Nested Forms and HABTM fails for existing model, no join record

I am trying to created a nested form with a HABTM association and I am
unable to find an approach that works for all cases.

What I am trying to achieve is a Project form that can:

1. Create a new project and create new company to associate with it

- This works perfectly

2. Create a new project with an existing company via autocomplete.

- This fails because it's looking for an existing record in the HABTM
join table *before* there is a project id.

Here is the SQL that comes out of this failed case:

SELECT "companies".* FROM "companies" INNER JOIN "companies_projects" ON
"companies".id = "companies_projects".company_id WHERE ("companies"."id"
IN ('6')) AND ("companies_projects".project_id = NULL )

In my Project model I am using:
accepts_nested_attributes_for :companies

Is this the wrong approach to tackling this use case? I've read that
this is only good for 1-1 or 1-n relationships. What is the preferred
method for doing something like this?

Thanks in advance,
David