Filtering For A Result - Advice

I'm creating an advanced search form and I have to filter the results
based on whether particular rows exist in other tables.

For example:
Project has_many ResearchProjects

:joins => "INNER JOIN research_projects_vw ON
research_projects_vw.fin_project_id = project_totals_vw.fin_project_id",
         :conditions => ["research_projects_vw.funding_type_code in
(?)", ns['funding_types'].split(',')])

Is there a good way of preventing getting a Cartesian product using this
join, or should i just be providing an option like :select => 'distinct
fielda, distinct fieldb' etc