Tag: Entity Framework

Entity Framework – Re-querying or fetching latest foreign key relationship records

In entity framework, you can often include foreign key or relationship collections in your LINQ request by using the ‘Include’ statement.

For example you may want to get a collection of Customers and their related Invoices. This would look something like:

var customersWithInvoices = DataContext.Customers.Include("Invoices")

However there may be times where this is not possible or you want to ensure that you have the latest related collection or you only want to fetch that collection when you need it.

To do this you need when the related records are a collection and re-query the navigation property for your parent entity (here ‘customer’) we use the Collection() method as follows:

try
{
DataContext.Entry(customer)
.Collection<Invoice>("Invoices")
.Query()
.ToList();
}
catch(Exception e)
{
Console.WriteLine(e);
}

Where ‘Invoices’ is the name of your navigation property. Now you can access the latest related objects.

e.g.

var countOfInvoices = customer.Invoices.Count();

To do this you need for a one-to-one related single entity then use the Reference() method instead as follows (Say we wanted the Country dial code of our customers Country):

DataContext.Entry(customer).Reference<Country>("CountryOfResidence").Query().FirstOrDefault();

Here the navigation property is called ‘CountryOfResidence”.

We can now navigate to the Country entity and get the dial code.

e.g.

var dialCode = customer.CountryOfResidence.DialCode;

Entity Framework – Setting integer keys client side gets ‘Cannot insert the value NULL’

I have just been banging my head against a wall trying to get a simple database record added to a simple table using Entity Framework! My table has a primary key field of type INT, but is NOT set as an identity field (i.e. Not set to increment SQL server side). I wanted the ability to set the Id in code.

An example would be a model like:

public Contact
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
}

then you would normally expect the standard EF code like this to work

var myNewContact = new Contact
{
    Id = 123,
    Name = "John Smith"
}
DataContext.Contacts.Add(myNewContact);
DataContext.SaveChanges();

However it always came back with the a DbUpdateException stating

“Cannot insert the value NULL into column ‘Id’, table ‘MyDatabase.dbo.Contacts’; column does not allow nulls. INSERT fails.”

It turns out that Entity Framework assumes that an integer [Key] field will always be generated server side. Maybe I should have known this, but actually found it very hard to locate this bit of information.

Once you know this the fix is very straight forward. You just need to decorate your model object with

[Key, DatabaseGenerated(DatabaseGeneratedOption.None)]

so the model above becomes:

public Contact
{
    [Key, DatabaseGenerated( DatabaseGeneratedOption.None )]
    public int Id { get; set; }
    public string Name { get; set; }
}

I hope this saves someone wasting as much time and loosing as much hair as I did on this simple issue!

One reason it took so long, is that I can’t get Entity Framework Profiler working with the Azure Emulator. If any has had any success with this, please let me know.