entity framework - "The property $propertyName is part of the object's key information and cannot be modified." on navigation property with shared composite key -

to preface question: problem not because i'm directly setting key property in model entity object (which cause of issue in other search results same exception message).

i'm making heavy use of composite keys in application, here's simplified version of current db schema (key fields in *asterisks*):

tenants( *tenantid*, ... ) categories( *tenantid*, *categoryid*, ... ) documents( *tenantid*, *documentid*, categoryid, ... ) 

the documents table has fk relationships both tenants , categories, both using same documents.tenantid column. documents.categoryid column nullable.

when this, exception:

tenant tenant = gettenant( 123 ); document doc = tenant.documents.first();  category newcategory = new category(); newcategory.tenantid = 123; dbcontext.categories.add( newcategory );  doc.category = newcategory; <-- exception thrown on line, without calling dbcontext.savechanges() @ all. 

i believe exception because setting category on document instance causes tenantid property set indirectly ef (because it's part of documents -> categories fk association.

what solution?

workaround update

i'm able hack creating new category entities saving them, identity values back, setting document properties directly:

tenant tenant = gettenant( 123 ); document doc = tenant.documents.first();  category newcategory = new category(); newcategory.tenantid = 123; dbcontext.categories.add( newcategory );  dbcontext.savechanges();  doc.categoryid = newcategory.categoryid  dbcontext.savechanges(); 

but ideally i'd work in single call savechanges() , using entity model navigation properties instead of scalar attribute properties.

for initial problem, worked-around using "workaround update" posted original posting.

however problem happened again different entity type (again, composite key involved in foreign-key) , noticed ef throws exception if call dbcontext.entry() on entity in graph while new entity in added state - not throw exception again if re-call entry() or savechanges(), , in fact saves new entities correctly in spite of initial exception - i'm thinking might bug in ef.

here's have now:

tenant tenant = gettenant( 123 ); document doc = tenant.documents.first();  category newcategory = new category(); newcategory.tenantid = 123; dbcontext.categories.add( newcategory );  doc.categoryid = newcategory.categoryid  try {     dbcontext.entry( doc ); } catch(invalidoperationexception) { }  dbcontext.savechanges(); 

it's ugly, works - , avoids having call savechanges twice.