I have database tables that look like this:

![Schema Diagram](https://i.stack.imgur.com/0iV2D.png)

A `Task` can be mapped to a `Module`, or not mapped at all (0...1). I'm using Entity Framework database-first, and when I generated the model from the database, the Task entity came through with Modules as a collection (0 or more). So I opened up my EDMX and changed the "Modules" navigation property on Task to 0...1.

![EDMX](https://i.stack.imgur.com/ND3fn.png)

Now, when I attempt to compile, I get this error:

> 
> Error 3003: Problem in mapping fragments starting at line 1241:Given the cardinality of Association End Member Task, it should be mapped to key columns of the table TaskModule. Either fix the mapping or change the multiplicity of this end.
> 
> 
> 

I don't understand what I need to do to fix this. I've looked at the association details and can't see the issue. I know I'm probably missing something stupid, but am totally stuck. Association properties:

![TaskModule Association](https://i.stack.imgur.com/DAcqr.png)

Visual Studio 2010 SP1, Entity Framework 4.3.1.0, SQL Server 2008 R2.