validation - Angular2 FormBuilder Validators: require at least one field in a group to be filled

Question

I have a form where I'm collecting phone numbers (mobile, personal, other). I need to have at least input populated. I'm trying to use Angular2 FormBuilder.

After much research I'm having a problem getting my head around this problem. I know I can do it using other methods but I was wondering if it's possible using FormBuilder Validators. If I add "Validators.required" then all 3 fields are required. Any suggestions or ideas?

phone: this._fb.group(
                    {
                        other: [''],
                        personal: [''],
                        mobile: [''],
                    }

Base on the hint from " JB Nizet", here's what I had to implement to make it work:

My group Validator (it still needs tweaking):

static phoneExists(group: FormGroup): { [key: string]: any } {

    if (null != group) {
        var other: AbstractControl = group.controls['other'];
        var mobile: AbstractControl = group.controls['mobile'];
        var personal: AbstractControl = group.controls['personal'];
        var data: Object = group.value;

        return (
            (other.valid && isEmptyInputValue(other.value))
            && (mobile.valid && isEmptyInputValue(mobile.value))
            && (personal.valid && isEmptyInputValue(personal.value))
            )
            ? { 'required': true }
            : null;
    }
}

My group change:

phone: this._fb.group(
                    {
                        other: [''],
                        personal: [''],
                        mobile: [''],
                    },
                    { validator: MyValidators.phoneExists }
                )

It took me a while, but the key is to add the key word "validator" and it will cause the group validator to fire.

In the HTML i added the following:

<small *ngIf="!myForm.controls.profile.controls.phone.valid" class="text-danger">
                                        At least one phone is required.
                                    </small>

I hope this help anyone else.

Answer 1

I use an atLeastOne function that creates a custom validator based on any existing validator:

import { FormGroup, ValidationErrors, ValidatorFn } from '@angular/forms';

export const atLeastOne = (validator: ValidatorFn) => (
  group: FormGroup,
): ValidationErrors | null => {
  const hasAtLeastOne =
    group &&
    group.controls &&
    Object.keys(group.controls).some(k => !validator(group.controls[k]));

  return hasAtLeastOne ? null : { atLeastOne: true };
};

The beauty is that you can use any validator with it and not just Validators.required.

In OP's case, it'll be used like this:

{
  phone: this._fb.group({
    other: [''],
    personal: [''],
    mobile: [''],
  }, { validator: atLeastOne(Validators.required) })
}