Organization Id Field as a Composite Primary Key

Question

We currently have a system where each client gets their own database when they sign-up. Not surprisingly this is getting out of hand.

We are preparing merge all these databases together to a single db. In order to do this we need to note which organization the row belongs. How best is this handled on the database and what pros/cons (Speed, Maintainability, etc) are there to the methods below?

Option 1: Make the Organization Id to all tables and make it part of the primary key (making all keys composite).

Option 2: Add Organization Id to all tables as a column with a Foreign Key to the Organization Table.

Option 3: Something else.

We are looking at moving to NHibernate with this move, if that has an effect on what is done.

Answer 1

Option 1 and 2 are not mutually exclusive and you should do both. A foreign key constraint on the organization helps ensure that your organization keys are valid. A composite primary key is the only way to achieve uniqueness, unless you want to re-key all your records, and it is also valuable for lookup performance. Alternatively, you'd have to create a new primary key column on the combined data, an identity column, for instance. Such a change would likely require more changes in other places.