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

Question

To preface this question: my problem is not because I'm directly setting a key property in my model entity object (which is the cause of the issue in other search results for the same exception message).
I'm making heavy use of composite keys in my application, here's a simplified version of my current DB schema (key fields in *asterisks*):
Tenants( *TenantId*, ... )
Categories( *TenantId*, *CategoryId*, ... )
Documents( *TenantId*, *DocumentId*, CategoryId, ... )
The Documents table has FK relationships with both Tenants and Categories, both using the same Documents.TenantId column. The Documents.CategoryId column is NULLable.
When I do something like this, I get the 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 is thrown on this line, without calling dbContext.SaveChanges() at all.
I believe the exception is because setting Category on the Document instance causes the TenantId property to be set indirectly by EF (because it's part of the Documents -> Categories FK association.
What is the solution?
Workaround Update
I'm able to hack it by creating the new Category entities then saving them, to get the IDENTITY values back, then setting the 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 like this to work in a single call to SaveChanges() and using the Entity Model Navigation Properties instead of scalar attribute properties.

Answer 1

For this initial problem, I worked-around it using the "Workaround Update" I posted to my original posting.
However this problem happened again for a different entity type (again, with a composite key involved in a foreign-key) and I noticed that EF throws the exception even if you call dbContext.Entry() on any entity in the graph while the new entity is in the Added state - but it does not throw the exception again if you re-call Entry() or even SaveChanges(), and in fact it saves the new entities correctly in spite of the initial exception - so I'm thinking this might just be a bug in EF.
Here's essentially what I 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, but works - and avoids having to call SaveChanges twice.