationships (navigation properties) automatically according to primary key and foreign key values when you load an entity from the database or when you attach it to the context.
In both code snippets you have loaded record which has a foreign key to your ForeignKeyTable. The context knows this value. (It doesn't matter btw if you have exposed the foreign key in your model. It will always be loaded, also without having a FK property in your model. You can see this when watching the SQL query.)
In both cases you attach afterwards a ForeignKey entity to the context which has as primary key the value of record.ForeignKeyId which the context already knows about. As a consequence EF will set the navigation property record.ForeignKey to this attached ForeignKey entity.
Obviously IsLoaded doesn't tell you if the entity is attached to the context because in both examples it is attached but one returns true and the other false. It also doesn't tell you if record.ForeignKeyId refers to an entity, because this is also the case in both examples.
It tells you apparently only that the entity has really been loaded from the database (and not only manually attached) (which also Intellisense says about IsLoaded). That's the only difference between your first and second example.
And it seems that lazy loading is not only controlled by the IsLoaded flag. If you attach an entity for the navigation property to the context, lazy loading doesn't happen anymore although IsLoaded is false.
What would happen if your last line in the second code snippet would actually trigger lazy loading? The ForeignKey object being loaded must have the same key as the ForeignKey object you have already attached (because record has this value as FK property ForeignKeyId). But because no two objects with same key can be attached to the context it must be the same object. But then there is no need to load it since such an object is already in memory and attached.
// Step 1.
var record = context.MyTable.First();
// Step 2.
var foreignKey = ForeignKeyTable.Where(x => x.Id == record.ForeignKeyId).Single();
// Step 3.
var entry = context.Entry(record);
// Step 4.
trace(entry.Reference(x => x.ForeignKey).IsLoaded);
// Step 5.
trace(record.ForeignKey.SomeProperty);
Retrieve some record (DB is queried). yes, and the resulting record is attached to the DbContext.
Retrieve a record that happens to be a foreign key property of record without using lazy loading like record.ForeignKey to retrieve it (DB is queried). yes. If you had wanted to eager load the foreign key in #1, you would have used context.MyTable.Include(m => m.ForeignKey).First(); That would have retrieved the record along with the fk in 1 query.
Get the details of the record entity. Kind of... it is the details of the entity in relation to the DbContext (what is attached / deleted / loaded / etc)
This is the part I'm unsure about. In my testing it outputs true. I'm guessing that IsLoaded doesn't know whether or not record.ForeignKey currently has a value, but knows that record.ForeignKey is already being tracked in the context based on it's knowledge of record.ForeignKeyId and the relationships that have been established. This means that the DbContext does not need to run another query to load the data for the foreign key. If you execute record.ForeignKey, the data is already there, and no additional trip to the db is required.
The db doesn't seem to be hit here, and I assume it's for the same reason IsLoaded returns true in 4. It knows that it's tracking the foreignKey object already, so it knows it doesn't have to do the lazy loading. The entitiy has already been loaded in step #2, so there was no additional trip needed to get it from the db.
Update after question edit
According to EF, the .Attach method on IDbSet:
Attaches the given entity to the context underlying the set. That is, the entity is placed into the context in the Unchanged state, just as if it had been read from the database.
In Entity Framework, when I've mapped my tables to the corresponding entities through the designer and get to actually using them, I'll find that an entity - Thing, who has a relationship (many to one, or one to one) with another object, say, Bob, for example, would produce the following three properties on Thing:
Bob
BobId
BobReference
And were I to set BobId, and save my entity, the next time I fetch this Thing, I'll be able to navigate the Bob property without trouble. I'm curious, however, if it is possible to configure EF to allow me to navigate the property without having to immediately save.
You can do something like this: (EF 4.1)
//Has to exists a record on Bob table with Id = 1
var thing = new Thing() { BobId = 1 };
var context = new YouContext();
context.Entry(thing ).State = EntityState.Unchanged;
context.Entry(thing ).Reference(x => x.Bob).Load();
and then thing.Bob is != null