新增單位管理以及一些功能修正

app/Models/Product.php

@@ -17,10 +17,10 @@ class Product extends Model
         'category_id',
         'brand',
         'specification',
-        'base_unit',
-        'large_unit',
+        'base_unit_id',
+        'large_unit_id',
         'conversion_rate',
-        'purchase_unit',
+        'purchase_unit_id',
     ];
 
     protected $casts = [
@@ -35,6 +35,21 @@ class Product extends Model
         return $this->belongsTo(Category::class);
     }
 
+    public function baseUnit(): BelongsTo
+    {
+        return $this->belongsTo(Unit::class, 'base_unit_id');
+    }
+
+    public function largeUnit(): BelongsTo
+    {
+        return $this->belongsTo(Unit::class, 'large_unit_id');
+    }
+
+    public function purchaseUnit(): BelongsTo
+    {
+        return $this->belongsTo(Unit::class, 'purchase_unit_id');
+    }
+
     public function vendors(): \Illuminate\Database\Eloquent\Relations\BelongsToMany
     {
         return $this->belongsToMany(Vendor::class)->withPivot('last_price')->withTimestamps();