Entity Framework and multi-tenancy database design

Question

I am looking at multi-tenancy database schema design for an SaaS concept. It will be ASP.NET MVC -> EF, but that isn't so important.

Below you can see an example database schema (the Tenant being the Company). The CompanyId is replicated throughout the schema and the primary key has been placed on both the natural key, plus the tenant Id.

Plugging this schema into the Entity Framework gives the following errors when I add the tables into the Entity Model file (Model1.edmx):

• The relationship 'FK_Order_Customer' uses the set of foreign keys '{CustomerId, CompanyId}' that are partially contained in the set of primary keys '{OrderId, CompanyId}' of the table 'Order'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_OrderLine_Customer' uses the set of foreign keys '{CustomerId, CompanyId}' that are partially contained in the set of primary keys '{OrderLineId, CompanyId}' of the table 'OrderLine'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_OrderLine_Order' uses the set of foreign keys '{OrderId, CompanyId}' that are partially contained in the set of primary keys '{OrderLineId, CompanyId}' of the table 'OrderLine'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_Order_Customer' uses the set of foreign keys '{CustomerId, CompanyId}' that are partially contained in the set of primary keys '{OrderId, CompanyId}' of the table 'Order'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_OrderLine_Customer' uses the set of foreign keys '{CustomerId, CompanyId}' that are partially contained in the set of primary keys '{OrderLineId, CompanyId}' of the table 'OrderLine'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_OrderLine_Order' uses the set of foreign keys '{OrderId, CompanyId}' that are partially contained in the set of primary keys '{OrderLineId, CompanyId}' of the table 'OrderLine'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.
• The relationship 'FK_OrderLine_Product' uses the set of foreign keys '{ProductId, CompanyId}' that are partially contained in the set of primary keys '{OrderLineId, CompanyId}' of the table 'OrderLine'. The set of foreign keys must be fully contained in the set of primary keys, or fully not contained in the set of primary keys to be mapped to a model.

The question is in two parts:

1. Is my database design incorrect? Should I refrain from these compound primary keys? I'm questioning my sanity regarding the fundamental schema design (frazzled brain syndrome). Please feel free to suggest the 'idealized' schema.
2. Alternatively, if the database design is correct, then is EF unable to match the keys because it perceives these foreign keys as a potential mis-configured 1:1 relationships (incorrectly)? In which case, is this an EF bug and how can I work around it?

Answer 1

On a quick scan of EF's error messages, it clearly doesn't like the way you're setting up compound keys, and I think it's probably nudging you in the right direction. Give some thought again to what makes your primary keys unique. Is the OrderID alone not unique, without a CompanyID? Is a ProductID not unique, without a CompanyID? An OrderLine certainly should be unique without a CompanyID, since an OrderLine should be associated only with a single Order.

If you truly need the CompanyID for all of these, which probably means that the company in question is supplying you with ProductID and OrderID, then you might want to go a different direction, and generate your own primary keys that are not intrinsic to the data. Simply set up an auto-increment column for your primary key, and let these be the internal OrderID, OrderLineID, ProductID, CompanyID, etc. At that point, the OrderLine won't need the customer's OrderID or CompanyID; the foreign key reference to the Order would be its starting point. (And the CustomerID should never be an attribute of an order line; it's an attribute of the order, not the order line.)

Compound keys are just messy. Try designing the model without them, and see if it simplifies things.

Answer 2

I think storing the company number in each of the tables is hurting you more than helping. I can understand why you want to do this (as the programmer/dba you can just go into any table and 'see' what data belongs to who which is comforting), but it is getting in the way of you setting up the database the way it should be.

Avoid the compound keys and your design gets a whole lot simpler.