Entity Framework 4 : Delete child from ParentChild Relationship – throws or what?

[topic from a discussion; addressing a colleague]

Imagine the very common situation where you have a parent-child relationship like the following, and you want to delete the child (Currency; or better just remove the relation by setting the parent’s child navigation property to null):

Having code (show in an intergration test) like this

[TestMethod]

public void RemoveChildEntityNavigationFromParentEntityProperty_WithManyToOneConstraint_Throws()

{

    // save parent and child with a 1:* relationship constraint

    int dbGeneratedCityId = -1;

    Currency chf = new Currency { Symbol = "CHF", Name = "Schweizer Franken" };

    City city = new City { Code = "ZRH", Name = "Zürich", Currency = chf };

    this.target.Add(city);

    this.target.Save();

    dbGeneratedCityId = city.Id;

 

    // get the parent again from the database and get rid of the child

    City reloadedCity = this.target.GetById<City>(new City { Id = dbGeneratedCityId });

    Assert.IsNotNull(reloadedCity.Currency);

    reloadedCity.Currency = null;

    this.target.Save();

 

    // assert that the child was properly deleted

    City reloadedUpdatedCity = this.target.GetById<City>(new City { Id = dbGeneratedCityId });

    Assert.IsNull(reloadedUpdatedCity.Currency);

}

 

Using SQLProfiler shows that the following queries are emitted for the first code block

 

Calling Save() in the second block throws the following exception:

Aha, sure, this is due to the “1” in the “1:*” relationship – no cities without a default local currency (not even south european EU contries … sorry, no, not funny).

Let’s change this to “0..1”

Running the same test again shows “green”

emitting the Update statement.

Please note:

• depending on the constraint, there may be no Delete(chf) necessary
• relationship constraints violations throw exceptions
• simply set the parent’s child navigation property to null
• loosening the constraint to “0..1” leaves the child entity in the database

In comparison we can do the following (with “0..1” relationship):

[TestMethod]

public void DeleteChildEntityFromParent_WithManyToOneConstraint_Throws()

{

    // save parent and child with a 0..1:* relationship constraint

    int dbGeneratedCityId = -1;

    Currency euro = new Currency { Symbol = "EUR", Name = "Euro" };

    City city = new City { Code = "ATH", Name = "Athens", Currency = euro };

    this.target.Add(city);

    this.target.Save();

    dbGeneratedCityId = city.Id;

 

    // get the parent again from the database and get rid of the child

    City reloadedCity = this.target.GetById<City>(new City { Id = dbGeneratedCityId });

    Assert.IsNotNull(reloadedCity.Currency);

   this.target.Delete(reloadedCity.Currency);  // Athens and no more Euro

    this.target.Save();

 

    // assert that the child was properly deleted

    City reloadedUpdatedCity = this.target.GetById<City>(new City { Id = dbGeneratedCityId });

    Assert.IsNull(reloadedUpdatedCity.Currency);

}

and get

Setting the constraint back to “1:*” throws too:

DeleteChildEntityFromParent_WithManyToOneConstraint_Throws threw exception:
System.Data.UpdateException: A relationship from the 'CityCurrency' AssociationSet is in the 'Deleted' state. Given multiplicity constraints, a corresponding 'City' must also in the 'Deleted' state.

Guessing the same behavior for NHibernate. Can anybody confirm this?