I have 2 tables with a master-detail relationship. We use a DataGridView to handle the detail records. The detail records have fields with lookup values (foreign keys). Instead of setting the ID value of the lookup tables we set the object value. This all works well, instead of one situation. If we add a detail row and set at least one of the lookup fields and remove this row before saving (submitchanges), we receive this error:
An attempt was made to remove a relationship between a TableX and a TableY. However, one of the relationship’s foreign keys (TableY.TableXID) cannot be set to null.
This problem was discussed in this question (scroll down to answer of Neil Barnwell), but it doesn’t have an answer for this particular problem. The solution “Call Datacontext.GetChanges” doesn’t help as Datacontext doesnt expose a method GetChanges and GetChangedset fails with same error.
It seems that if you set an object of the entity and remove the new record before any submit Linq still thinks you want to add the new record. In this case you have to set all objects that have FK references to null.
See this post which describes the problem and solution in detail: http://social.msdn.microsoft.com/Forums/en/linqtosql/thread/4170c7bb-f727-4e6a-9190-fc268ae4ce4b