Question

clients :has_many :project,:roles

I have implement but its not a good implementation..

Answer 1

joins can be used several levels deep, for example that Activity belongs to Carrier
Activity.joins(:carrier => {:projects => {:client => :roles}})

will generate sql along the lines of

SELECT * from activities
JOIN carriers on activities.carrier_id = carriers.id
JOIN projects on carriers.id = projects.carrier_id
JOIN clients on projects.client_id = clients.id
JOIN roles on roles.client_id = clients.id

to which you can add your conditions. You’ll need to qualify you column names, i.e. use projects.id instead of just id so that your database knows which id you want.