Stuck with the same bug

So, this thread is just a continuation of previous topic (Always getting not null object even though there are no objects with these parameters; - #10 by komi94).

Basically everything will be clear what the bag is from screenshots that I will leave. Also I will leave snippets of models, template and controller.

-First insertion of the grade:

-Second insertion of the grade (from different subject):

-Models:

app/models/class.js:

import DS from 'ember-data';

export default DS.Model.extend({
	name: DS.attr('string'),
	subjects: DS.hasMany('subject'),
	students: DS.hasMany('student')
});

app/models/student.js:

import DS from 'ember-data';

export default DS.Model.extend({
	firstName: DS.attr('string'),
	lastName: DS.attr('string'),
	grades: DS.hasMany('grade'),
	class: DS.belongsTo('class')
});

app/models/subject.js:

import DS from 'ember-data';

export default DS.Model.extend({
	title: DS.attr('string'),
	grades: DS.hasMany('grade'),
	class: DS.belongsTo('class')
});

app/models/grade.js:

import DS from 'ember-data';

export default DS.Model.extend({
	ocene: DS.attr('array'),
	subject: DS.belongsTo('subject'),
	student: DS.belongsTo('student')
});

-Template:

<br> <br> <br> <br> <br>
<center>

<table>

<h2> Chosen class: {{model.name}} </h2>

<br>

<i> Current students in class: </i> <br>

<ol> 
{{#each model.students as |student|}}
  <li> {{student.firstName}}  {{student.lastName}} </li>
{{#if showS}}
<br> Current subjects in class: 
{{#each model.subjects as |subject|}}
  <br> <b> {{subject.title}} </b>
   Choose grade: 
  <select onchange={{action (mut ocena) value="target.value"}}>
    <option value="1"> 1 </option>
    <option value="2"> 2 </option>
    <option value="3"> 3 </option>
    <option value="4"> 4 </option>
    <option value="5"> 5 </option>
  </select>
  <br>
  <button {{action 'unesiOcenu' student.id subject.id}}> Insert mark for student </button>
{{/each}}

  {{#each student.grades as |grades|}}
			<table>
				<th> {{grades.subject.title}} </th>
				<th> {{grades.marks}} </th>
			</table>
		{{/each}}
  {{/if}}
{{else}}
  <b> Currently there are no students for the class! </b>
{{/each}}
</ol>

</table>
  <button class="choose" {{action 'showSubjects'}}> Show subjects </button> 
</center>

-Controller:

findGradesFor: function(student, subject){
			var grades = this.store.peekAll('grade');

			grades.filter((grade) =>{
				return grade.get('student') === student && grade.get('subject') === subject;
			});

			return grades.get('length') ? grades : null;
		},

and action:

unesiOcenu(student, subject){
			var mark = this.get('ocena');

			console.log(student.get('firstName') + "" + student.get('lastName'));
			console.log(subject.get('title'));

			var filteredGrades = this.findGradesFor(student, subject);

			if(filteredGrades.get('firstObject')){
				filteredGrades.get('firstObject').get('marks').pushObject(mark);
				filteredGrades.save();
			}else{
				this.store.createRecord('grade', {
					marks: [mark],
					student: student,
					subject: subject 
				}).save().then(function(ocena){
					student.get('grades').pushObject(ocena);
					student.save();
					subject.get('grades').pushObject(ocena);
					subject.save();
				});
			}
		}