
Today's cryptic Entity Framework message ...

*The operation failed: The relationship could not be changed because one
or more of the foreign-key properties is non-nullable. When a change is
made to a relationship, the related foreign-key property is set to a
null value. If the foreign-key does not support null values, a new
relationship must be defined, the foreign-key property must be assigned
another non-null value, or the unrelated object must be deleted.*

The reason for this was a simple override to GetHashCode(). Entity
Framework seems somewhat pedantic about how hash codes work.


The relationship could not be changed because one or more of the foreign-key properties is non-nullable


Today I tried creating a two-level hierarchy using Entity Framework 4
but using the same discriminator column for both layers of the
hierarchy.  It seems you cannot do this but instead must have a separate
column for each layer of the hierarchy.  Makes some sense but the error
message wasn't very helpful.

I have a table-per-hierarchy model, one abstract class (Node), a column
that differentiates the sub-classes (NodeType), and three sub-classes
(NodeClass, NodeProperty, NodeIndividual) with conditions (NodeType=1,
2, 3), it's working just fine.

I now want to add another sub-class which itself has three sub-classes.
 So I create the new sub-class marking it abstract (NodeLiteral), I
create the three derived types inheriting from it (NodeiteralInt,
NodeLiteralDouble, NodeLiteralDateTime) and add conditions on them based
on the same discriminator column used in the first level of inherited
classes (NodeType=101,102,103).

When I compile I get:-

Error 3032: Problem in mapping fragments starting at lines ... (classes)
... are being mapped to the same rows in table Node. Mapping conditions
can be used to distinguish the rows that these types are mapped to.

Since I DO have mapping conditions that are distinct for each of these
classes the error message is somewhat confusing.

If I add another column and use that as the discriminator for the second
level of inheritance it works.

So if you are trying to model a two or more layer hierarchy in EF don't
try to use a single column for all your conditions, create a column for
each layer of the hierarchy instead.


The Blog of Ian Mercer.

Topic: entity-framework

The relationship could not be changed because one or more of the foreign-key properties is non-nullable

Error 3032: Problem in mapping fragments : mapped to the same rows in table