sql access control

-- get a collaborator ID and a document id
-- it should return nothing when the access is not allowed
-- it should return one entry when access is allowed
-- e.g.   
-- get get bob and the object Kotti. Is bob allowed to see it (yes, bob is owner of the project)
-- get eve and the object kotti. Is eve allowed to see it (no, eve is neither collaborator nor ownwer)

-- SCHEMA is:
-- Documents: This is what people request. They might not have access to the document, we wanna check this. Documents belong to projects
-- Projects: Collect documents. Each project has an owner
-- Users: Can own projects or be collaborators on them
-- Collaborators: many-to-many match between users and projects

-- here tested with user id 2 and document id 2 but it could be any other id. 
SELECT
documents.id as doc_id,
projects.id as proj_id,
documents.name as doc_name,
projects.name as proj_name,
join_users_projects.id_user as collaborator
 FROM documents 
 JOIN projects on documents.project = projects.id
 JOIN join_users_projects on join_users_projects.id_project = projects.id
 WHERE collaborator = 2
 AND doc_id = 2