Improve queries in placements policies

app/policies/placements/mentor_policy.rb

@@ -3,7 +3,7 @@ class Scope < ApplicationPolicy::Scope
     def resolve
       return scope if user.support_user?
 
-      scope.where(id: Placements::MentorMembership.select(:mentor_id).where(school: user.schools))
+      scope.where(id: user.schools.joins(:mentor_memberships).select(:mentor_id))
     end
   end
 end

app/policies/provider_policy.rb

@@ -3,7 +3,7 @@ class Scope < ApplicationPolicy::Scope
     def resolve
       return scope if user.support_user?
 
-      scope.where(id: Placements::Partnership.select(:provider_id).where(school: user.schools))
+      scope.where(id: user.schools.joins(:partnerships).select(:provider_id))
     end
   end
 end

app/policies/school_policy.rb

@@ -3,7 +3,7 @@ class Scope < ApplicationPolicy::Scope
     def resolve
       return scope if user.support_user?
 
-      scope.where(id: Placements::Partnership.select(:school_id).where(provider: user.providers))
+      scope.where(id: user.providers.joins(:partnerships).select(:school_id))
     end
   end
 end