Cannot insert duplicate key when directly assigning Foreign Key with EF - entity-framework

Question

Model:
class TableA
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID {get;set;}
}
class TableB
{
[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public int ID {get;set;}
public int TableAID {get;set;}
[ForeignKey("TableAID")]
public virtual TableA TableA {get;set;}
}
Code:
var copyB = sourceDb.TableB.Single(B => B.ID == TableBID_Param); //EG. B.ID == 17
copyB.TableAID = TableAID_Param; //EG. B.TableAID = 5;
destDb.TableB.Add(copyB);
destDb.SaveChanges(); //Error occurs here
I have changed the Foreign Key ID, and the Primary Key should be regenerated

Answer 1

The problem here, is the the copyB object is:
still associated with sourceDb
likely lazy loading TableA (if TableAID has a value)
The TableA navigation property will take precedence over the TableAID foreign key property. DestDb will try to insert the TableA object, and this likely already exists (possibly from a previous successful execution of this flawed code).
You should quarantine your objects between contexts, in a similar way that you should use a separate ViewModel when sending data to a web client - you have control how it gets serialised. This also makes it clear to a future developer that a new Primary Key is expected (not implied)
Extend TableB:
class TableB
{
public string SomeData {get;set;} //Let's pretend there's something interesting to clone
public TableA ContextClone()
{
var clone = new TableA();
clone.SomeData = SomeData;
//Nothing else get's assigned - it's up to the caller to map ForeignKeys
}
}
New Code:
var sourceB= sourceDb.TableB.Single(B => B.ID == TableBID_Param); //EG. B.ID == 17
var clone = sourceB.ContextClone();
clone.TableAID = TableAID_Param; //EG. B.TableAID = 5;
destDb.TableB.Add(clone);
destDb.SaveChanges();