I have a simple controller method (via ajax) that inserts a new entity into the db and then queries the database for all of the same type of records and returns a list of those records(json).
What I can't figure out is that when I insert the first record and query for my list of records, any child entities are not lazy loaded. However, when I add a second or any number of subsequent records my list includes all child entities lazy loaded as expected. Here is some code:
public class Person
{
public int Id {get;set;}
public string Name {get;set;}
public int StateId {get;set;}
public virtual State State {get;set;}
}
public class PersonController : Controller
{
public ActionResult CreatePerson(PersonModel model)
{
var person = model.ToPersonEntity();
_personService.InsertPerson(person);
var people = _personService.GetAllPeople();//on first person inserted this list does NOT have State loaded
var personAddresssList = people.Select(x => x.ToPersonAddressFormat());
return Json(personAddressList, JsonRequestBehavior.AllowGet);
}
}
There isn't really any more relevant code as this is a pretty simple operation. I fixed the problem by using .Include(x => x.State) in my linq query but I've never had to do this as long as my properties were marked 'Virtual'.
The only thing I can think of is that EF still has my original person as a tracked entity and that when I pull up the list of persons and the only person is the one I just inserted, it uses the cached entity which would not have any child properties attached to it yet. If this is true then when I load a list of more than one person, some funky black magic in EF says "I see 2 items in the list I won't use the cached person I just inserted".
Any ideas?
Related
I'm new at code first in entity framework and reading up on relationships, I see everyone does it differently. It might be because of earlier versions, might be the same or might be because of performance.
Let's say I have two tables Company and User.
I would set the company-to-user relationship like this:
public List<User> Users { get; set; } = new List<User>();
Then if I from the user-to-company perspective needed to find the company, I would have this in the User:
public Company Company { get; set; }
And do this query:
return await _clientContext.Users.Where(x => x.Company.Id == companyId).ToListAsync();
Or I could have this:
public int CompanyId { get; set; }
[ForeignKey("CompanyId")]
public Company Company { get; set; }
And have this query:
return await _clientContext.Users.Where(x => x.CompanyId == companyId).ToListAsync();
Also some define Company with the keyword virtual like this:
public virtual Company Company { get; set; }
I'm not sure if every scenario is the same and doing x.CompanyId instead of x.Company.Id would actually be the same. What is used normally?
I generally recommend the first option using Shadow Properties for FKs over the second when using navigation properties. The main reason is that with the second approach there are two sources of truth. For instance with a Team referencing a Coach, some code may use team.CoachId while other code uses team.Coach.CoachId. These two values are not guaranteed to always be in sync. (depending on when you happen to check them when one or the other is updated.)
Updating references between entities via a FK property can have varied behaviour depending on whether the referenced entity is loaded or not.
What is the expected difference between if want to update a team's coach:
var teamA = context.Teams.Single(x => x.TeamId == teamId);
If Team has a Coach navigation property and a CoachId FK reference I could do...
teamA.CoachId = newCoachId;
If TeamA's old coach ID was 1, and the newCoachId = 2, what do you think happens if I have code that lazy loads the coach before SaveChanges?
var coachName = teamA.Coach.Name;
You might expect that since the Coach hadn't been loaded yet it would load in Coach #2's name, but it loads Coach #1 because the change hasn't been committed even though teamA.CoachId == 2. If you check the Coach reference after SaveChanges you get Coach #2.
Depending on whether lazy loading is enabled or not you can get a bit strange behaviour by setting a FK property where navigation properties are nulled. Even when eager loading, changing a FK property will potentially trigger a new lazy load if that new entity isn't already tracked:
var teamA = context.Teams.Include(x => x.Coach).Single(x => x.TeamId == teamId);
teamA.CoachId == newCoachId;
var coachName = teamA.Coach.Name; // Still points to Coach #1's name as expected.
context.SaveChanges();
coachName = teamA.Coach.Name; // Triggers lazy load and return new coach's name.
Saving a FK against an entity that has eager loaded the reference does not automatically re-populate referenced entities. So for instance if you have lazy loading disabled, the same above code:
context.SaveChanges();
coachName = teamA.Coach.Name; // Potential NullReferenceException on teamA.Coach.
This will potentially trigger a null reference exception unless the new coach happens to be tracked by the DbContext prior to SaveChanges being called. If the DbContext is tracking the entity, the new reference will be swapped in on SaveChanges, otherwise it is nulled. (With lazy loading this is covered by the new lazy load call after it was nulled)
When working with navigation properties my default recommendation is to hide FK properties as Shadow Properties. (For EF6 this means using .Map(x => x.MapKey()). For relationships where I only care about the ID, I will expose the FK with no navigation property. So, one or the other. (Such as lookups or bounded contexts where I want raw speed.)
I will deviate sparingly from this for exposing FKs for relationships I may inspect by ID frequently, and treat it as read-only, but still have infrequent need of the navigation property. An example of this would be CreatedBy / CreatedByUserId. Many queries may inspect the CreatedByUserId for data filtering, while some projections may want the CreatedBy.Name etc. A record's CreatedBy doesn't change so I avoid potential pitfalls of the data getting out of sync.
Your second scenario is used normally.
i.e.
public int CompanyId { get; set; }
[ForeignKey("CompanyId")]
public Company Company { get; set; }
And have this query:
return await _clientContext.Users.Where(x => x.CompanyId == companyId).ToListAsync();
I have an entity A which holds a Queue<XYZ> B property. I want to persist it in a MySQL database. When migrating, the table is created correctly - each entity A has a table depicting Queue<XYZ> B. However, when I want to query the data from the database using:
A entityA = await _context.A.Include(entityA => entityA.B).FirstOrDefaultAsync((...));
Then I get such an error:
System.InvalidCastException: Unable to cast object of type 'System.Collections.Generic.Queue'1[API.Models.XYZ]' to type
'System.Collections.Generic.ICollection'1[API.Models.XYZ]'.
When I change the Queue<XYZ> B to List<XYZ> B then it works fine.
Is it impossible to automatically persist a queue to a database with Entity Framework?
Or do I have to do some extra work?
I have an entity A which holds a Queue B property. I want to
persist it in a MySQL database. When migrating, the table is created
correctly - each entity A has a table depicting Queue B. However,
when I want to query the data from the database using:
A entityA = await _context.A.Include(entityA => entityA.B).FirstOrDefaultAsync((...));
According to your description and the query statement, I have created a DeparmentViewModel with the Queue property, and reproduced the error when query the database using EF core.
public class DepartmentViewModel
{
[Key]
public int DepId { get; set; }
public string DepName { get; set; }
public Queue<EmployeeViewModel> EmployeeViewModels { get; set; }
}
To solve this issue, it seems that we could re-generate a Queue object based on the query result, like this:
DepartmentViewModel queryresult = _context.DepartmentViewModels
.Include(c=>c.EmployeeViewModels)
.Where(c => c.DepId==2) //
.Select(c=> new DepartmentViewModel()
{
DepId = c.DepId,
DepName = c.DepName,
EmployeeViewModels = new Queue<EmployeeViewModel>(c.EmployeeViewModels.ToList())
}).FirstOrDefault();
Besides, I also found that if using the Queue, when insert new data, it might cause the following error:
"System.InvalidOperationException: 'The type of navigation property 'EmployeeViewModels' on the entity type 'DepartmentViewModel' is 'Queue' which does not implement ICollection. Collection navigation properties must implement ICollection<> of the target type.'"
So, in my opinion, when you create Navigation Properties, try to use List<T> or ICollection<T>, instead of using Queue<T>.
So with entity framework I'm trying to update two existing entities.
There I've the main object something like:
public class MainObject
{
public string Name { get; set; }
public virtual SmallObject Part { get; set;}
}
public class SmallObject
{
public string Name { get; set; }
}
In the repository I first check if the SmallObject already exists in the database by:
MainObject.Part = (from s in repoSmallObject.GetAll()
where s.name == MainObject.Part.Name
select s).FirstOrDefault();
Then finally I call the update method in my GenericRepository
repoMainObject.Update(MainObject)
which is defined as a generic repository method:
dbSet.Attach(entity)
context.Entry(entity).State = EntityState.Modified;
context.SaveChanges();
But the relationship doesn't get updated. Why is that? Both objects are attached to context not?
*Edit: The two repo's are injected with the same Context.
And strangely enough the Add method works and also updates the relationship.
When you set
context.Entry(entity).State = EntityState.Modified;
you need at least to set the state after and before updates (i.e. context.Entry(mainObject).CurrentValues and OriginalValues) so EF can build the right UPDATE query (with right WHERE clause).
It works if you set
context.Entry(entity).State = EntityState.Added;
because EF needs just to generate an INSERT query.
I don't know exactly why you need it but usually I prefer to attach the object to the DbSet and modify the properties so EF handles various states.
dbSet.Attach(MainObject)
MainObject.Part = (from s in repoSmallObject.GetAll()
where s.name == MainObject.Part.Name
select s).FirstOrDefault();
(In your case does not work because MainObject.Part.Name does not change)
The attached object should have the same values of the database otherwise you have a concurrency exception.
BTW, why you don't read the old object (MainObject) from the DB than work on it???
I'm using EF5 code first.
We have a method
LogHistoryTracking(DbEntityEntry entity)
to log changes when SaveChanges is called.
At SaveChanges, we get the changed entities and pass into LogHistoryTracking
var changedEntities = ChangeTracker.Entries().ToList();
But when I access
changedEntity.OriginalValues.PropertyNames
there is no properties for foreign keys object (only foreign key Id - but how can we get the data when there is only id here?).
I also tried to google for a solution, but this issue might be not so popular.
There is this article, but it does not work.
Appreciate any help. Thanks.
If you want to have your entity properties to be accessible you must 'Include' them prior to accessing them. Like in the following example which gets the orders of the first cutomer :
var orders = context.Customers
.Include("Orders")
.First().Orders;
In this example if you do not call .Include("Orders") you will not have Customer.Orders. The same goes if you have foreign key and forget to include the navigation property of the foreign key. This is because the key (the ID) is part of the object and the navigation property is not.
Let us see one real world example :
public class Employee : Entity
{
public virtual int CompanyUserId { get; set; }
public virtual CompanyUser CompanyUser { get; set; }
//... cut out for brevity
}
If you get the employees like this :
var employees = context.Employees;
You will not be able to access employees[0].CompanyUser after
context.SaveChanges() because of lazy loading. The connection is disposed after context.SaveChanges(), so no more data fetching.
But if you call :
var employees = context.Employees
.Include("CompanyUser")
.ToArray();
You will be able to access employees[0].CompanyUser.SomeProperty right away before context.SaveChanges regardless lazy loading because ToArray() will execute the query and fetch the entities with the "includes".
If you call :
var employees = context.Employees
.Include("CompanyUser");
Then you will have employee[0].CompanyUser.SomeProperty even after context.SaveChanges() with Lazy Loading because you have told EF to include "CompanyUser" property before executing the query. On execution EF will include the named property.
UPDATE
Intercepting DbContext can be done in at least two different ways.
First - override SaveChanges() or SaveChangesAsync because it is virtual:
public class MyDbContext : DbContext
{
public event Action<MyDbContext> SavingChanges = _ => { };
public override int SaveChanges()
{
this.SavingChanges(this);
return base.SaveChanges();
}
}
Second way without direct override is by hiding the DbContext inside interface like this one (this is from real project) :
public interface IUnitOfWork : IDisposable
{
void Commit();
}
Third way (somewhat different) is by intercepting the Db calls.
Fourth way exists but it depends on what IoC you use. If you use Castle Windsor you can use interceptors. I suppose that with every IoC there is its own way of intercepting this.
I have a one to many relationship as outlined below. In some parts of the business layer there are queries of the Item table and in others the Client table (as well as its Items). LazyLoading and ProxyCreation are both false, reference loop handling is set to ignore.
public class Client {
public virtual ICollection<Item> Items { get; set; }
public string Name {get;set;}
}
public class Item {
public virtual Client TheClient {get;set;}
public string ItemProp {get;set;}
// another 10 properties or so
}
myitems = dbContextScopeX.Items.Include(x => x.Client).ToList();
The view has a list of items with the need to show the Client's Name (in my example). I am looking for item.Client.Name ultimate, however when myitems gets queries/serialized it contains:
myitems.Client.Items
If I set the attribute [JsonIgnore] on the Client's Item property it never comes through the graph which I need it to in other places. Is there a way to get myItems.Client.Name without having to get myitems.Client.Items in the query or without having to create an anonymous projection for the Item array?
Project the Item properties you want (be they simple or complex type) along with just the Client name into an anonymous type and serialize that.
myitems = dbContextScopeX.Items.Include(x => x.Client)
.Select(i=>new {
ItemProp = i.ItemProp,
ItemCollection = i.ItemCollection,
...
ClientName = i.Client.Name
}).ToList();
Only caveat is you have to do some manual work if you want to deserialize this back into entities.