Updating a many-to-many relation with Entity Framework - entity-framework

Question

I'm struggling a bit with a many-to-many relation update using Entity Framework. My tables looks like this:
CREATE TABLE Agreement
(
ID int NOT NULL IDENTITY(1,1),
CONSTRAINT [PK_Agreement] PRIMARY KEY CLUSTERED (ID),
);
CREATE TABLE Price
(
ID int NOT NULL IDENTITY(1,1),
ProductPrice decimal(18,5),
CONSTRAINT [PK_Price] PRIMARY KEY CLUSTERED (ID),
);
CREATE TABLE AgreementPriceLine
(
AgreementID int NOT NULL,
PriceID int NOT NULL,
CONSTRAINT [PK_AgreementPriceLine] PRIMARY KEY NONCLUSTERED (AgreementID, PriceID),
CONSTRAINT [FK_AgreementPriceLine_Agreement]
FOREIGN KEY (AgreementID) REFERENCES Agreement (ID)
ON DELETE NO ACTION ON UPDATE CASCADE,
CONSTRAINT [FK_AgreementPriceLine_PriceID]
FOREIGN KEY (PriceID) REFERENCES Price (ID)
);
Which Entity Framework mapped like so (using generate model from database, the .edmx file):
public partial class Agreement
{
public Agreement()
{
this.Prices = new HashSet<Price>();
}
public virtual ICollection<Price> Prices { get; set; }
}
public partial class Price
{
public Price()
{
this.Agreements = new HashSet<Agreement>();
}
public virtual ICollection<Agreement> Agreements { get; set; }
}
Now, if I want to update two prices on an agreement, how would I go about that? I tried the following:
public void UpdateAgreementPriceLines(List<Price> prices, Agreement agreement)
{
try
{
using (var ctx = new MyEntities())
{
if (agreement != null)
{
// Make sure the number of prices are equal before attempting to "update" anything
if(agreement.Prices.Count == prices.Count)
{
// Clear old prices
agreement.Prices.Clear();
// Add new prices
foreach (var price in prices)
{
agreement.Prices.Add(price);
}
ctx.SaveChanges();
}
}
}
}
catch (Exception e)
{
Logging.Instance.Fatal(e.ToString());
}
}
But it's rather 'hacky' to first empty the collection and then add the new prices. Also, I can't get it to work, the prices are not removed/added at all (and no exception is caught)
Any help/hint on this is greatly appreciated :-)
Thanks in advance.

Answer 1

You need to remove the items from the colletion, like this:
agreement.Prices.Remove(priceItem);
To remove all objects you can do the following:
var prices = agreement.Prices.ToList();
agreement.Prices.RemoveRange(prices);
If you don't want to load all items to the memory before the removal method, you can do the following:
//supposing that you want to deleted the Price which id = 10
Price deletedPrice = new Price { PriceId = 10 };
Context.Entry(deletedPrice).State = EntityState.Deleted;
Context.SaveChanges();