POST nested object Rest Spring MVC AngularJS

I am trying to resolve a problem with saving an entity that has one-to-one relationship.

I have a Tenant object that has a field Apartment. Every time when I try to save a new Tenant object I get 400 error although the params seems ok to me:

Params:

Response:

If I remove Apartment from the input form the Tenant gets to be saved without a problem.

How can I save an entity along with it's relation???

RestController

@RequestMapping(method = RequestMethod.POST)
    public ResponseEntity<?> create(@Valid @RequestBody Tenant tenant, BindingResult result) {
        if (result.hasErrors()) {
            ApiResponse message = new ApiResponse();
            message.setErrors(result.getFieldErrors());
            return new ResponseEntity<ApiResponse>(message, HttpStatus.UNPROCESSABLE_ENTITY);
        }
        try {
            service.saveTenant(tenant);
        } catch (ConstraintViolationException e) {
            result.rejectValue("apartmentNumber", "error.apartment", DUPLICATE_VALUE);

            Map<String, String> test = new HashMap<>();
            for (FieldError fieldError : result.getFieldErrors()) {
                test.put(fieldError.getField(), fieldError.getDefaultMessage());
            }
            return new ResponseEntity<Map<String, String>>(test, HttpStatus.CONFLICT);
        }
        return new ResponseEntity<Tenant>(tenant, HttpStatus.CREATED);
    }

JSP page - the input in question

<div class="form-group">
    <label for="inputEmail3" class="col-sm-3 control-label">Mieszkanie</label>
    <div class="col-sm-9">
        <select name='apartment' ng-model="ctrl.tenant.apartment" ng-required='true' class="form-control">
          <option ng-repeat="item in ctrl.apartments" value="{{item}}"
          ng-selected="ctrl.tenant.apartment.description == item.description">
            {{item.description}}</option>
        </select>
        <p class="help-block">
            <span class='error' ng-show="myForm.tenant_apartment.$invalid">
              Pole wymagane</span> <span class='error'>{{errors.tenant_apartment}}</span>
        </p>
    </div>
</div>