c# - Entity Framework error, only when deleting "multiple" Parent then Child row -

i'm building , mvc project using entity-framework 6 (code first).

my database data models (code first):

public class parentnode {    public int parentnodeid { get; set; }     public string someparentdata { get; set; }     //fk    [required]     public int childnodeid { get; set; }    public virtual childnode childnode { get; set; } }   public class childnode {    public int childnodeid { get; set; }     public string somechildishdata { get; set; }     //references table/model    public virtual icollection<parentnode> parentnodes{ get; set; } } 

the issue: deleting multiple childnodes

• i won't rely on cascade delete
• parentnode->childnode has many 1 relation. in case need delete 1 childnode, first delete parentnode childnode ()
• deleting 1 childnode done this:

deleting 1 childnode: no problem (if 1 present in list)

deleting multiple childnodes: fk null reference error occurs

{  ...  foreach(parentnode parenttodelete in parentnodelist)  {    db.entry(parenttodelete).state = entitystate.deleted;    db.savechanges();       }   foreach(childnode childtodelete in childnodelist)  {    db.entry(childtodelete).state = entitystate.deleted;    db.savechanges();   //error exception occurs here!!  }  ... } 

error message:

an exception of type 'system.invalidoperationexception' occurred in entityframework.dll not handled in user code

additional information: operation failed: relationship not changed because 1 or more of foreign-key properties non-nullable. when change made relationship, related foreign-key property set null value. if foreign-key not support null values, new relationship must defined, foreign-key property must assigned non-null value, or unrelated object must deleted.

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table rows state when error occurred

parent

pk: restaurantmenucategoryid

fk: restaurantmenunameid

child

pk: restaurantmenunameid

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as can see parent table still has reference child row restaurantmenunameid == 2, whilst expected behaviour parent reference should have been deleted before accessing child table.

my observations:

• by looking @ database, seems when db.savechanges() occurs parent node, though delete query in database hasn't occurred, code keeps on moving asynchronously. synchronous behaviour should used, instead of fire , forget?
• an other possibility dbcontext isn't updated after parent delete, have been true if looking @ db parent rows deleted "still there". going on?

question: please let me know problem is, main topics related issue?