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CannotDeleteDefaultValueException

Thrown when attempting to delete the base unit while other units of the same type exist.

Namespace​

JobMetric\UnitConverter\Exceptions\CannotDeleteDefaultValueException

HTTP Status Code​

400 Bad Request

Constructor​

public function __construct(
    int $code = 400,
    ?Throwable $previous = null
)

Parameters​

ParameterTypeDescription
$codeintHTTP status code (default: 400)
$previousThrowable|nullPrevious exception for chaining

Understanding the Restriction​

The base unit cannot be deleted while other units exist because:

  1. Other units' values are defined relative to the base unit
  2. Deleting the base would leave other units without a reference point
  3. All conversion calculations would become invalid

Example Scenario​

Weight Type:
├── Gram (value = 1) ← BASE UNIT
├── Kilogram (value = 1000) ← relative to gram
├── Milligram (value = 0.001) ← relative to gram
└── Ton (value = 1000000) ← relative to gram

If gram is deleted:

  • What does value = 1000 for kilogram mean?
  • There's no reference point for conversions
  • The entire type becomes unusable

When Is It Thrown?​

This exception is thrown when:

  1. You try to delete a unit with value = 1 (base unit)
  2. AND other units of the same type still exist

Example: Invalid Delete Attempt​

use JobMetric\UnitConverter\Facades\UnitConverter;

// Weight type has: gram (base), kilogram, ton

$gram = UnitConverter::findUnitByCode('g');

// Trying to delete the base unit - WRONG!
UnitConverter::destroy($gram->id);
// Throws: CannotDeleteDefaultValueException

When Delete IS Allowed​

You CAN delete the base unit if it's the only unit in that type:

use JobMetric\UnitConverter\Facades\UnitConverter;

// Assume: Weight type has ONLY gram (no other units)

$gram = UnitConverter::findUnitByCode('g');
UnitConverter::destroy($gram->id);
// Success! Type is now empty

Handling the Exception​

In Controllers​

use JobMetric\UnitConverter\Exceptions\CannotDeleteDefaultValueException;
use JobMetric\UnitConverter\Facades\UnitConverter;

class UnitController extends Controller
{
    public function destroy(int $id)
    {
        try {
            $response = UnitConverter::destroy($id);
            return response()->json(['message' => 'Unit deleted']);
        } catch (CannotDeleteDefaultValueException $e) {
            return response()->json([
                'error' => 'cannot_delete_base_unit',
                'message' => $e->getMessage(),
                'hint' => 'Delete all other units first, or use changeDefaultValue() to set a different base unit.',
            ], 400);
        }
    }
}

In Admin Panels​

Show a clear warning when users try to delete the base unit:

// Check if unit can be deleted
public function canDelete(Unit $unit): array
{
    // Base unit with other units in type
    if ((float) $unit->value === 1.0) {
        $otherUnitsExist = Unit::where('type', $unit->type)
            ->where('id', '!=', $unit->id)
            ->exists();
        
        if ($otherUnitsExist) {
            return [
                'can_delete' => false,
                'reason' => 'This is the base unit. Delete other units first or change the base unit.',
            ];
        }
    }
    
    // Check if unit is used in relations
    if ($unit->unitRelations()->exists()) {
        return [
            'can_delete' => false,
            'reason' => 'This unit is in use by other models.',
        ];
    }
    
    return ['can_delete' => true];
}

Solutions​

Solution 1: Change the Base Unit First​

If you need to delete the current base unit, first designate another unit as the base:

use JobMetric\UnitConverter\Facades\UnitConverter;

// Current: Gram (base), Kilogram, Ton

// Step 1: Make Kilogram the new base
$kilogram = UnitConverter::findUnitByCode('kg');
UnitConverter::changeDefaultValue($kilogram->id);

// After: Gram (0.001), Kilogram (base), Ton (1000)

// Step 2: Now you can delete Gram
$gram = UnitConverter::findUnitByCode('g');
UnitConverter::destroy($gram->id);
// Success!

Solution 2: Delete All Other Units First​

If you want to completely remove a unit type:

use JobMetric\UnitConverter\Facades\UnitConverter;
use JobMetric\UnitConverter\Models\Unit;

// Get all weight units
$weightUnits = Unit::where('type', 'weight')
    ->orderByDesc('value') // Delete non-base units first
    ->get();

foreach ($weightUnits as $unit) {
    // Skip base unit for now
    if ((float) $unit->value === 1.0) {
        continue;
    }
    
    UnitConverter::destroy($unit->id);
}

// Now delete the base unit (it's the only one left)
$baseUnit = Unit::where('type', 'weight')->first();
UnitConverter::destroy($baseUnit->id);

Solution 3: Bulk Delete Entire Type​

If you need to delete all units of a type at once:

use JobMetric\UnitConverter\Models\Unit;

// WARNING: This bypasses business logic checks
// Use only when you're certain no relations exist

Unit::where('type', 'weight')->delete();

⚠️ Warning: Direct deletion bypasses the exception and could leave orphaned unit_relations records.

Prevention​

1. Disable Delete Button for Base Units​

// Frontend
function canDeleteUnit(unit, allUnitsOfType) {
    // Is base unit?
    if (unit.value === 1) {
        // Check if other units exist
        const otherUnits = allUnitsOfType.filter(u => u.id !== unit.id);
        if (otherUnits.length > 0) {
            return false;
        }
    }
    return true;
}

2. Show Base Unit Badge​

<!-- In unit list -->
<tr v-for="unit in units">
    <td>{{ unit.name }}</td>
    <td>
        {{ unit.value }}
        <span v-if="unit.value === 1" class="badge badge-primary">Base Unit</span>
    </td>
    <td>
        <button 
            @click="deleteUnit(unit)"
            :disabled="!canDeleteUnit(unit)"
            :title="canDeleteUnit(unit) ? 'Delete' : 'Cannot delete base unit'"
        >
            Delete
        </button>
    </td>
</tr>

3. Provide Change Base Option​

Instead of delete, offer to change the base unit:

// In your controller
public function changeBase(Request $request, int $currentBaseId)
{
    $request->validate(['new_base_id' => 'required|exists:units,id']);
    
    $response = UnitConverter::changeDefaultValue($request->new_base_id);
    
    return response()->json([
        'message' => 'Base unit changed successfully',
        'new_base' => $response->data,
    ]);
}