associations - Rails - counting instances where boolean is true -

i trying make app rails 4.

i have project model , project invitations model.

projects has many project invitations project invitations belong projects 

in project show, im trying count how many invitations have been sent , how many have been accepted.

the first part works fine. acceptances, have attribute in project_invitation table called :student_accepted. if true, want count record.

<%= @project.project_invitations.size %>             <% if @project.project_invitations.student_accepted == true %>                 <%= @project.project_invitations.size %>                 <% else %>                 'no'             <% end %> 

it gives error:

undefined method `student_accepted' #<activerecord::associations::collectionproxy []> 

i have tried:

<% if project.project_invitations.student_accepted == true %>  <%= project.project_invitations.size %> 

it gives error:

undefined local variable or method `project' #<#<class:0x007fc01d9dcbe8>:0x007fc01de04248> 

im struggling understand how reference attributes though associated models. have read several books assume background knowledge. i've had helpful input on related questions (below), still not grasping concept.

http://stackoverflow.com/questions/32916133/rails-how-to-show-attribute-of-an-associated-model  http://stackoverflow.com/questions/32898541/rails-how-to-show-attributes-from-a-parent-object 

can see i've done wrong?

you can find number of invitations students have accepted with:

@project.project_invitations.where(student_accepted: true).count 

rails's active record query interface guide explains how works.

the reason received undefined method 'student_accepted' #<activerecord::associations::collectionproxy []> error because calling student_accepted on activerecord::associations::collectionproxy, object rails creates define collection of records.

if wanted iterate on collection do:

<% @project.project_invitations.each |invitation| %>   # here can call `invitation.student_accepted` <% end %> 

this necessary because project has many invitations.