clients.js

// routes/company/clients.js
import Ember from 'ember';

export default Ember.Route.extend({
  model(params) {
    let company = this.modelFor('company');
    
    // this
    return this.get('store').query('client', {
      company: company.get('id')
    });
    
    // or simply do this
    return company.get('clients');
  }
});

company.js

// routes/company.js
import Ember from 'ember';

export default Ember.Route.extend({
  model(params) {
    return this.get('store').findRecord('company', params.company_id);
  }
});

routes.js

this.route('company', { path: '/company/:company_id' }, () => {
  this.route('clients');
});